[Joesurf79]

I have some questions for which I seem to get conflicting answers, and I'm wondering where I can find a good book on exhaust system design theory / tuning.


I have lived under the impression that on small displacement 4 cyl N/A motors, once you get past the headers / cats / front pipe - the section of the exhaust generally refered to as the "cat-back" had little to no effect on power production or delivery. Thus headers that were properly designed to scavenge / use exhaust pulses were the most important piece of the puzzle, followed by reducing catalytic converter restrictions - the catback was just for gas routing and sound managment purposes. By the time the gasses got that far down the pipe, there was no "gain" to be had - the velocity had dropped off too far.


Are the assumptions correct? I ask because I recently installed an Ace header and catless front pipe + tune (on a track only, NOT road use BRZ), and planned to fab my own 2.5" stainless, minimal weight dump pipe with a small resonator and 6" magnaflow canister muffler to cut the noise a little.


As I poked around with some guys that are familiar with spec miata racing whom I respect, they say they have seen quite an effect (10+ hp gains / losses) on the dyno when testing different diameters and tubing lengths of the exhaust system AFTER the headers... In the low HP NA car world, 10 hp is a huge difference!


This gives me pause - I don't want to fab up a short section of exhaust after the front pipe and end up losing 20+ pounds of exhaust weight...but also losing 10 hp as well!


Thoughts? Resources? Experiences? Any good books that anyone can recommend?