ok, i know custom exhausts have been talked to death about on here but i was looking through the may issue of scc and thought id bring it up. they completely dismissed all the issues of too big exhaust size. they said that needing backpresure for low end torque is just an old myth. they suggest 2.5 inch piping for 2.6 - 3.0 liter engines. the president from Isuzu Performace wrote back saying that these recommendations are too small and that he has done testing on all different cars to test it. he recommends .25 to .5 bigger. what do you guys think?? why do all the aftermarket companies chose 2.5 for us???

 

it depends on the RPM range you're looking at also.
a 3L engine that "only" runs to 6500rpm won't need much mroe than a 2.5" exhaust. you'll lose tons of torque down low. there are a bunch of people that have had a complete 3" system made up and they lose tons of low end.. the benefit is that they have more high end.

in our engines, the 2.5" seems to be the best overall size. enough flow for high HP, but also enough constriction for high tq on the bottom end.

 

I second that thought. It all depends on where you want power and aht kind of throttle response . If you don't mind having to stand on it for a min to get the revs up, go big. If you want quick shifting power where you aren't at redline all the time, keep it around 2.5" all the way back. Be advised that this all goes out the window with turbo and SC cars. Both want large diameter to get rid of gases asap, especially turbo. 4" after the turbo is not uncommon on engines equiped with large compressors.

 

I agree with Matt, unless you are running nitrous in which case upping the diameter a half inch could be benificial at WOT. I would personally stick with 2.5" though. I am going to keep mine 2.5 even with nitrous...

 

thanks guys. answered my question

 

you guys are just too smart

 

actually greddy use's 60mm exhaust piping which is a lil bit smaller than 2.5 inch

but i`d go with 2.5 inch its about the perfect size for the max

 

i will continue to agree to disagree, but hey, it's all good!