Mob inspired me

Roland69 said:

anyone remember the old school Thrush glass packs

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is that what ya had on the z24? lol

88 Fox GT said:

Not saying that I don't believe the whole backpressure thing, but of course that chart is going to say that their headers make more power than other companies headers.

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The thing is; It's repeatable. I just happened to choose the Anderson test because I'd come across it most recently. It's also one of the few in memory that compares apples to apples. 1 5/8 in shorties to 1 3/4 shorties on a single combination.

This isn't the first time that I've heard the back pressure = torque phenomena. There's certainly something to having different sized exhaust components on different engine combinations that yield desired results. Changing the inside diameter of the exhaust in either direction "CAN" give a desirable result but it's not a product of increasing back pressure. It's a product of increasing the velocity of the exhaust within the exhaust tube. I'm sure that everyone has heard (at one time or another) the term "tuned exhaust".

Tuned exhaust isn't achieved by tossing a restriction somewhere into the exhaust plumbing. If that were the case, we'd all have some sort of restrictors stuffed into our tail pipes. If we all had the equipment to do it, we'd be varying the diameter of various tubes within our exhaust systems to find the optimum size to both allow the exhaust to exit the engine with as little hinderance as possible and keep the velocity of exiting gasses as fast as we possibly could. This is what I'd aluded to above when I was talking about exhaust scavenging.

As with the induction tract, if you go TOO large then you'll get lazy air within the exhaust plumbing. What's most desirable is having an exhaust system where the spent gasses from one cylinder are assisting to help "suck" the gasses from the next firing cylinder, out of the exhaust system. Tuning the exhaust perfectly to keep those exhaust gasses at high speed keeps the venturi effect of the expended exhaust moving quickly out of the tail pipe.

It's this same thing that's brought about the advent of stepped tubes in long tube headers. It's also why long-tubes tend to be more effecient and provide more power than shorties (although sometimes the gain may be negligable again, dependant upon the suitability of the exhaust for the combination).

Now having said all of that; I don't think that stepping up to some 1 7/8s headers would net much in the way of gains (provided there some headers of that size to be had in a set of shorties), but who knows. What I am reasonably certain of, is that I were to put a set of OEM shorties onto that same combination that I definitely see a performance loss in both torque and horsepower on that very combination. I'm also reasonably certain that if I were to put that same exhaust system (1 3/4 etc.) onto a stock 89 motor (for instance) that it would be to large and would not perform as well as the OEM exhaust does on an unmodified motor. I believe that exhaust system to be too much for a stock motor and that the lazy exhaust scavenging would show its head on the dyno.

For the most part, I think Ford did pretty well matching the exhaust to the engine combo when they built these cars. They DO have the equipment to ensure that it all works together as well as it should. Modifying the motor however, also requires opening up the exhaust to accomodate it. We've allheard before that your exhaust system doesnt "give" you power. It frees up what already in the motor.