Exhaust

[JMo60]

Im thinking about welding up a 2.5" all mandrel exhaust to my car. Im still using the factory exhaust without a cat, or mufflers. What kind of performance help have you guys seen while going from stock 2.25" to a say 2.5" system, with smoother transitions in the bends?

[19Cutlass94]

I know going from my like 1-3/4" or like a 1-7/8" pipe ( not sure, the pipe was kinda oval ) to a 2.5" was huge. But it does it loud and raspy so plan for these things!

[JMo60]

so we are lookin at making it sound like shit? That sucks, because with no cat, stock resonator, and 8 inch resonators before the exhaust tips, it sounds so nice.. If its gonna make it crappy, i might not even do it... i donno

[Ortie666]

All I have to say is the bigger the better.. I ran 2.5" full duals, custom mufflers with headers on both my cars.. always great performance.. My recent car witch is has an iron head 2.8, edelbrock intake, 400cfm tunnelram carb, and 1.6 rockers. Now I had gottin an exhast inspection because on it because they were side dumps and i had the baffle's out witch made straight pipes. well even with the baffles back in it wouldent pass because it's a chevette and the passenger compartment goes to the rear bumper.. So i was forced to change the exhast to a single 2.5 to a thrush turbo muffler then under the axle and out the back.. well it was quiet, and it passed..Thats the good news, the shitty news is that I lost an easy second off my quarter mile, I wasnt able to do all throttle drifts in 2nd gear any more, and it quit reving at 6000rpm(<this could be a good thing).. On the day it passed the inspection that nite I took it all off and ran open headers. I couldent beleve the diffrence it would make power to 7200, light the tires in 2nd and i beet my buddys turbo rabit with a 1.8T audi motor... Now mabye little civic's need some back pressure, but if u plan on making power or going fast exhast is a key element..
P.S. try desktop dyno, its quite acurate

[JMo60]

so, i guess i might do it.. is there any advantage in going larger than 2.5". Too big kills velocity through the system, so i was wondering what everyone else does... i have been running through the stock exhaust for way too long. The worst part is where it splits to duals, its a very poor design.

[Ortie666]

yeah, larger than 2.5 may be a waste if you just have stock manafolds, if u had headers i would say biggers better

[sharkey]

with stock manifolds a lot of the scavaging is left up to the exaust system itself. with headers, the headers themselves do all the scavanging (and much more efficently) and anything after the collector is a restriction. with headers, bigger is usualy better, but after a point you wont see much gain. for most people here, 2.5" is plenty big enough for an n/a engine.

[JMo60]

i have S&S headers, and using nitrous... so possibly even larger... i think a good 2.5 inch system with better bends would be real good

[IsaacHayes]

Cool, I will go 2.5" if I do get mine done. I was thinking 2.25" since I have a 3100 but not a 3400. But only difference is the 3400 has a tiny bit bigger intake valve and .3l more displacement. My motor has the same intakes (actually better plenum). So 2.5" for me. The downpipe on my car is terrible double walled tiny thing.

2.5" should work great for you. Superdave is running 3" but he has a 3500 with long tube headers and a super rowdy cam. He is putting out enough power to need the 3". I have no proof but I think 3" would be too big for you unless it's only a drag car and you'll be spraying nitrous every time. A daily driver 2.5" should support your HP just fine.

Check this link out for pipe size.

[19Cutlass94]

Ive got a 3100 ( for now ) and the 2.5" made a HUGE difference. The downpipe is actually 2.5" 409 double walled and i just used that over cause the flange is about 1" thick. 3" would deff be too big

[IsaacHayes]

Did you measure the ID of downpipe? I think you'd be surprised how small it is inside... The outside looks large, but that's not what matters... Regardless, if you are running that small downpipe and the 2.5" back still made a difference, there is something to be said!

I think I'm going to have to do this soon. The intake side of the motor is opened up, but not the exhaust. Normally I'd get the stuff and do it myself, but I have no welder, and no time. So I'm going to have to fork the money and hope they do a good job.

[Ortie666]

Has anyone else tryed runs with open headers? Even just unbolt the collector and just wire up the Exhast off to the side?? Im just curious on the gain other people see

[IsaacHayes]

There is a guy with a 3400 swap in a beretta un-tuned who ran with an open downpipe (cutout) and picked up a bunch of speed. Very fast run for OBD1.5 setup. I can't remember the thread where it was stated, but 3400-modified was posting his times and mentioned it.