| As a general rule-of-thumb, changing to a larger diameter exhaust allows the escaping exhaust gases to expand more quickly which cools the gases faster, and consequently reduces the velocity of the exhaust gases, and also the timing of the exhaust pressure wave pulses.
These pressure waves are generated when the exhaust valves open. The exhaust gases flow at around 300 fps, but the pressure waves travel much faster - at the speed of sound (that is dependent on gas temperature and therefore on pipe diameter.)
The general idea is to use this pulse energy for tuning. You want to time the negative wave pulse reflections to coincide with the period of valve overlap. This allows the low pressure to help to pull-in a fresh intake charge as the intake valve is opening, and help to remove the residual exhaust gases before the exhaust valve closes. The effectiveness of this scavenging effect changes, depending upon engine rpm.
Too small a pipe diameter gives too high an exhaust gas velocity meaning that that the system is too restrictive (which reduces top end power), whereas too low an exit velocity tends to make the power curve excessively peaky by reducing low end torque.*On the track, high end power is more important. On the street, low end torque is more useful.
So, the best exhaust system is the one that gives the most useful power curve. So, YOU need to be the judge of what compromises you’re willing to accept, and then build the engine-exhaust combination that will give it to you.
So simply put ...
1. A large diameter full exhaust system, at best, will not help a stock engine make more power. Usable performance will suffer.
2. Adding a large diameter full exhaust system, WILL help a modified engine (i.e. larger displacement, larger intake and exhaust valves, flowed heads, higher compression pistons, etc.) make more power.
In other words, build a balanced system. Starting with an set of overly-large exhaust pipes probably isn’t the way.  |