Every motor will have different exhaust requirements, and for the most part, trial and error. While changing one or more of the following has an effect +/- a certain percentage... (pipe size, crossover, crossover type, headers, manifolds, muffler type, muffler size, collector size, primary size, muffler inlet/outlet position) Each has it's own positive or negative effect on a combination, the key is either having the knowlegde on what works where, or the money to have it known.
There are certain instances where a "non restrictive" or zero backpressure exhaust may be required..Nascar?
I am sure there is a huge difference between the exhaust system from a superspeedway track at 2.5 miles (Atlanta, Michigan, Talladega) versus 1/2 mile tracks like Richmond, Bristol and Martinsville. And I am sure there is a difference between those two exhaust set ups and a road course such as Watkins Glen. Each track has a specific range where the motor needs to operate at peak HP/torque, thus one would believe the exhaust system would be modified in one detail or another. There may even be a different exhaust used for restrictor plate racing.
I agree that an open exhaust (headers or dumps) produce the most HP. The key is finding the right combination that works for the vehicle. The exhaust requirements could be the same for the street or track for the same car, but at the track, more HP could be achieved with different mufflers, size etc.
Maybe there is a grey area with terminology. There is and can be many variations to increase or decrease a system +/- 1-2 HP or more per change. You could possibly gain up to (as example) 40-60 HP (over a good system) with the proper setup if you have the time and money to have changes made. I have heard guys buying a certain exhaust system from Pypes and making exhaust mods at the track and over time and reducing their E.T's with other recalibrations.
The systems out there, for the most part appeal to the general class. You could squeeze more performance out of it, sure.
