Timing information by Lars - Corvette Forum : DigitalCorvettes.com Corvette Forums
 
 
Go Back   Corvette Forum : DigitalCorvettes.com Corvette Forums > C3 Corvette Forums > C3 Corvette
Register Forums Garage Garage Mark Forums Read Auto EscrowInsurance Advertise

Notices

C3 Corvette
C3 Corvette General | Technical | Performance | Aftermarket | Discussions

Other sections:
Corvettes Classifieds
Corvette Detailing & Car Care
Corvette Audio/Video/Radar
General Automotive/Shop/Tools
Corvette Performance Driving/Racing

( Sponsored by: Zip Corvette Parts )

Shops/Tuners
Custom Image Corvettes
A&A Corvette
Corvette tuner

Interior
Corvette aftermarket products

Insurance



Parts & Products
Race Ramps
Edelbrock
ATI/Procharger
Corvetteguys.com
Melrose Motorsports
Parts Taxi
Airaid
Pfadt Racing
Madvette Motorsports
Hi-tech Custom Concepts
Corvette Garage
Corvette Parts and Accessories
Corvette Car Care Products
Corvette HID

Tracks/Schools
Bob Bondurant School of High Performance Driving
Corvette driving school

Wheels/Tires
Cray Wheels

Services
BADWERKS.com
Reply
 
Thread Tools
Old 05-06-2007, 06:54 PM   #1
big2bird
DC Crew
 
big2bird's Avatar
 
Posts: 5,916
Member #58543
Member since: Mar 2007
Location: Anaheim,Ca.,USA

My Corvette(s)
1981 T-Top Caprice powered Disco Buggy

Thanks: 0
Thanked 1 Time in 1 Post
Timing information by Lars

Technical Information Bulletin Rev. G 9-11-05
How to Set Your Timing for Peak Performance

by Lars
Lafayette, CO


“90% of all “carb problems” are ignition timing problems.”
(Lars Grimsrud)

This tech paper will discuss setting the timing on a Chevy V8. This procedure also applies to other GM V8s.

The procedure outlined here differs from the Service Manual, and is based on my years of experience doing this work in the quickest, least painful, most economical way while keeping the level of quality high. It is recognized that other people will have different methods of doing things, and may disagree with specific methods and procedures that I use.

How to Set the Timing
When you think about it, setting the timing at idle speed makes no sense at all: You don’t operate your car at idle, and timing changes as the rpm changes. Fact is, the timing spec at idle speed is provided as a simple way for most people to set the timing, and is not a good procedure for optimum performance.

Small block Chevys (and most other GM performance V8 engines) perform best when the total timing (full centrifugal advance plus the initial timing setting with vacuum advance hose disconnected) is all in by 2,500 – 2,800 rpm and is set to about 36 degrees. If you have an adjustable timing light, this is very easy to check. If you don’t, you need to scribe a 36-degree mark on your harmonic balancer. Here’s how:

Measure the circumference of your harmonic balancer using a sewing tape measure (or other flexible tape measure). Get it as accurate as you can. Take this measurement and divide by 10. The number you get is the distance to 36 degrees. Measure this distance CLOCKWISE from your existing harmonic balancer timing mark as viewed from the front of the engine and place a clear mark on the balancer.

Remove your distributor cap and rotor. If you have a points-style distributor with the stock, factory, heavy springs in place, remove one of the springs. Disconnect the vacuum advance. Install the rotor and cap. Loosen the distributor hold-down clamp bolt just enough so that the distributor can by turned, yet leave it snug enough that the distributor will hold its position.

Start the engine. If you’re using an adjustable timing light, set the light to 36 degrees advanced. Now rev the engine while observing the timing marks with the light. You will notice that the stock line on the balancer will move up towards the timing plate as rpm increases. Continue to increase rpm until the line does not move any further (centrifugal advance is “pegged out”). Once the timing is “pegged out,” the line on the balancer should line up with the “0” mark on the timing tab. Rotate the distributor to achieve this.

If you’re using a non-adjustable light, perform the same process, but align your new 36-degree mark with “0” mark on the timing tab.

Shut it down.

Pop the cap and rotor and re-install the spring, if you removed it. Put everything back together, but leave the vacuum disconnected. Start it up. For future reference, make a note of the timing setting at idle. This is your new curb idle timing spec. Now give the engine a few quick rev’s past 3,000 rpm and verify that the full timing (36 degrees) is coming in. If it’s not, you need to change to a softer set of springs until you get full 36-degree advance before 3000 rpm. (NOTE: A stock set of springs will often not allow full centrifugal advance to come in before redline rpm. If you have heavy stock springs installed, don’t rev the engine beyond its limits to try to force full advance in.) I suggest obtaining Mr. Gasket kit part number 927 or 928: Use the gold springs on HEI systems. For points-style systems, use one black spring and one silver spring – these springs will get your total timing all in by 2500-2800 rpm, providing very good throttle response and power. The black & silver spring combo can also be used on MSD distributors if you widen out the spring hook ends.

Hook up the vacuum. Re-set your idle speed and idle mixtures if necessary to lower the idle speed. Now do a road test.

The 36-degree 2500 rpm advance curve is optimum for performance, but may require premium fuel. Lug the car around, and punch the throttle at low rpm while listening for detonation (“engine knock”). If you’re getting any audible knock, you MUST retard the timing. Retard the timing in 2-degree increments until engine knock stops. Engine knock will seriously damage engine components if not corrected. If you get no knock, you may see slightly improved performance at 38 degrees total timing. This is particularly true if you’re running at high altitude.

If you have no engine knock under acceleration, but the car “chugs” or “jerks” at cruising speed (light throttle application), you are getting too much vacuum advance on top of the mechanical advance. You may need to change out the vacuum advance diaphragm with a unit producing no more than 16 degrees of vacuum advance. See my paper on “Vacuum Advance Control Units Facts and Specs” for more info on this.

Your timing is now set for best possible performance. Make note of the new setting, and use this for your future tune-up work.

Lars’ Suggested Timing Specs for GM V8 Performance Applications:

• 36 degrees total timing (vacuum advance hose disconnected), all “in” by 2500 rpm
• 18 degrees initial timing at idle (vacuum advance hose disconnected). Note that it may not be possible to achieve the 18-degree initial spec with the 36-degree total without modifying the distributor advance stop system. It is more important to achieve the 36 total than to hit an exact 18 initial. However, if your initial timing is very low (below 12 degrees) with the 36 total, it is important that you repair or modify your distributor in order to achieve correct engine performance
• 16 degree vacuum advance control unit with a pull-in spec that allows the full range of vacuum advance to be pulled in at the engine’s idle manifold vacuum level. Connect to manifold vacuum for most applications (this will allow the engine to idle with actual timing at idle of 34 degrees).

Questions, Comments & Technical Assistance
If you have questions or comments regarding this article, or if you notice any errors that need to be corrected (which is quite possible since I’m writing this from memory…), please feel free to drop me an e-mail. Also, if you need any technical assistance or advice regarding this process, or other maintenance issues, feel free to contact me:

[email protected]
(This has been re-printed with the authors permission. Mods. Please consider this for a "stickey." It is priceless information).

Last edited by big2bird; 05-07-2007 at 08:38 PM. Reason: Author request
big2bird is offline   Reply w/quote Quick reply to this message
Sponsored Links
Advertisement
 
Old 05-06-2007, 07:41 PM   #2
big2bird
DC Crew
 
big2bird's Avatar
 
Posts: 5,916
Member #58543
Member since: Mar 2007
Location: Anaheim,Ca.,USA

My Corvette(s)
1981 T-Top Caprice powered Disco Buggy

Thanks: 0
Thanked 1 Time in 1 Post
How to install your distributor

Technical Information Bulletin Rev. C 9-11-05
How to Install your Chevy Distributor
by Lars
Lafayette, CO

(Point-Style Distributors. HEI systems can be installed using similar techniques, but photos in this article do not apply)


A distributor can actually be dropped into a block in virtually any orientation and made to function by re-arranging the spark plug wires to match the installation. You will see this approach to distributor installation quite frequently, and it is a sure-fire tip-off that the engine builder/distributor installer didn’t have a clue about how to do the job right.
GM always installed the distributor in a specific orientation, and always used the same distributor cap “tower” for the #1 spark plug. By following this procedure, your distributor will be installed in the correct factory position for a professional appearance.

1. Bring the engine up to Top Dead Center on the Compression Stroke and align the timing mark on the harmonic balancer with the 8-degree mark on the timing chain cover (or wherever you want the engine to fire. 8 Degrees is a good starting point for an initial start-up, but you can set it anywhere from 6 to 12 degrees before top center).

To Find Top Dead Center on the Compression Stroke with the engine in the car:
a. Remove the #1 spark plug
b. Disconnect the coil wire from the distributor cap and ground it
c. Have a helper plug the #1 spark plug hole with a finger.
d. With the starter, slowly “bump” the engine over until the helper feels air being forced by his
finger.
You are now coming up on the compression stroke. Align the timing marks as noted above.

To Find Top Dead Center on the Compression Stroke with the engine out of the car:
a. Remove the valve cover on the driver’s side of the engine to expose the valves for cylinder #1.
b. Rotate the crankshaft until the timing mark approaches top dead center. Observe the exhaust
valve.
c. If the exhaust valve is moving as you are approaching top dead center, you are on the exhaust stroke.
You need to rotate the crankshaft one more time.
d. If neither valve is moving as you approach top dead center, you are on the compression stroke.
Align the timing marks as noted above.
2. Install the rotor to the distributor.
3. Hold the distributor body in the orientation show in figure 1 relative to the engine/block and drop the distributor straight down into the block. Pay no attention to rotor orientation at this time. If the rotor is aligned with the oil pump driveshaft, the distributor will drop all the way down and seat. If the rotor does NOT line up, the distributor will not drop all the way down.
4. If the distributor does NOT drop all the way down (chances are best that it won’t), pull the distributor up out of the block just enough to disengage the rotor from the camshaft gear, and turn the rotor a little bit. Drop it down again. Repeat this until the distributor drops all the way down and the rotor engages with the oil pump.
5. The distributor will now be all the way into the block, but the rotor will not be properly aligned. You can now pull the distributor up until the cam gear disengages, turn the rotor JUST A HAIR (half a cam tooth), and drop it straight back down again. The rotor will now move one tooth over, and the chamfer on the oil pump shaft will allow the oil pump to line back up. The distributor will drop all the way back in again, with the rotor moved over one tooth. (If it doesn’t work, try rotating the rotor the opposite direction.) Repeat this operation (I call it “walking the distributor”) by lifting the distributor up, slightly moving the rotor, and dropping it back in until you’ve “walked” the rotor around to its correct position as shown in the figure below. Once you get the technique down, you can do this very quickly – much quicker than trying to align the oil pump driveshaft with a screwdriver while looking down the hole. The screwdriver technique also requires that you pull the distributor ALL THE WAY OUT to fiddle around with the screwdriver several times until you get it right. So try my “walking” technique: it’s quick and accurate.
6. Once you have “walked” the rotor into position, you should be able to obtain the orientation of the distributor body and the rotor as shown in Figure 1. Install the distributor hold-down clamp and bolt. Snug it, but leave it loose enough that you can rotate the distributor smoothly.
7. Attach an Ohm-Meter (continuity tester) between the distributor primary lead wire (the wire coming out of the bottom of the distributor body) and ground (any point on the engine). Rotate the distributor body SLIGHTLY clockwise from the orientation shown in Figure 1 until you read continuity (points are closed – giving continuity to ground). Now, SLOWLY rotate the distributor body counter-clockwise until the points JUST break open (loss of continuity on the ohm meter). The instant the points break open is the ignition firing point. Tighten your distributor hold-down bolt at this point. Your distributor body and rotor should now be aligned like Figure 1 (or VERY close).
8. Slip your distributor cap onto the distributor. Notice which “tower” is the #1 plug wire. With a felt marker, place a little mark on the distributor body at the #1 tower position. Pull the cap back off, and verify that the rotor is pointing to this mark (or VERY close). If it’s not, you’re most likely off by a tooth. Repeat the installation steps.
9. If everything is aligned (and it will be if you followed these steps), install the cap and install the plug wires as shown in figure 2.
10. Start the engine. It will fire and run immediately if the above steps have been followed.
11. Set the dwell to 30 – 31 degrees (always set dwell before setting timing. Changing the dwell changes the timing).




Figure 1: Distributor & Rotor Correctly Installed at #1 Firing Position







Figure 2: Correct Spark plug Wire Order and Placement




Alternate C3 Distributor Installation Orientation
Due to the relationship between the distributor tach drive cable and the firewall on the C3 Corvette (1968 – 1974), GM issued a Service Bulletin allowing an alternate distributor installed orientation in order to straighten out the tach drive cable and to promote longer cable life.

The distributor orientation shown in the above photos installs the distributor so that the vacuum advance control unit pokes out in front of the distributor shielding at about a 7:00 o’clock position as seen from the front of the engine. In order to straighten out the cable and still retain the shielding, GM allowed the distributor housing to be rotated clockwise 45 degrees. This rotation pokes the vacuum advance out from behind the shielding, pointing it towards the rear flange of the passenger-side valve cover at about the 9:00 o’clock position as seen from the front of the engine. This orientation straightens out the tach drive cable. When doing this, the spark plug wires were all shifted 1 tower counter-clockwise in the cap, so that the plug wires retained their same relative positions to the engine/vehicle (#1 wire was placed in the previous #2 cap location, thus retaining the #1 wire as the forward, passenger-side plug wire).

It is recommended that C3 distributor installations be done according to the Service Bulletin Alternate Orientation in order to increase cable life and to simplify cable & distributor installation.




(Courtesy “paul67” from CorvetteForum.com)


How to Set the Timing
When you think about it, setting the timing at idle speed makes no sense at all: You don’t operate your car at idle, and timing changes as the rpm changes. Fact is, the timing spec at idle speed is provided as a simple way for most people to set the timing, and is not a good procedure for optimum performance.

Small block Chevys (and most other GM performance V8 engines) perform best when the total timing (full centrifugal advance plus the initial timing setting with vacuum advance hose disconnected) is all in by 2,500 – 2,800 rpm and is set to about 36 degrees. If you have an adjustable timing light, this is very easy to check. If you don’t, you need to scribe a 36-degree mark on your harmonic balancer. Here’s how:

Measure the circumference of your harmonic balancer using a sewing tape measure (or other flexible tape measure). Get it as accurate as you can. Take this measurement and divide by 10. The number you get is the distance to 36 degrees. Measure this distance CLOCKWISE from your existing harmonic balancer timing mark as viewed from the front of the engine and place a clear mark on the balancer.

Remove your distributor cap and rotor. If you have a points-style distributor with the stock, factory, heavy springs in place, remove one of the springs. Disconnect the vacuum advance. Install the rotor and cap. Loosen the distributor hold-down clamp bolt just enough so that the distributor can by turned, yet leave it snug enough that the distributor will hold its position.

Start the engine. If you’re using an adjustable timing light, set the light to 36 degrees advanced. Now rev the engine while observing the timing marks with the light. You will notice that the stock line on the balancer will move up towards the timing plate as rpm increases. Continue to increase rpm until the line does not move any further (centrifugal advance is “pegged out”). Once the timing is “pegged out,” the line on the balancer should line up with the “0” mark on the timing tab. Rotate the distributor to achieve this.

If you’re using a non-adjustable light, perform the same process, but align your new 36-degree mark with “0” mark on the timing tab.

Shut it down.

Pop the cap and rotor and re-install the spring, if you removed it. Put everything back together, but leave the vacuum disconnected. Start it up. For future reference, make a note of the timing setting at idle. This is your new curb idle timing spec. Now give the engine a few quick rev’s past 3,000 rpm and verify that the full timing (36 degrees) is coming in. If it’s not, you need to change to a softer set of springs until you get full 36-degree advance before 3000 rpm. (NOTE: A stock set of springs will often not allow full centrifugal advance to come in before redline rpm. If you have heavy stock springs installed, don’t rev the engine beyond its limits to try to force full advance in.) I suggest obtaining Mr. Gasket kit part number 927 or 928: Use the gold springs on HEI systems. For points-style systems, use one black spring and one silver spring – these springs will get your total timing all in by 2500-2800 rpm, providing very good throttle response and power. The black & silver spring combo can also be used on MSD distributors if you widen out the spring hook ends.

Hook up the vacuum. Re-set your idle speed and idle mixtures if necessary to lower the idle speed. Now do a road test.

The 36-degree 2500 rpm advance curve is optimum for performance, but may require premium fuel. Lug the car around, and punch the throttle at low rpm while listening for detonation (“engine knock”). If you’re getting any audible knock, you MUST retard the timing. Retard the timing in 2-degree increments until engine knock stops. Engine knock will seriously damage engine components if not corrected. If you get no knock, you may see slightly improved performance at 38 degrees total timing. This is particularly true if you’re running at high altitude.

If you have no engine knock under acceleration, but the car “chugs” or “jerks” at cruising speed (light throttle application), you are getting too much vacuum advance on top of the mechanical advance. You may need to change out the vacuum advance diaphragm with a unit producing no more than 16 degrees of vacuum advance. See my paper on “Vacuum Advance Control Units Facts and Specs” for more info on this.

Your timing is now set for best possible performance. Make note of the new setting, and use this for your future tune-up work.

Lars’ Suggested Timing Specs for GM V8 Performance Applications:

• 36 degrees total timing (vacuum advance hose disconnected), all “in” by 2500 rpm
• 18 degrees initial timing at idle (vacuum advance hose disconnected). Note that it may not be possible to achieve the 18-degree initial spec with the 36-degree total without modifying the distributor advance stop system. It is more important to achieve the 36 total than to hit an exact 18 initial. However, if your initial timing is very low (below 12 degrees) with the 36 total, it is important that you repair or modify your distributor in order to achieve correct engine performance
• 16 degree vacuum advance control unit with a pull-in spec that allows the full range of vacuum advance to be pulled in at the engine’s idle manifold vacuum level. Connect to manifold vacuum for most applications (this will allow the engine to idle with actual timing at idle of 34 degrees).

(This has been re-printed with the written permission of the author).

Last edited by big2bird; 05-07-2007 at 08:39 PM. Reason: Author request
big2bird is offline   Reply w/quote Quick reply to this message
Old 05-06-2007, 07:44 PM   #3
big2bird
DC Crew
 
big2bird's Avatar
 
Posts: 5,916
Member #58543
Member since: Mar 2007
Location: Anaheim,Ca.,USA

My Corvette(s)
1981 T-Top Caprice powered Disco Buggy

Thanks: 0
Thanked 1 Time in 1 Post
Vacuum Advance

Technical Information Bulletin Rev C 5-22-05
Distributor Vacuum Advance Control units
Specs and facts for GM Distributors

by Lars
Lafayette, CO

I’ve been seeing a lot of discussion and questions regarding distributor vacuum advance control units; what do they do, which ones are best, what was used on what, etc., etc. To clarify some of this, I thought I’d summarize a few facts and definitions, and provide a complete part number and specification listing for all vacuum advance control units used by Chevrolet on the points-style distributors. I’m also providing a listing of the specs for all other GM (non-Chevrolet) control units, but without the specific application listed for each (it would take me a bit too much time to research each part number by application across each of the GM Motor Divisions – it took me long enough to compile just the Chevy stuff…!). This latest revision to this paper also includes the HEI listings (the HEI distributors use a longer control unit, so the non-HEI and HEI vacuum advance control units CANNOT be interchanged).

As always, I’m going to include the disclaimer that many of these are my own comments and opinions based on my personal tuning experience. Others may have differing opinions & tuning techniques from those presented here. I have made every attempt to present factual, technically accurate data wherever possible. If you find factual errors in this information, please let me know so I can correct it.

Background
The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response. Once the engine load increases, this “over-advance” condition must be eliminated to produce peak power and to eliminate the possibility of detonation (“engine knock”). A control unit that responds to engine vacuum performs this job remarkably well.

Most GM V8 engines (not including “fast-burn” style heads), and specifically Chevys, will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed.

The additional timing produced by the vacuum advance control unit must be tailored and matched to the engine and the distributor’s mechanical advance curve. The following considerations must be made when selecting a vacuum advance spec:

How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15 inches Hg, a vacuum advance control unit that needs 18 inches to peg out would be a poor selection.

How much centrifugal advance (“total timing”) is in effect at cruise rpm? If the distributor has very stiff centrifugal advance springs in it that allow maximum timing to only come in near red-line rpm, the vacuum advance control unit can be allowed to pull in more advance without the risk of exceeding the 52-degree maximum limit. If the engine has an advance curve that allows a full 36-degree mechanical advance at cruise rpm, the vacuum advance unit can only be allowed to pull in 16 more degrees of advance.

Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times.

Does your engine require additional timing advance at idle in order to idle properly? Radical cams will often require over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.

Thus, we see that there are many variables in the selection of an appropriate control unit. Yet, we should keep in mind that the control unit is somewhat of a “finesse” or “final tuning” aid to obtain a final, refined state of tune; we use it to just “tweak” the car a little bit to provide that last little bit of optimization for drivability and economy. The vacuum advance unit is not used for primary tuning, nor does it have an effect on power or performance at wide open throttle.

With these general (and a little bit vague, I know…) concepts in mind, let’s review a few concepts and terms. Then it’s on to the master listing of specs and parts…..:

Part Number
There are many different sources for these control units. Borg Warner, Echlin, Wells, and others all sell them in their own boxes and with their own part numbers. Actually, there are very few manufacturers of the actual units: Dana Engine Controls in Connecticut manufactures the units for all three of the brands just mentioned, so it doesn’t make much difference who you buy from: They’re made by the same manufacturer. The part numbers I have listed here are the NAPA/Echlin part numbers, simply because they are available in any part of the country. For Wells part numbers (Autozone), drop the “VC” prefix and use a “DV” prefix.

ID#
Every vacuum advance control unit built by Dana, and sold under virtually any brand name (including GM), has a stamped ID number right on top of the mounting plate extension. This ID, cross referenced below, will give you all specifications for the unit. So now, when you’re shopping in a junkyard, you’ll be able to quickly identify the “good” vs. the “bad” control units.

Starts @ “Hg
Vacuum is measured in “inches of Mercury.” Mercury has the chemical symbol “Hg.” Thus, manifold vacuum is measured and referred to as “Hg. The “Start” spec for the control unit is a range of the minimum vacuum required to get the control unit to just barely start moving. When selecting this specification, consideration should be made to the amount of vacuum that a given engine produces, and what the load is on the engine at this specification. For example, an engine with a very radical cam may be under very light load at 7 inches Hg, and can tolerate a little vacuum advance at this load level. Your mom’s Caprice, on the other hand, has such a mild cam that you don’t want the vacuum to start coming in until 9 – 10 inches Hg. For most street driven vehicle performance applications, starting the vacuum advance at about 8” Hg produces good results.

Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking about these control units, it is important that you know whether the person you’re talking to is referring to the distributor degrees, or if he’s talking crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet, will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees (which is what you “see” with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft advance. This would be an 8-degree vacuum advance control unit.

Max Advance @ “Hg
This is the range of manifold vacuum at which the maximum vacuum advance is pegged out. In selecting this specification, you must consider the vacuum produced at cruise speed and light throttle application. If your engine never produces 20” Hg, you better not select a control unit requiring 21” Hg to work.

The following listing (Non-HEI) is as follows: The first two part number listings are the two numbers that are most commonly used in a Chevrolet performance application. The “B1” can is the most versatile and user-friendly unit for a mild street engine. As you can see, it was selected by GM for use in many engines due to its ideal specs. The “B28” can was used on fuel injected engines and a few select engines that produced very poor vacuum at idle. The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above). The third listing is one of my personal favorites, and you can see from the specs that it is an excellent choice for most performance engines and street engines with moderate cams, since most moderate performance engines will idle with at least 13 inches of vacuum.

After this, the listing is by Echlin part number. The Chevrolet applications are listed first by application, followed by a complete listing of all of the units used on any GM product (all GM units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do).

Non-HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC680 B1 1959 – 63 All Chevrolet 8-11 8 @ 16-18
1964 Corvette exc. FI
1964 Impala, Chevy II
1965 396 High Perf.
1965-67 283, 409
1966-68 327 exc. Powerglide
1967-68 All 396
1969 Corvette 427 High Perf.
1969 396 Exc. High Perf.
1969 Corvette 350 TI
1969-70 302 Camaro
1970 400 4-bbl
1970 396 High Perf.
1970 Corvette 350 High Perf.
1973-74 454 Exc. HEI

VC1810 B28 1965 409 High Perf. 3-5 8 @ 5.75-8
1965 327 High Perf.
1966 327 High Perf.
1964-67 Corvette High Perf. FI

VC1765 B20 1965 396 Impala High Perf 5-7 8 @ 11-13
1966-67 Corvette Exc. High Perf.
1966-67 Impala 427 Exc. High Perf.
1966-68 327 Powerglide Exc. High Perf.
1969 307 All
1969-70 396, 427 Camaro, Chevelle High Perf.
1970 400 2-bbl
1970 307 MT
1973 Camaro 350 High Perf.

--------------------------------------------------------------------------------------------------------------------------------------------

VC1605 B9 1965 impala 396 Exc. High Perf. 7-9 10.3 @ 16-18
1965 327 All Exc. FI
1969 327 Camaro, Chevelle, Impala
1969-70 Corvette 350 Exc. High Perf.
1969-70 350 4-bbl Premium Fuel
1970 350 Camaro, Chevelle, Impala High Perf.
1971-72 350 2-bbl AT
1971-72 307 All

VC1675 B13 1968 327 Camaro Powerglide 9-11 8 @ 16-18
1968 327 Impala AT
1968 307 AT
1968 302, 307, 327, 350 Camaro, Chevy II
1970 350 Camaro, Chevelle Exc. High Perf.

VC1760 B19 1969 350 Camaro, Chevelle, Impala 4-bbl 5.5-8 12 @ 14-18
1969-70 350 2-bbl

VC1801 B21 1971 350 2-bbl 7-9 10 @ 16-18
1971-72 400, 402
1971-72 307 AT

VC1802 B22 1971-72 350 4-bbl 7-9 8 @ 14-16


Other Part Numbers & Specs:

VC700 B3 8-10 11.5 @ 19-21
VC1415 M1 6-8 10 @ 13-15
VC1420 M2 5-7 11 @ 16-17
VC1650 B12 8-10 10 @ 15-17
VC1725 B18 8-10 12 @ 13-16
VC1740 A5 6-8 12 @ 15-17.5
VC1755 A7 8-10 12.5 @ 18-20.5
VC1804 B24 6.5-8.5 10 @ 12-14
VC1805 M13 6-8 12 @ 14.5-15.5
VC1807 B25 5-7 8 @ 13-15
VC1808 B26 5-7 8 @ 11-13
VC1809 B27 5-7 9 @ 10-12
VC1812 B30 5-7 12 @ 11.75-14



The following listing (HEI) is as follows: The first four part number listings are the 4 numbers that are most commonly used in a Chevrolet performance application. The “AR12” can is the most versatile and user-friendly unit for a good performance street engine, and it’s very similar to the “B20” can used on the early distributors. The AR 15 and AR23 are almost identical, with only slight variations in their “start-stop” specs. The “AR31” can is the HEI equivalent to the “B28” Hi-Perf can used on the early engines: The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)

After this, the listing is by Echlin part number. All GM HEI vacuum advance units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do.

HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC1838 AR12 1975 350 Buick 7-9 7 @ 10-12

VC1843 AR15 1977 305 All Exc. Hi Alt, Exc, Calif. 3-5 7.5 @ 9-11
1974 400 All w/2-bbl
1977 305 El Camino
1976 262 Monza Exc. Calif
1976 350 Vette Hi Perf, Incl. Calif
1975 350 Z-28
1977 305 Buick Skylark

VC1853 AR23 1976 350 All Calif. 5-7 7.5 @ 11-12.5
1976 350 Vette Calif., Exc. Hi Perf
1976 400 All, Exc. Calif
1975 350 4-bbl
1974 350 All w/1112528 Distr.
1978 350/400 Heavy Duty Truck, Exc. Calif, Exc. Hi Alt.

VC1862 AR31 2-4 8 @ 6-8

--------------------------------------------------------------------------------------------------------------------------------------------

VC1703 N/A 1978-79 Vette Special Hi Perf N/A N/A
1979 305 El Camino Calif.
1978-79 350 Blazer & Suburban
1979 Buick 305/350

VC1825 AR1 1976 454 Caprice, Impala 3-5 9 @ 6-8
1975 454 Caprice, Chevelle, Monte, Suburban

VC1826 AR2 5-7 12 @ 10-13

VC1827 AR3 5-7 9 @ 9-11

VC1828 AR4 1975-76 350 Buick & Olds 6-9 10 @ 12-14
1976 350 Pontiac

VC1831 AR7 6-8 12 @ 14-16

VC1832 AR8 1975-76 455 Buick Electra 4-6 12 @ 12-14

VC1833 AS1 1975-76 500 Cadillac Exc. Calif. 4-6 14 @ 15-16

VC1834 AR9 4-6 13 @ 13-16

VC1835 AS2 1975-76 350 Olds 5.5-7.5 12 @ 15-17

VC1836 AR10 1977 305 All Hi Alt, Exc. Calif. 3-5 9 @ 11-13
1977 350 All exc. Calif.
1977 350 Vette Exc. Calif, Exc. Hi Perf
1976 305 All Exc. Calif
1976 350 All Exc. Vette, Exc. Calif
1976 350 Vette Exc. Calif., Exc. Hi Perf
1975 262, 350 All w/2-bbl carb
1975 350 All 4-bbl w/ 1112880 & 1112888 Distr.
1977 305 Chev Truck Light Duty
1975-76 350 El Camino 2-bbl

VC1837 AR11 1976 305 Blazer, Exc. Calif 6-8 12.5 @ 10.5-13.5
1976 350/400/455 Pontiac 4-bbl

VC1839 AR13 4-6 12 @ 11-13

VC1840 AR14 1975-76 350/400/455 Pontiac Firebird 6-8 10 @ 9-12

VC1841 AS3 1975-76 500 Cadillac Calif. 5-7 10 @ 13-14

VC1842 AS4 1976 350 Olds Cutlass 5-7 12 @ 13-15

VC1844 AR16 3-5 12 @ 13.5-15.5

VC1845 AS5 1978-79 425 Cadillac w/F.I. 4-6 14 @ 14-16
1977 425 Cadillac

VC1846 AR17 1977 301 Buick Skylark 3-6 13 @ 10-13
1977 301 Pontiac

VC1847 AS6 1978 403 Motor Home 4-6 12 @ 12-14
1977-79 350/403 Buick LeSabre Hi Alt, Riviera, Olds
1977-79 350/403 Pontiac Hi Alt

VC1848 AR18 4-6 12 @ 9-12

VC1849 AR19 4-6 12 @ 7-10

VC1850 AR20 1977 350/400 Pontiac 4-6 10 @ 8-11

VC1851 AR21 1977-79 350 Buick LeSabre, Century 5-7 12 @ 11-13
1978-79 350 Pontiac

VC1852 AR22 77-78 305/350/400 Chev Truck, Heavy Duty 7-9 5 @ 12-14
1975-76 350/400 Chev Truck Heavy Duty

VC1854 AR24 3-5 13 @ 10-13

VC1855 AS7 1977-79 260 Olds Cutlass 3-5 15 @ 10-12

VC1856 AR25 3-6 15 @ 10-14

VC1857 AR26 3-6 12 @ 13-16
VC1858 AR27 1978-79 305 All 3-6 9 @ 11-13
1978 350 Camaro
1978 305 Chev Truck, M/T, Light Duty
1978 350 Chev Truck Hi Alt
1978 305/350 Buick & Olds
1978-79 305 Pontiac

VC1859 AR28 1979 350 Vette Exc Hi Perf 3-6 10 @ 9-12
1978-79 305 w/1103282 Distr., Incl. El Camino A/T
1979 350 Camaro, Impala, Nova, Malibu, Monte
1979 350 Suburban
1979 350 Buick Century
1978 305/350 Buick & Olds
1978-79 305 Pontiac Hi Alt.

VC1860 AR29 3-6 12 @ 10-13

VC1861 AR30 1978-79 301Buick 3-5 13 @ 11-13
1979 301 Olds
1978-79 301 Pontiac

VC1863 AR32 2-4 10 @ 11-13

VC1864 AR33 1978 305 Chev Truck, A/T, Light Duty 4.5-6.5 13 @ 11-13

VC1865 AR34 1973-74 350 Vette Special Hi Perf 3-5 15 @ 8.5-11.5

VC1866 AS8 1978-79 425 Cadillac w/carb 3-5 14 @ 13-15

VC1867 AS9 2-4 10 @ 8-10

VC1868 AR35 1979 305 Chev Truck & El Camino 2-4 10 @ 6-9
1979 305 Buick & Olds
1979 305 Pontiac A/T

VC1869 AS10 2-4 12 @ 8-11


(This has been re-printed with the written permission of the author).

Last edited by big2bird; 05-07-2007 at 08:40 PM. Reason: Author request
big2bird is offline   Reply w/quote Quick reply to this message
 
Old 05-06-2007, 07:45 PM   #4
big2bird
DC Crew
 
big2bird's Avatar
 
Posts: 5,916
Member #58543
Member since: Mar 2007
Location: Anaheim,Ca.,USA

My Corvette(s)
1981 T-Top Caprice powered Disco Buggy

Thanks: 0
Thanked 1 Time in 1 Post
Vacuum explained

Technical Information Bulletin Rev. New 9-10-03
Carburetor Vacuum Ports:
Manifold, Ported and Venturi Vacuum Explained


by Lars
Lafayette, CO


This tech paper will discuss the concepts of 3 different types of vacuum sources, and will briefly discuss their potential uses and applications in tuning GM V8 engines.


Overview
The airflow through a carburetor and through an engine’s intake system creates various pressure regions due to a variety of effects. The low pressure regions are sources for “vacuum,” used for signal sources and power sources for operating accessories.

“Vacuum” in an engine is not truly a “vacuum.” Rather it defines a lower-than-atmospheric-pressure area in the engine or in the carburetor. Lower-than-atmospheric pressure is measured in “inches of Mercury.” Mercury has the chemical symbol “Hg,” so the terminology for the measurement becomes “in. Hg.” To visualize how this measurement works, imagine a U-shaped glass tube partially filled with Mercury. The one end of the tube is exposed to the atmosphere. The other end of the tube is attached to your low pressure source. The low pressure on the one end of the tube will cause the Mercury to rise up the tube. The amount that it rises is “in. Hg.”

A funny terminology issue arises with this: Most people would describe that the low pressure area is “sucking” the Mercury up the U-shaped tube. To be technically correct, there is no such thing as “suction.” What is actually happening is that the low-pressure area is allowing the high pressure on the other end to “push” the Mercury up the tube. Thus, when you “suck” on a straw in a milkshake, you are not sucking: You are creating a low pressure region in your mouth, and atmospheric pressure is “pushing” the milkshake up the straw and into your mouth.


How Vacuum is Produced
In an engine and carburetor there are two different mechanisms (processes) for producing vacuum. The two are interesting in their differences.

The most commonly recognized vacuum source in an engine is “manifold vacuum.” Manifold vacuum is created in the intake manifold of an engine due to the pistons’ intake downstroke on one end, and the restriction created by the partially open carburetor throttle plates on the other end. If the throttle plates are closed tightly and the pistons are moving quickly, a very high vacuum is created. If the throttle plates are opened more, creating a larger “leak path,” less vacuum is created. If the pistons are moving slowly and the throttle plates are wide open, the pressure in the manifold will be very close to atmospheric pressure – no intake vacuum. Thus, intake vacuum can be used as a signal source to determine how hard an engine is working.

Less recognized, and more frequently misunderstood, is the term “venturi vacuum.” Venturi vacuum is produced in an entirely different manner, and it behaves completely independently of manifold vacuum. To understand venturi vacuum, you have to think back to your high school physics class and the Bernoulli Effect: As a fluid (liquid or gas) moves through a tube, the areas of low velocity produce high pressure, and the areas of high velocity produce low pressure. Thus, as the tube necks down and becomes narrow, the fluid flowing through the narrow section has to move faster, and pressure in the narrow section is lower than the pressure in the larger section. Venturi vacuum, and the Bernoulli Effect, occurs in the venturi of the carburetor: As air enters the carb, it passes through the “necked-down” area of the venturi. As it passes through the venturi, the air accelerates. This fast-moving air creates a low pressure area right in the middle of the venturi, and this low pressure area is used to discharge fuel from the float bowl of the carb into the air stream. Remembering our discussing in the Overview of this article, the fuel is not “sucked” out into the venturi: The low pressure area in the venturi allows atmospheric pressure on top of the fuel in the float bowl to “push” the fuel up and out of the venturi discharge nozzles. The venturi vacuum varies only by the total amount (“mass”) of airflow passing through the venturi and its velocity: The faster the air is passing through (more air), the higher the venturi vacuum will be. This is completely independent of the manifold vacuum.


How Vacuum is Used
Manifold vacuum is very easy to tap and utilize: Simply drill a hole in the intake manifold and stick a hose in it. This vacuum can then be used to power accessories (headlight doors and heater controls), or it can be used as a signal source based on how hard the engine is working. One such signal source application is distributor vacuum advance: When the engine is working lightly (high vacuum produced), the ignition is advanced, and as the engine loses vacuum due to the throttle being mashed to the floor, vacuum is lost and ignition is retarded. Pretty simple.

But what if you want to “switch” the vacuum on and off based on whether the engine is just idling or in cruise mode? Enter the term “Ported Vacuum.”

When emissions became a priority to vehicle manufacturers, a method had to be found to reduce emissions at idle. The amount of Hydrocarbons emitted out of the tailpipe can be drastically altered by changing the timing: Retarding the timing reduces Hydrocarbon emissions. But retarded timing adversely affects gas mileage at cruise. So a method was needed to retard timing at idle, yet maintain it at normal levels for cruise. The solution was seen to “turn off” the vacuum advance at idle, yet have it operate normally under all other operating conditions. To do this, a small hole was drilled in the carburetor throttle body just above the position of the throttle plate at idle (NOT in the venturi area), and this hole was connected to a vacuum nipple on the carb. When the throttle plates are closed at idle, they act as an “off” switch to block the drilled hole from manifold vacuum. As the throttle plates are opened up, the hole becomes fully exposed to manifold vacuum, and normal manifold vacuum is realized at the nipple. Thus, you have a manifold vacuum “on-off” switch, turning manifold vacuum “off” at idle, and restoring it to normal operation once the throttle plate is cracked open. Vacuum advance can be eliminated at idle for good emissions, and instantly restored to normal operation at cruise. At both cruise and Wide Open Throttle (WOT), manifold vacuum and ported vacuum are exactly the same: There is high vacuum at cruise, and virtually no vacuum at WOT. The difference in vacuum occurs only at idle.

So if there is no manifold vacuum or ported vacuum at WOT, how can vacuum open the secondaries on a vacuum secondary carb at WOT? Manifold or ported vacuum is not used to open the secondaries: The secondaries are opened by venturi vacuum. On a true vacuum secondary carb (Holley and BG are the only real vacuum secondary carbs – Q-Jets and AFBs are not vacuum secondary carbs), there is a small hole drilled right into the middle of the venturi wall on the passenger side primary venturi. This passage runs over to the vacuum diaphragm, and the low pressure created in the primary venturi from very high airflow through the venturi is used to open the secondary throttle shaft. The more air that passes through the primary side, the more the secondary diaphragm will open. It is strictly a function of airflow through the primary venturi and the low pressure that this creates in the venturi. Manifold vacuum can be non-existent, yet if the airflow through the carb is high, the secondaries will be pulled open by the venturi vacuum. Note, however, that there is no external nipple on any carb for venturi vacuum: The only source is the small drilled hole in the venturi, and this hole only runs to the secondary diaphragm through an internal passage.

Thus, we see that we have 3 types of vacuum in the engine: Manifold Vacuum, Ported Vacuum (simply a “switched” manifold vacuum source), and Venturi Vacuum. Of these, only Manifold Vacuum and Ported Vacuum can be utilized for tuning purposes.


Tuning with Vacuum
I see a lot of discussion and confusion regarding the use of Ported Vacuum versus Manifold Vacuum for distributor vacuum advance. Once understood, the tuner can effectively utilize one source or the other, depending on the tuning requirements of the vehicle. There is no right or wrong answer on which source to use, but using the correct source for the tuning requirements of a given engine can have a big effect on off-idle and near-idle performance characteristics.

First, be sure to locate and read my paper titled “Distributor Vacuum Advance Control Units.” This contains a lot of technical information related to this issue that I won’t be repeating under this paper heading.

The timing advance curve requirements for an engine will vary a bit from one engine to another depending on cam, compression ratio and other efficiency factors. But in general terms, most GM V8s will produce peak power at WOT with 36-38 degrees of ignition timing. Peak fuel economy and drivability at cruise is achieved with about 52-54 degrees of advance. Best idle quality has a much wider range depending on cam & engine, but tends to be in the 12-24 degree range. Lowest emissions usually occur with timing in the 4-8 degree range.

When tuning, it is important to realize that the upper limits on timing are determining factors for how to set things up: You want the total WOT timing (the maximum timing the engine will see with vac advance disconnected and with the centrifugal advance fully deployed) to be not over 38 degrees. 36 is the best setting for most applications. Once this has been set, it automatically determines what your initial advance ends up being unless you physically alter the length of the advance curve. In most cases, once total advance has been set to 36, the initial advance will end up being about 12 degrees-or-so. And, since most vacuum advance control units pull in about 16 degrees of vacuum advance at cruise speed (where the full centrifugal advance will also be deployed), the 36-degree setting will produce 52 degrees of total combined advance at cruise with the vac advance fully deployed.

But what if your engine/cam combination idles best at 26 degrees advance? Radical cams often require quite a bit of advance at idle. If you simply bump the initial timing up from 16 to 26, your total WOT advance will go from 36 to 46. The total combined timing at cruise will go from 52 to 62. This is not acceptable, and can result in severe engine damage from detonation at WOT, and the car will chug and jerk at cruise from the over-advanced condition. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2 in. Hg below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.

If, however, your engine idles best in the 12-16 degree range due to a mild cam, plug the vacuum advance control unit to a ported vacuum source to eliminate the vacuum signal at idle. You will still obtain the 36-degree WOT total, and you’ll still have 52 at cruise. Also, if you need to pass an emissions test, use the ported source to reduce your hydrocarbons.

By playing with the total length of your centrifugal advance curve, selecting between ported or manifold vacuum, and carefully selecting a matched vacuum advance control unit that meets your specification requirements, you can achieve an optimum idle, excellent off-idle throttle response, and the best fuel economy possible.


Questions, Comments & Technical Assistance
If you have questions or comments regarding this article, or if you notice any errors that need to be corrected (which is quite possible since I’m writing this from memory…), please feel free to drop me an e-mail. Also, if you need any technical assistance or advice regarding this process, or other maintenance issues, feel free to contact me:

[email protected]
(This has been re-printed with the written permission of the author).

Last edited by big2bird; 05-07-2007 at 08:40 PM. Reason: Author request
big2bird is offline   Reply w/quote Quick reply to this message
Old 05-06-2007, 08:43 PM   #5
Eddie 70
DC Crew
 
Eddie 70's Avatar
 
Posts: 1,343
Member #5191
Member since: Dec 2003
Location: Kingston, Tn, USA

My Corvette(s)
70 Vert, 02 EB Z06

Thanks: 4
Thanked 1 Time in 1 Post
Good compilation of tech papers.
Eddie 70 is offline   Reply w/quote Quick reply to this message
70 Vert, Hardtop, Keisler 5 Speed, Steeroids, Stayfast Top

Test Fire Of LS6
LS6 With Magnaflow Not Tuned


Vette Pic
Old 05-07-2007, 06:02 AM   #6
matchframe
DC Pit Crew Boss
 
matchframe's Avatar
 
Posts: 9,731
Member #1213
Member since: Jun 2003
Location: Dickinson, (Houston) Texas USA

My Corvette(s)
1978 Modification in progress

Thanks: 419
Thanked 43 Times in 37 Posts
This is GREAT info!!

Thanks for posting it!
matchframe is offline   Reply w/quote Quick reply to this message
Old 05-07-2007, 07:57 AM   #7
Double_Take
DC Crew
 
Double_Take's Avatar
 
Posts: 4,607
Member #48594
Member since: Jul 2006
Location: Glenview, IL

My Corvette(s)
1970 Coupe (sold), 1970 coupe

Thanks: 0
Thanked 5 Times in 3 Posts
Great info! Its exactly the info I need right now. My engine has been running rather poorly since I bought it (Stupid bubba tuner!) I bought a spring kit and Im going to throw the new springs in tuesday and see if that steadies things. Im also think my vacuum advance is too low after reading this artical. The engine runs well with the vacuum advance plugged, but when I take it for a drive it bogs off idle and likes to die at stop lights.

Double_Take is offline   Reply w/quote Quick reply to this message
Old 05-07-2007, 10:10 AM   #8
69vettester
DC Crew
 
69vettester's Avatar
 
Posts: 108
Member #59856
Member since: May 2007
Location: Bradenton FL

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by big2bird View Post

[email protected]
(This has been re-printed with the authors permission. Mods. Please consider this for a "stickey." It is priceless information).


Moderators please consider these papers in a permanent "sticky" section on the C3 tech forum. They have Been around many years and used by hundreds to tune their new engines and rebuilds
Thanks
69vettester is offline   Reply w/quote Quick reply to this message
69 vette roadster. Hooker sidemounts, 450hp 383 roller engine BG speed demon 750 carb, Msd pro billit Dist, 6a Spark module, Richmond 6spTrans, Dewitts Radiator .
Old 05-07-2007, 12:45 PM   #9
lsejlowe
DC Pit Crew
 
lsejlowe's Avatar
 
Posts: 10,260
Member #6680
Member since: Jan 2004
Location: Omaha, NE

My Corvette(s)
black '69 350/350 4spd

Thanks: 8
Thanked 63 Times in 59 Posts
Quote:
Originally Posted by 69vettester View Post


Moderators please consider these papers in a permanent "sticky" section on the C3 tech forum. They have Been around many years and used by hundreds to tune their new engines and rebuilds
Thanks


any chance we could find a link to the figures that aren't appearing right now? If we could get them uploaded to the gallery here on DC we'll never have to worry about broken links and whatnot.
lsejlowe is offline   Reply w/quote Quick reply to this message
Optimists think the glass is half full.
Pessimists think the glass is half empty.
Engineers realize it's twice as big as it needs to be.
Old 06-06-2007, 09:31 PM   #10
Danomatic
 
Posts: 22
Member #9499
Member since: Feb 2004

Thanks: 0
Thanked 0 Times in 0 Posts
Would be nice if a knowledgeable individual would post some timing tables (less than 2000rpm) for those of us using a PCM and a non-spec cam (how about 224/230 on a 108/116 in a 383 ).

Might save me some tuning time.
Danomatic is offline   Reply w/quote Quick reply to this message
Old 06-27-2007, 09:20 PM   #11
lars
DC Crew
 
Posts: 4
Member #60073
Member since: May 2007
Location: Lafayette CO

My Corvette(s)
1964, 1985

Thanks: 0
Thanked 0 Times in 0 Posts
Timing below 2000 rpm is irrelevant in a performance application provided the engine idles smoothly and has good throttle response off idle. You want the timing curve to start coming in just above idle speed and pull a straight line to 36 degrees at 2500-3000 rpm; you can draw it on a piece of graph paper. Cam specs are also fairly irrelevant - you want to hit 36 degrees total as quickly as the engine can take it without detonatrion regardless of cam, but the 2500-3000 spec works pretty good for every application I have done. A larger cam will want more initial timing than a small cam, but that's about the only setuyp difference. What's a PCM...?

Original copies of the papers with diagrams and correct formatting are available by simply e-mailing me a request for the paper(s) you want:
[email protected]
Lars
lars is offline   Reply w/quote Quick reply to this message
Old 07-08-2007, 03:09 PM   #12
BarryK
DC Crew
 
BarryK's Avatar
 
Posts: 4
Member #17178
Member since: Aug 2004
Location: Newark, DE

My Corvette(s)
'65 L76 Coupe 1978 L82

Thanks: 0
Thanked 0 Times in 0 Posts
besides emailing Lars for the papers, I have the current versions of most of Lars papers hosted up on my website's tech articles page;

http://lbfun.com/Corvette/Tech/vettetech.html

feel free to download and utilize whatever tech articles I have and you may need
BarryK is offline   Reply w/quote Quick reply to this message
Old 07-08-2007, 06:01 PM   #13
SmokinBBC
DC Crew
 
SmokinBBC's Avatar
 
Posts: 1,079
Member #60689
Member since: May 2007
Location: Greensboro, NC

My Corvette(s)
70 LS5 Cpe MT

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by lars View Post
Timing below 2000 rpm is irrelevant in a performance application provided the engine idles smoothly and has good throttle response off idle. You want the timing curve to start coming in just above idle speed and pull a straight line to 36 degrees at 2500-3000 rpm; you can draw it on a piece of graph paper. Cam specs are also fairly irrelevant - you want to hit 36 degrees total as quickly as the engine can take it without detonatrion regardless of cam, but the 2500-3000 spec works pretty good for every application I have done. A larger cam will want more initial timing than a small cam, but that's about the only setuyp difference. What's a PCM...?

Original copies of the papers with diagrams and correct formatting are available by simply e-mailing me a request for the paper(s) you want:
[email protected]
Lars
Hey Lars..good to see you here.
SmokinBBC is offline   Reply w/quote Quick reply to this message
If you ever here me say "While I'm at it", smack me over the head with a 2x4.
Old 07-08-2007, 06:02 PM   #14
SmokinBBC
DC Crew
 
SmokinBBC's Avatar
 
Posts: 1,079
Member #60689
Member since: May 2007
Location: Greensboro, NC

My Corvette(s)
70 LS5 Cpe MT

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by BarryK View Post
besides emailing Lars for the papers, I have the current versions of most of Lars papers hosted up on my website's tech articles page;

http://lbfun.com/Corvette/Tech/vettetech.html

feel free to download and utilize whatever tech articles I have and you may need

Barry....3 posts in 3 years...you gotta get out more often boy

Thanks for updating your site!
SmokinBBC is offline   Reply w/quote Quick reply to this message
If you ever here me say "While I'm at it", smack me over the head with a 2x4.
Old 07-08-2007, 06:19 PM   #15
BarryK
DC Crew
 
BarryK's Avatar
 
Posts: 4
Member #17178
Member since: Aug 2004
Location: Newark, DE

My Corvette(s)
'65 L76 Coupe 1978 L82

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by SmokinBBC View Post
Barry....3 posts in 3 years...you gotta get out more often boy

Thanks for updating your site!
well, you know how it is, I don't like to take over a room, so to speak....

besides, my wife doesn't let me out of the basement very often. I escaped tonight and she doesn't know i'm loose.
BarryK is offline   Reply w/quote Quick reply to this message
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Corvette Forum : DigitalCorvettes.com Corvette Forums forums, you must first register.
Please enter your desired user name (12 CHARACTERS MAXIMUM), your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
City / State / Country?
Where you live
What kind of Corvette(s) do you own?
This field is not required.
Insurance
Please select your insurance company (Optional)

Log-in


Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Carfax Not Always the Facts poudre News Room 7 08-11-2007 03:05 PM
Big Brother watching Onstar? Can we let this die please? Youngs98 Stingray's "Bad Shark" Lounge 74 12-16-2006 10:03 AM
A bit disappointed (performance) basketcase C3 Corvette 16 03-11-2005 06:20 AM
setting Ignition timing Marine Vette General Corvette Topics 4 10-09-2004 08:13 PM
engine timing grumpyvette C4 Corvette 0 01-30-2004 02:48 PM

Powered by vBadvanced CMPS v3.2.2

All times are GMT -6. The time now is 06:14 AM.




Powered by vBulletin® Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Garage Plus vBulletin Plugins by Drive Thru Online, Inc.
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
© 2003-2011, DigitalCorvettes.com - All Rights Reserved