I doubt this sir. I'm no where near the car expert that many here are, and you sound like you know more than I on this stuff, but all the evidence out there proves what I said correct. I honestly have no technical backing to support this. My experience is on aircraft as a CC. But, from all things, doesn't this make since??
Think about it... makes more since. Why throw a 4" exhaust on a 318 when it can't really use it? Thats all I'm saying. I know most here have built up motors and all, but again, is bigger necessarily better?
You're misunderstanding your question.
The issue is not the backpressure but the velocity. I've posted several threads in the past year where I've discussed this at length, but here it is again in 10 minutes or less:
You're understanding the cross sectional area concept pretty well, but you are using the wrong terms. A complete lack of back pressure is actually the optimum design, provided you have the correct cross sectional area and primary length. Unfortunately noise and emissions laws affect this, so every factory exhaust is a compromise, and when you throw the economic factor in there, cheaper and smaller always wins. Still, the OEMs are able to tune some pretty respectable performance into those things. Look at our stock 5.9 motors, choked down with multiple cats, a poor exhaust system, and a prehistoric EFI system, yet it gives us 245/345. Thats really pretty good for a smog motor that pretends to be a hotrod.
Pipe cross sectional area and pipe length are the two biggest factors (collector design notwithstanding) in figuring the major benefit of header design, which is exhaust velocity. Velocity of spent gases is more important than volume of spent gases, because a lot of spent gases moving slowly is not beneficial to performance. Velocity as it increases causes a corresponding increase in cylinder scavenging, where you evacuate more spent gases and provide more room for fresh combustibles. By increasing the velocity while using a performance cam (higher overlap than stock) you are also assisting the intake side during the overlap by using the exhaust gas velocity to create a vacuum that pulls the intake charge in. However, larger primary diameter, larger collector diameter, and larger exhaust pipe diameter or any combination of those three can actually cause performance loss, unless you augment the intake side and give it more intake charge.
The backpressure=performance concept occurred when people who were running stock or nearly stock vehicles decided to have their local exhaust shop "slap on some duals" to mimic the performance cars of the 60's. They would notice a drop in performance. I noticed it when I went to true duals on my 81 Z/28, even though I'd installed an Edelbrock Performer manifold and cam package, a Holley carb, and a recurved HEI distributor. Where did my acceleration go, especially out of the hole? Everyone knows that duals adds power, right?
In my case, its because I eliminated the performance barriers that existed in the exhaust, and then didn't recalibrate my A/F. I needed to rejet my carb, or even go to a bigger one, to take advantage of all that extra flow. Otherwise it was running way too lean. A carb can't rejet on its own, and most garden mechanics of the day didn't know what was going on. The exhaust shops would simply say, "Yep, changed out lots of factory y pipe systems for true duals and seen this happen almost every time. You lost too much backpressure."
So the myth took root and stuck. The only truth to it is that on a carbureted motor, reducing backpressure can lean out the motor so much that you burn up the exhaust valves. Fuel also cools the exhaust, the more you use the cooler it runs.
On a modern EFI engine, your O2 or Lambda sensor will read the spent gases and then change the injector pulse width to alter the A/F until its within spec. If you have a factory EGT sensor on there, it can really do some fine tuning.
At the end, only an EFI motor can self-adjust for the increase in velocity ("loss of backpressure"). A carbureted motor will suffer from increased velocity until you adjust it yourself (rejet or upgrade the carb to a larger one).
