First rule is exhaust pipe must be able to flow at least as much air out as it is taking in, then you have to allow for expansion due to heat, then any automobile exhaust is a comprise as to where you want what to happen. Then a different cam grind will respond to the same exhaust system in a different manner. Years ago they discovered that the over all length of pipe caused a different response from the engine. Then you get involved in the effect of reverberation of sound which also has an effect on flow. This is why so many different designs have been used for so many years. Those of us old enough to remember when Headman made 1 5/8 tube headers for 283 cu in chevys then everybody started making large tube headers for the larger small blocks and many guys went to the large tubes only to discover that they slowed down. All the math and all the designs in the world won't give you a fool proof answer on exhaust systems because it all depends on who, what, when, where, and how the vehicle is made and operated. We can site many examples of what will work here or there but that doesn't mean it will have a positive effect on what we are doing. As was mention earlier many engines are designed to operate in a small operating range under fairly constant conditions but our cars aren't one of them. So that leaves us with two options. Use a design that is generally accepted and be happy or spend the bucks for dyno time and several different systems to find what works best for us. Then change tire size or cams or anything else and discover that if we want the ultimate power we get to start all over again designing an exhaust system. In top fuel, nascar, indy cars where 5 hp makes a difference this is critical but in our street driven cars I don't beleive it is. My advice would be pick a system that looks and sounds the way you want and put it on your car.
Big Bill
