ok i did a search and saw nothing on the first few pages so it hasnt been brought up recently but im sure it has at one stage

is 2.5" the best size for i6 engines or will smaller or larger be better

im running lukey extractors and 2.5" off that to 2.5" cat and rest of exhaust and muffler is 2.5" too

the reason im asking this is because i can get 3" cat and stainless pipe and bends for next to nothing jus needs to be welded up and i know a guy who can, so basicly want to know weather this is worth it or not. I know it would be definatly for turbo application which could or could not come in the future....

cheers boys

PS has anyone seen or got dyno reading of differant size exhaust as comparison?

i have 2.5in on my EF.... cat back (with pacey hedders) and runs like a dream... looking at getting rid of last muffler for a bit more of a note....

2 1/2" exhaust pipe is the best.
But larger mufflers would be better from what i've been reading lately. That's because a straight peice of 2 1/2" pipe flows say for example 200cfm, were a straight through 2 1/2" muffler flows less than that (for examples sake lets say 120cfm), but a straight through 3" muffler may flow 190cfm, so using a 2 1/2" exhaust pipe and 3" mufflers will keep a nice constant flow rate, but it might not like the varying sizes. I'm wanting to find out a bit more about this...

so lets say i had extractor's a flange to 2.5" pipe then 3" cat and 3" pipe all the way back, the first pipe should restrict the system to 2.5" flow and the rest being 3" will let it breath about that too...

if you want to do some thing like that you would run the 2.5 pipe right to the muffler. then fit a 3 inch muffler and 3 inch tail pipe.

if you run 3 inch ipe from the extractors you fool the pulse tunning of the pipe
giving in efect and open extractor. not what you want for the street.