Bumping this good thread :cheers
3.5" exhaust has been good enough for ~450rwkw 600+rwhp *dynodynamics* (not one hit wonder but repeated many times). SMB mufflers.
I will find an extract from a Garrett Engineer who wrote up a fantastic few paragraphs in relation to turbo cars and what effect down stream turbine pressure has on all up system pressure (when considering turbine expansion ratio). *over anyones head yet?*
In a nut shell the larger turbo/turbine you run the less effected it is to exhaust back pressure (post turbo) as the turbine expansion ratio to flow the said given amount of gas is LESS :Typing So drain pipe "I ran over a dust bind Honda exhaust" is only really needed on a small turbo working its ring hole out! as its at a very high expansion ratio to make the power you need/want to have.
I use a 3.5" diameter system on my own car, with a catalytic convertor, and middle muffler and a big rear muffler and plumbe back exhaust. Turbine outlet pressure is 4psi range at 7500rpm and honest 440bhp output. Exhaust manifold pressure is around 17.5psi and inlet boost pressure is 18psi (better than the fammed 1:1 pressure relationship people strive for), car is whisper quiet too for a Rotary Turbo at this power level. The catalyst imparts over 2.5psi to this post turbine pressure level, without that you could run 700+bhp worth of air flow through the system and still stay at 1:1 pressure with the right sized turbine (housing and wheel).
Exhausts dont need to be as large nor loud as some people think to make good power :3gears-rh