I'm curious... Every once in a while I read from people here that "exhaust backpressure is necessary for low-end torque" or maybe "don't go 3 inches all the way back on a 305 b/c you need to keep some backpressure to make low-end torque".

I don't understand - can anyone explain why backpressure could result in more torque? And if this is true, how do you decide how much is enough/too much/not enough?

 

Well, try this next time you're at the hardware store to kind of give you the idea behind this:

Go to the section where they keep the PVC pipes. Now look at the one inch diameter stuff; the really long bulk stock. Make sure no one's around to see you because you're about to look weird.. Blow through the pipe. Just a hard puff. Right after that puff, you should feel the "momentum" of the air inside try to pull the air out of your lungs. Go to a bigger diameter and try the same thing. Can't get that same effect, right?
This is basically what's going on inside the engine's exhaust system. When the pipe is too big, that backpressure isn't there after an exhaust stroke to help "pull" the gasses out of the cylinder. That means the leftover non-combustable gasses that aren't pulled out are still there to displace combustable air/fuel on the intake stroke, resulting in less power. At higher RPMs, the effect isn't as pronounced.

But in my case, as well as many other 305 guys here, we're building for that future 350 or 383 or anything >305, so we usually go over that in anticipation of future modifications.

Anyway, like I said that's a simplified way of explaining your question.

 

"Well put!"
Anyway, how about those of us who have bumped the compression to 10.50:1 in our 305's, would the larger pipe help in this case, or would it be about the same with some loss of back pressure? My motor revs so fast it reminds me of the ole 302 DZ, in the butt dyno.

 

It's not the "backpressure" that provides the scavenging pulse. If that was the case, you wouldn't need headers, you could just put a restriction in the tube and have as much or as little backpressure as you wanted. It's the vacuum created behind the outgoing pulse travelling down the tube. The idea of backpressure comes from the need for the tube to provide some restriction to keep the pulse from dissipating too soon and destroying this vacuum.

 

Well, it might have been misleading to use that word "backpressure" when talking about the scavenging effect, but they're indirectly related. You're right, Apeiron.

[This message has been edited by Jza (edited November 30, 2001).]

 

They are NOT related!

What Apeiron is describing (and you with your PVC analogy) is maintaining gas velocity to produce the scavaging. Having too great a flow area reduces the velocity, reducing the post-pulse "vacuum" effect and therefore cylinder filling for the next cycle, reducing cylinder pressure upon ignition, reducing torque produced, and requiring higher rpms to get gas velocity up to where it does begin to produce the scavaging effect and increase torque, etc. (remember, horsepower is just torque times rotational speed).

In practice, you want to match the primary tube size to the RPM and cylinder volume of your engine (compression ratio has next to nothing to do with it). For example, a high-reving 305 can use as large a primary tube as a low-reving 400, but will seem lacking in torque - as much from the cam timing/duration as from the primary tube size.

Let's stop this talk about "back pressure = torque", and get an understanding of the physics involved. We'll all be much better off for it.

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4, 2300 stall TC. Ported World 305 heads, Crane PowerMax 2050 cam. ZZ3/4 intake, oil pump, pan & baffle. Accel HEI SuperCoil & module. Hooker 2055 headers, 3" Catco cat & 3" catback w/dual-opposite Flowmaster 80. 2.93 limited slip. Spohn SFCs waiting to be installed. AMSOIL syn lubes bumper-to-bumper. Daily year-round driver. Best ET, speed TBD...
57 Bel Air, my 1st car. '66 396, 9.7:1 forged TRWs, Weiand Action+, Holley 750VS w/4150 conversion, GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlocks, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Idles smooth @ 700 RPM in D. Best 15.02/95.06 @ 5800' Bandimere (corrected 13.93/102.4 @ sea level).

 

double post


[This message has been edited by Jza (edited November 30, 2001).]

 

https://www.thirdgen.org/messgboard/...ML/005393.html

An earlier post i made on this topic that IMO sums it up pretty well.

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Ed Maher - Moderator @ The TPI & Carb Boards
92 Z28 Convertible - Quasar blue / Tan top
305 TPI A4 2.73 - 14.7 @ 93.6
Stock except ported plenum and dual cats
-=ICON Motorsports=-

- Definitely prototypes, high powered mutants of some kind. Too weird to live, too cool to die

 

Thanks Ed. I got tired of explaing that. Its too damn long to repete all the time. Is it in the thirdgen FAQ somewhere?

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-Tas
'89 Formula WS-6

305, TBI, 700R4, P.A.W. 14x3 open element with K&N, Milodon 160* thermo, functional Formula hood, cross-flow Flowmaster, '99z28 rear pipes and tips, Hooker 1-5/8" 50 state legal headers, Dynomax 3" I pipe (PN 44063 and 43248)
Super GRK_Taz World
F-Body Dual Exaust
EFI & Intake Options

 

9-1/2 months later, it still hasn't gotten through.

 

This may sound stupid but I just had an idea. What would happen if we used a big 4-inch pipe from the headers back and had an electric blower sucking the exhaust out of the cylinders and blowing out of the exhaust? Would create more torque and HP?

 

Only if you had a turbo.

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-Tas
'89 Formula WS-6

305, TBI, 700R4, P.A.W. 14x3 open element with K&N, Milodon 160* thermo, functional Formula hood, cross-flow Flowmaster, '99z28 rear pipes and tips, Hooker 1-5/8" 50 state legal headers, Dynomax 3" I pipe (PN 44063 and 43248)
Super GRK_Taz World
F-Body Dual Exaust
EFI & Intake Options

 

Well, it sounds like a neat idea and all, but the problem is, there is more exhaust gas than intake charge by volume. Thus, it would take a bigger blower to suck the exhaust out at a given HP level than it would to blow it in. Thus, just get a regular blower.
Plus, at a certain point, even if you did have some wild blower to pull out the exhaust, you would start pulling raw intake charge straight through the cylinder during overlap. Thus you'd hit a case of no more gains even possible.

 

Then you could say your blower really "sucks"!!!!!

AJ