Well the only thing on exhaust I've come across is ones for S4 cars. I have an S5 so I can run a three inch exhaust with no problem right?
Charles

 

I'm not expert but 3" is a little to much for a stock port engine. Sure it will work well at high rpms but most people don't drive at 6-8000 rpm all the time. 2.5" will do the same thing with the added bonus of maintaining a higher exhaust gas velocity in the mid part of the rpm range. That is were I do most of my driving. Now the next question is will you notice a difference from one to the other? Probably not unless you did some dyno testing under controlled conditions(same day, same outside temp. same this, same that, etc....).

Personally on my racecar I'm starting with a 2.5" pipe coming off the header collector. I might use a 2.5" pre-silencer with a 3" Supertrapp at the back of the car or possibly one big 3" straight thru muffler at the end.

Notice I didn't use the most misleading word in the exhaust vocabulary - Back pressure

 

with a 2.5 straight through exhaust systemu will not have enough back pressure when its needed.. you will need to wire open your 6 ports.. i would recommend 2.5 inch for the n/a.. unless your going to have a full street ported monster then i would think about 3"

 

No need to worry about back pressure for port activation on a series 5. I agree w/ FPrep.....too big for street. It will add gobs of high end power, but the reduction of that much back pressure will SERIOUSLY criple your low end torque. I run a 2.5 from header all the way to muffler on the street, and it is great.

 

hes got a series 5 so back pressure isnt needed for the 6 ports its run off the air pump.

iv got 2.5 inch headers myself and plan on a 2.5 all the way back. once i can afford to replace the 2 1/4 thats coming off the headers right now.

 

no.. 3" is too big as far as I know. It makes very very slight increases up high, and kills low & mid power.

 

just run 2.5 all the way to the back. you won't be disappointed.

 

Sweet. 2.5 is what I'll use then. Thanks guys.
Charles