Originally posted by JSmith:
cooling of exhaust gasses is a big issue, since as a gas cools it does contract, but its density also increases and its this increase in density that restricts flow.
If a larger pipe is used then the ultimate gas temperature will decrease as the gas expands to fill the pipe, not only that but the larger pipe has more external surface area that will radiate heat away and further cool the gasses.
Idealy a pipe size should be selected that will allow the "optimum" gas velocity to occur when the engine hits the RPM at which it makes its max power. If the pipe is too large, at low RPM it will actually over scavenge and may go as far as to pull raw fuel from the combustion chamber which will decrease power and waste fuel.
Now I'm confused. Pulling raw fuel from the combustion chamber? Loss of high end torque? Everything I've experienced so far contradicts all this. I know nothing of exhaust, just that I thought larger pipe results in more power. When I had the three inch pipe installed, maybe the fact that the pipe starts two inches after the cat may play a role and may have turned my entire exhaust into one big muffler.
