While not a "veteran" of this forum, several decades of riding and racing (plus 2 years of custom-building various exhaust headers) have taught me some lessons about making power with exhaust systems that I will pass along here.
EXHAUST MODS:
The objective of nearly all engine mods that are intended to increase power do so by increasing
air flow in, and/or exhaust flow out of the engine. This is to done to provide more oxygen to the combustion chamber to support the burning of more fuel which = more power. Turbos, superchargers, oxygen-bearing fuels (nitromethane), nitrous oxide, cams, head porting, etc. all contribute more oxygen to the fuel burning process. For any given engine design, the engine's power
potential is proportional with the volume of air it can flow. This means that (with all other factors being equal) a 1,300cc Busa engine will theoretically make the same maximum power as a Busa engine that has been bored to 1,400cc... IF THEY FLOW THE SAME VOLUME OF AIR. This notion is resisted by some motorheads but is nonetheless true.
Efficient exhaust systems increase power primarily by reducing
engine pumping losses which can be thought of as flow resistance. Manufacturers have to compromise their exhaust designs to meet environmental (pollutants) and social (noise) requirements which leaves room for aftermarket exhaust builders to offer more efficient designs. Some factory and aftermarket designs are better than others but the factories are really doing a good job these days in view of the constraints mentioned above.
The physics of exhaust dynamics is a daunting and convoluted subject and beyond my intentions here. Suffice it to say that aftermarket performance designs are (or should be) oriented toward reducing so-called "back pressure". The reduction of back pressure decreases engine pumping losses and thereby frees
existing power. It is true that some engines suffer a torque-loss at low RPM when back pressure is reduced. This is because the engine is tuned to "X" amount of back pressure. If properly re-tuned to the newly-reduced back pressure, most modern engines show the same or greater power at low RPM
plus substantially more power at mid and high RPM.
The selection of an exhaust system should be guided by what you want your engine to do. Some aftermarket designs focus upon maximizing torque across the entire powerband, some focus upon appearance (cosmetic exhausts), others focus upon maximizing high RPM power, but all of them represent some kind of
design compromise because of a fundamental limitation of Physics--both flow
volume and flow
velocity cannot be optimal at all RPM for a given tube size. Hence the compromise.
I personally go for exhaust designs that offer the highest
average torque across the powerband. Why? Because For a vehicle to cover "X" distance as quickly as possible, it is not the highest peak power generated by the engine that is most critical. It is the highest
average power generated across the distance that typically produces the quickest time. When comparing two power curves on a dynamometer chart (assuming other factors remain constant), the curve containing the highest average power is the one that will typically cover the distance in the least time.
If you are interested in more details, an article that I authored on the subject is located here (scroll down the page):
Exhaust Theory
