Exhaust diameter

[jay28]

At what point does exhaust diameter start to impact on performance? Ive fitted a 2" system in place of my 2.5" side exit. I know the more free flowing the better but what sort of power is 2" good for?

[Phoenix Autosport]

imho unless you have replaced the turbo elbow with something more free flowing it wont make any odds as that is the bottle neck in the system, re max power its more about flow rates than the actual diameter, design being more important than size

[Tutuur]

Some rally expert once told me the exhaust is 'diameter in mm=3.06x(rootxflyhp).'
Everything above that is where exhaust gasses are going to see more back press due to velocity.

Obviously there are more variables like the heat of the gasses and bends in the exhaust but this is a rough rule of tumb.

Considering that 2" (internal)would be enough for 250hp

[jay28]

Enlarged elbow. Seems odd not having noise! It seems much quieter, just need to put a downward sweep on the back box. I cut the tailpipe off leaving a couple of inches sticking out as I have a smoothed bumper and quite like the look of it. May get over speed bumps without wincing every time now!