Originally Posted by
GU357
Rotel ur partially right about the air being needing to move and that exhaust diameter is related to desired rev range, but.
The reason u want a thinner exhaust at Low Revs is so the exhaust travels fast enough to create a low pressure drag behind the exhaust pulse.
if u dont have that drag then the engine doesnt really want to rev at that speed (or turn over at all).
I dont think it moves the power range but rather emphasizes and magnifies it in certain ranges.
but all things being equal means you lose it down low or up high.
If u have a high reving engine ie Rotary, then u want a big pipe because u dont want the pipe to be constrictive of the exhaust gas that can be output, so ur pumping more exhaust so u will need a bigger pipe to maintain the same flow rate.
When tuning an exhaust you aim for volumetric efficiency, and 0 back pressure.
the width of the exhaust must be suited for the peak RPM's where you have peak HP but also where you still have torque.
to attain no backpressure u want a straight pipe with no bends or constrictions (ie CATs) thats why u get HFC's n that.
*another good trick to make exhaust move from ur headers is too wrap it in heat wrap! because hot gasses move faster.
-the whole back pressure thing is a misconception because someone that that by putting a big exhaust on his car he wouldhave less backpressure then he drives it n is like this sucks, no backpressure = no torque, rather then saying wider is not better (in this case).