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Super Moderator
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Discussion Starter · #1 ·
THE FACT is that your engine only KNOWs dynamic compression, the piston compresses nothing untill the valves are both closed so with the correct cam choice you can run almost any static,compression ratio

ok read this very carefully
BTW with that crane #119661 I run 11:1 compression with no trouble at all on the street useing 93 octane and only when racing do I use just a little octane booster because I also run nitrous

now notice both valves close at about 77 degrees abdc with the crane 119661 cam, now look at this chart

when compared to the cam card youll see that the piston on a 350chevy only starts compressing the cylinder volume at about 2.4" from TDC for an effective true displacement of about 190cubic inches of displacement
your going to be trying to ge a dynamic compression of about 7.9" but that cam will require about an 11:1 compression ratio to fall to about 7.9:1 dynamic
(need more info,..look below)
read these lessons (#1-8)
The LSA, or lobe separation angle, is ground into the cam and cannot be changed. It is the angle that separates the intake and exhaust lobe for a particular cylinder, and is measured in camshaft degrees. The intake lobe centerline is measured in crankshaft degrees. The #1 intake lobe centerline is usually between 100° to 110° ATDC and is what you use to degree the cam. The cam manufacturer will publish the specs for the cam based on a given intake lobe centerline. Comp Cams, for instance, produces a large number of cams with 110_° LSA ground 4° advanced, so they list the specs for the cam with a 106° intake lobe centerline. You can calculate the ILC by adding the intake opening angle in °BTDC, the intake closing angle in °ABDC, plus 180° for the distance from TDC to BDC. Divide by 2 and subtract the intake opening angle and you will have the ILC. For example a 12-430-8 Comp Cam lists IO at 34°BTDC, IC at 66° ATDC, so 34 + 66 + 180 = 280. 280/2 = 140. 140 - 34 = 106° ILC

Figure 3 is a picture of both an intake and an exhaust lobe of a camshaft, seen end-on. It shows the relationship between the lobes, shows the overlap area, and illustrates this next section.
As stated in lesson 2, overlap has a great deal to do with overall engine performance. Small overlap makes low-end torque but less high-end power. Large overlap reduces low-end torque but increases high-end power.
Overlap is determined by two other cam specifications, Duration and Lobe Center Angle.
Duration is the time, measured in crankshaft degrees, that a valve is open. A duration of 204 degrees means that while the valve is open, the crankshaft rotates through 204 degrees.
Duration is measured on two "standards," "advertised duration" and "duration at 0.050"." Advertised duration is measured from when the valve just starts to lift off its seat to when it just touches the seat again. This is measured in different ways by different manufacturers. Some measure when the valve lifter is raised 0.004", some at 0.006", and some at different points yet. So the industry agreed to another standard that was supposed to make it easier to compare cams. In this standard, the duration is measured between the point where the lifter is raised by 0.050", and the point where it is lowered again to 0.050".
The 0.050" standard is great for side-by-side "catalog" comparisons between cams. But if you use engine prediction software on your computer, the software is much more accurate when you can feed it "advertised" duration numbers.
Lobe Center Angle is the distance in degrees between the centers of the lobes on the camshaft.
To increase duration, cam makers grind the lobes wider on the base circle of the cam. This makes the lobes overlap each other more, increasing overlap. More duration = more overlap.
To increase overlap without changing duration, cam makers will grind the lobes closer together, making a smaller lobe center angle. Less lobe center angle = more overlap.
Overlap and duration are the two big factors in cam design. More overlap moves the power band up in the engine's RPM range.
Longer duration keeps the valves open longer, so more air/fuel or exhaust can flow at higher speeds. It works out that increasing the duration of the camshaft by 10 degrees moves the engine's power band up by about 500 rpm.
A smaller lobe separation increases overlap, so a smaller lobe separation angle causes the engine's torque to peak early in the power band. Torque builds rapidly, peaks out, then falls off quickly. More lobe separation causes torque to build more slowly and peak later, but it is spread more evenly over the power band. So a larger lobe separation angle creates a flatter torque curve.
So you can see how a cam maker can tailor the camshaft specs to produce a particular power band in an engine--

Short duration with a wide separation angle might be best for towing, producing a strong, smooth low-end torque curve.
Long duration with a short separation angle might be suited for high-rpm drag racing, with a high-end, sharp torque peak.
Moderate duration with wide separation angle might be best suited for an all-around street performance engine, producing a longer, smoother torque band that can still breathe well at higher RPM.
Remember, there's always a compromise made in this process.

One last item to consider is the lobe centerline. The lobe centerline is the angle of the lobe's center peak, measured in crankshaft degrees when the piston is at Top Dead Center (TDC). In general (but not always), when a cam is installed "straight up," the intake lobe centerline and the lobe separation angle are the same.
The lobe centerline can be altered when the camshaft is installed, by advancing or retarding the camshaft's position in relation to the crankshaft. Advancing the camshaft by 4 degrees will move the power band about 200 RPM lower in the RPM band. Retarding the cam by 4 degrees will likewise move the power band 200 RPM higher in the RPM band. This allows you to fine-tune the engine's performance according to your needs.

personally I try to stay close to 110 degrees on most carb engines and 112 degrees on EFI engines because I value a wider torque curve more than a few hp only close to peak rpm

if cams are a mystery please take the time to read these, it will get you a good start (read LESSONs 1-8)

first the math
(1)youll be limited to about 1.2 hp per cid on engine size
(2) YOULL BE LIMITER to about 6400rpm with HYDROLIC flat tappet cams and about 7500rpm with SOLID LIFTER flat tappet cams
(3)piston speeds that exceed 4000fpm usually lead to trouble
(4)dynamic compression ratios of over about 8.3:1-8.5:1 with aluminum heads or about 7.8:1-8:1 with iron heads can cause detonation problems

(5)the formula for matching POTENTIAL HP to INTAKE PORT FLOW is (.257 x port flow at max cam lift x 8(3 of CYLINDERs)= POTENTIAL hp
look closely at the duration used for each MATCHING rpm range. ALSO KEEP IN MIND THE DCR AND OVERLAP MUST MATCH look here these are the valve timeing overlap ranges that are most likely to work correctly trucks/good mileage towing 10-35 degs overlap
daily driven low rpm performance 30-55degs overlap
hot street performance 50-75 degs overlap oval track racing 70-95degs overlap
dragster/comp eliminator engines 90-115 degs overlap
but all engines will need the correct matching dcr for those overlap figures to correctly scavage the cylinders in the rpm ranges that apply to each engines use range

2,409 Posts
I've been studying this subject. The extra info in the links is much appreciated.:thumbsup:

Super Moderator
8,600 Posts
Discussion Starter · #3 ·
"What is that minimum allowable clearance after any cam timing change as opposed to the original 0.100" settings??? "

IDEALLY you should HAVE GREATER THAN .100 clearance to start with AFTER PLANING OUT YOUR CAM SWAP,AND INSTALLING THE CAM, so that changing the timing (ADVANCE/RETARD within about plus or minus 6 degrees from that SPLIT OVERLAP POINT still allows that .100 minimum piston to valve clearance.
BTW youll normally find that the valves reach their closest point to the pistons at about 10 degrees before and after TDC, NOT at TDC
Ive seen guys go as tight .060 but thats nuts in the long haul if your expecting that engine to live very long at high rpms

look carefully at this chart

now look at this cam card

at two points those valves are damn close to the piston
if that lifter is .050 off the base at 11 degress BTDC the ROCKER ARMS 1.6 ration lifts the valve .080 thousands, the piston is only .035 thousands down the bore, with a common .025 deck and .021 head gasket that means the edge of the valve is .001 thousands froum touching the piston without valve relief cuts for additional clearance on a flat top piston. heat expansion allone would cause them to hit BEFORE the TIMEING EVER THOUGHT OF GETTING ADVANCED (yes Im well aware the valve enter the cylinder at a 23 degree angle and that you really have SLIGHTLY MORE clearance but a few thousands makes no differance here

Premium Member
11,627 Posts
If you don't quit posting all this great info, I may never get my motor finished. So many things to consider. Really tho, great stuff, thanks. :)

38 Posts
wow id sure like your guys response to my question. A cam that has been custom selected for my new stroker has around 230ish duration. I was told by sooo many people i would need around 240-50 to get to 7500. The engine builder simply told me to check the duration on a ls6 (z06) cam. something like 206 and 218. and that sucker pulls to 6500. basically that was enough of an answer. guys is there anything else that affects rpm capability besides duration?? how the heck does the z06 pull to 6500 with 206/218 duration??? i may be wrong on the actual specs but i think i saw it somewhere around there. MY LT4 heads flow 309 + cfm @ .600 lift. cam is about under there in lift #s. thiink it was 301 @ .550 lift. volume is 190-195 cc. mech roller too.

all i wanted was to make peak around 6300-6600 and still have it make good power to 7300-75500 rpms.
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