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Discussion Starter #1
Hi.

I wounder when 3" size exahust is not enough?
My new engine is going in and should produce somwhere around [email protected]
My exhuast would look like this:
Hooker longtube headers going in a hooker 3inch y-pipe to a singel strait 3" pipe (instead of a cat ). Then a set of magnaflow dual 2.5 cat back system.
I worrie that the singel 3" will kill much HP.

Care do give me some feedback on this?

regards
 

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i would do dual exhaust instead of the y pipe if it were me
 

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Exhaust Flow Revisted Again

Orginally posted by grumpyvette 3-14-04:

The jury is still out on “backpressure” although on street applications where some mid-range is desirable, it can help."

wrong ...your confuseing backpressure with the need to build an exhaust system designed to scavage the cylinders efficiently at low rpms but not be restrictive at higher rpms, if you install to large an exhaust you can loose flow velocity which hurts scavaging thus torque, but backpressue has no significant useful effect. (other than in some extreme cases where at low rpms it might prevent over scavaging the cylinders which does not hurt power but might hurt mileage, yet any system that is that restricted at low rpms will have little chance of making good high rpm power. the stock exhaust is designed to scavage in the 2000-4500rpm range, most headers are designed for efficient scavaging in the 4000rpm-6000rpm range. the stock L98 has only 245 hp but it accesses it much lower in the rpm range making the car more responsive , but give an Lt1-4 the correct gearingand a distance to get into its effective rpm range and the differance in hp becomes obvious

"One of the issues I’m most concerned with is loss of mid-range from my engine. I love the way the L98 will hump from a roll, especially with my 6 speed. A down-shift at the right speed is quite a thrill, especially when seeing the guy next to you fall behind. I realize some of this has to do with the length of the air intake plenums vs the LT1 and LT4 engines, but it could also be due to backpressure from the exhaust. I’m not sure. "

intake runner length and cross sectional area,,matched to compression and CAM timing NOT BACK PRESSURE are why the L98 has more low rpm torque the far shorter intake runners on the LT1-4 are designed to be effective higher in the engines rpm range. L98 has an effective runner length close to 25" long, the LT1-4 runners are effectively about 7" long, play with these calculators. keep in mind that the L98 has small high speed ports to maximize the 1500rpm-4000rpm range the cars are geared to run in the majority of the time

http://www.rbracing-rsr.com/runnertorquecalc.html

http://www.newcovenant.com/speedcrafter/calculators/intake.htm

http://www.bgsoflex.com/intakeln.html

while theres no direct linear relationship between port flow and hp there is a relationship.
the ballpark formula is
.257 x port flow x 8 = potential hp

EXAMPLE
if your intake port flows 250cfm
.257 x 250 x 8 = yealds APPROXIMATELY a 514hp figure before your heads become the limiting factor

this ballpark formula is based on average results FROM WELL TUNED HOT ROD TYPE ENGINEs USEING DYNO FLYWHEEL RESULTS
NOT true race only engines or street engines
also keep in mind that the results youll get will differ and to get those numbers it requires the engine to operate in its best rpm range with a cam and compression levels that match and all other components must also match
if your heads flow 279 cfm at .700 lift but your intake only flows 240cfm and your cam has a max lift of .550 your not going to get the max potential HP results
for the ballpark formula to work you must have a tuned exhaust, a cam that matches the compression ratio and all other parts must flow at least close to as well as the heads at the max figures
also keep in mind that the ports cross sectional area should keep the airflow in those ports in approximately the 200fps-300fps ranges
airflow speeds that very greatly from that 200fps-300fps will not tend to give best results, thats why huge ports that flow exceptionally well don,t work well on smaller displacement engines
port lenth also has a large effect on the rpm range that the ports can effectively pack the cylinders at due to harmonics in the collum of air
PORT SIZE FLOW AND THE RELATION TO CAM DURATION, and your displacement and the tuned rpm that your headers operate best in for scavageing the cylinders also comes into play here!


FIRST, This will not be anything more that a brief glimpse into a subject that takes years to understand and I’m sure there are a few people on the site that can give more exact info! This is meant to apply to the 350-383 sbc engines most of us are useing
My purpose is merely to give an idea as to the relationship between the factors and yes IM ignoring several minor factors to make things easier to understand
But lets look a a few concepts

(1) There are 720 degrees in a 4 cycle engines repetitive cycle of which between about 200degrees to about 250 degrees actually allow air to pass into the cylinder, (the valves open far enough to flow meaningful air flow) and the piston has a maximum ability to draw air into that cylinder based mostly on the engines displacement and the inertia of column of air in both the intake port and the suction (or negative pressure the PROPERLY designed headers provide) this produced a max air flow thru the ports, the greater the volume of fuel/air mix effectively burn per power stroke the greater the engines potential torque production, the faster you spin an engine the greater the NUMBER OF POWER STROKES PER MINUTE, and up to the point where the cylinder filling effectiveness starts falling off due to not enough time available to fill that cylinder the torque increases, above that rpm or peak torque it’s a race between more power stokes and lower power per stroke

As air enters an engine it normally travels thru both an intake system and the cylinder heads intake port to eventually pass into the cylinder thru the valve. The valves in a normal small block corvette engine are between 1.94 and 2.08 in diameter, that’s between 2.9sq inches and 3.4 sq inches of area, but because the valves require a seat that at a minimum are about 85%-90% of that flow area we find that the intake port even with out any valve has a max flow of not more than about 90% of the flow thru a port of valve size. Or in this case 2.46 sq inches-2.9 sq inches of port area, Since you gain little if any flow having a port that’s substantially larger than the valves AT NORMAL ATMOSPHERIC pressures and since you can’t substantially increase the valve sizes for several mechanical reasons you must improve efficiency, this is done in two major ways, you can match the intake port length and cross sectional area to the engines most efficient rpm range on the intake side, to build a positive pressure behind the intake valve as it opens and match the exhaust length and diameter on the exhaust side to provide a negative pressure to help draw in more volume this will require the cam timing match that same rpm range of course. By experimentation its been found that air flow port speeds in the 200-320 cubic feet per minute range are about the best for a chevy V-8 now lets say you have a 383. 383/8=47.875 cubic inches per cylinder, the rpm range most used is 1500rpm-6000rpm so that’s where are cam and port size must match, you can do the math , (47.875 x ½ engine rpms = cubic inches, divided by your cams effective flow duration, (use 210-235) as a default for a stock cam) x 720 degrees/1728 (the number of cubic inches in a cubic foot) to get the theoretical max port flow required (I will save you the trouble its 250cfm-275cfm at max rpms and about 2.4-2.9 sq inches of port cross section, depending on where you want the torque peak, or use this handy calculator,

Intake Runner Area = Cylinder Volume X Peak Torque RPM 88200
Or this helpful site
http://www.newcovenant.com/speedcrafter/calculators/intake.htm
Either way you’ll find that you’ll want a port size in the 2.4sq –2.9 sq inch area
Now use this calculator to figure ideal port length, REMEMBER youll need to add the 6” in the cylinder head to the intake runner length to get the total length and you can,t exceed the engines REDLINE RPM which with hydrolic lifters seldom is higher than 6400rpm

http://www.bgsoflex.com/intakeln.html


Ever wonder why your engines torque curve gets higher with the engines rpm level until about 4000rpm-5500rpm(DEPENDING ON YOUR COMBO) but fades above that rpm level?
well it depends on several factors, first as long as the cylinders can fill completely you get a good fuel/air burn so you get a good cylinder pressure curve against the piston each time the cylinder fires, THE ENGINES TORQUE CURVE INCREASES WITH THE NUMBER OF EFFECTIVE POWER STROKES PER SECOND, at very low speeds there’s not enough air velocity to mix the fuel correctly or produce a effective ram tuning effect but as the rpms increase the cylinders fill very efficiently until the rpms reach a point where the cylinders just don’t have the time necessary to flow
enough air through the valves to fill the cylinders , remember a 5000rpm the intake valve out of 720 degs. in each cycle opens for about 250degs of effective flow even with a hot roller cam, now that’s only about 35% of the time and there’s 41.6 intake strokes per second , that’s only 1/60th of a second for air to flow into the cylinder
Its your engines ability to fill the cylinders that increases your power and the more efficiently you do that the higher the rpm level you can accomplish that at the more power your engine makes, remember the formula for hp is (torque x rpm/ 5252=hp) so moving the torque curve higher in the rpm range increases hp but at some point the time available to fill the cylinders becomes so short that efficiency begins to drop off rapidly, the peak of efficiency is reached normally in the 4500rpm-5500rpm range, and as rpms increase its a race between more power strokes per minute trying to raise the power and the increasingly less effective percentage of cylinder filling dropping the power.
Volumetric Efficiency
The volumetric efficiency of a 4-stroke engine is the relationship between the quantity of intake air and the piston displacement. In other words, volumetric efficiency is the ratio between the charge that actually enters the cylinder and the amount that could enter under ideal conditions. Piston displacement is used since it is difficult to measure the amount of charge that would enter the cylinder under ideal conditions. An engine would have 100% volumetric efficiency if, at atmospheric pressure and normal temperature, an amount of air exactly equal to piston displacement could be drawn into the cylinder. This is not possible, except by supercharging, because the passages through which the air must flow offer a resistance, the force pushing the air into the cylinder is only atmospheric, and the air absorbs heat during the process. so, volumetric efficiency is determined by measuring (with an orifice or venturi type meter) the amount of air taken in by the engine, converting the amount to volume, and comparing this volume to the piston displacement.
this increases until the torque peak then falls as the rpms increase. Here is a rough guide to match duration to port flow at different rpm level

if you’ve been following along you’ll find that you’ll need intake ports about 2.3-2.9” sq inches in cross section, and between 12” and 21 “ long (DEPENDS ON WHERE THE ENGINE IS DESIGNED TO MAKE MAX HP) and cam timing in the [email protected] to [email protected] lift range, as the rpms or displacement increase either the port flow or the cams duration must increase or the engines cylinder fill efficiency rpm will drop!
Now this is important, as the port flow efficiency goes up though the use of longer and larger intake ports the cam duration could remain the same or even be lower and you get more efficient cylinder filling as the rpms increase, that’s why high efficiency port designs like on the LS1 can use lower duration cams to flow similar total air flow thru the ports than the lower efficiency ports like the old fuelie heads could but at some point all ports reach max flow and an increase in the time the valves remain open at higher rpms increases the cylinder fill efficiency and that increases the engines ability to make torque at that rpm range

we have all heard it, " you need massive low rpm tq" "you need a screaming high rpm hp peak" well heres some info,
More in-depth description:
http://www.revsearch.com/dynamometer/torque_vs_horsepower.html
http://www.dynacam.com/Product/Torque_vs__Horsepower/torque_vs__horsepower.html
http://vettenet.org/torquehp.html
http://auto.howstuffworks.com/horsepower4.htm
http://homepage.mac.com/dgiessel/engine/hpvstq.html

first thing to keep in mind is that theres no such "thing" as horsepower, horsepower is a mathmatical formula for the RATE at which TORQUE can be applied the formula for hp is (tq x rpm/5252=hp
example
450 ft lbs of torque at 3000rpm=257hp
450 ft lbs of torque at 6000rpm=514hp
because the torque at the higher rpm useing gearing can be applied faster
here read this

http://www.69mustang.com/hp_torque.htm

http://www.ubermensch.org/Cars/Technical/hp-tq/

http://vette.ohioracing.com/hp.html

where most guys go wrong is in not correctly matching the cars stall speed and gearing to the cars tq curve, if you mod the engine for increased high rpm performance but fail to also match the stall speed and gearing to that higher rpm tq curve much of the potential improvement is wasted.
example


in the close to stock engine above, the engine should be geared to stay in the 3500rpm-5000rpm range for max acceleration (lower in the rpm range if mileage is a big factor)

in the moded engine above the rpm range moved to 4000rpm-6500rpm requireing differant rear gears and slightly higher stall speeds to gain max acceleration in the same car,
you should readily see that a trans that shifts at 5000rpm will work in the first example but would waste most of the power curve in the second example,where shifting at 6500rpm under full power acelleration would make more sence.
a 3.08 rear gear and 700r4 trans matches the first example well but it would take a swap to a 3.73-4.11 gear to allow the engine in the second example to keep its most effective power band matching that second power curve well.

links youll need to figure out correct rear gear ratios

http://www.miata.net/garage/tirecalc.html

http://www.wallaceracing.com/reargear.htm

http://users.erols.com/srweiss/calcmph.htm

http://users.erols.com/srweiss/calcrpm.htm

http://users.erols.com/srweiss/calcrgr.htm

http://www.prestage.com/Car+Math/Ge...io/default.aspx

http://www.geocities.com/z28esser/speed.html

http://server3003.freeyellow.com/gparts/speedo.htm

http://www.pontiacracing.net/trannyratios.htm

http://www.tciauto.com/tech_info/gear_ratios.htm

C'ya
D
 

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Grumpy is definitely up on his performance. He knows more about backpressure than I know about performance in general!
 

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i was trying to tell my friends that backpressure is not good for max power..... they just wouldnt listen :huh:
 
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