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OK folks. Steve and I are going round and round about which is better for a boosted vehicle looking to get the most gains and have a nice quick spool. Money is not really a concern here but we've gone head to head about which is better for the vehicle - 2.5" or 3" exhaust. I want to know definitively which one would be better and why. Here's my logic, and this is not backed up by fact, just supposition.

Mad balla Foos-style 3" exhaust
Benefits

• Minimal back pressure - the whole point is to get the exhaust out the ***-end of the car as fast as possible, right? That's why you see folks at the strip with that big 4" DP sticking out the side of the bumper.
• minimal backpressure means faster spool which equals more WHP earlier in the RPM range

Drawbacks

• Pain in the *** to run 3" piping on a Honda

Steve's busted 2.5" ghetto-back exhaust
Benefits

• Easier to run than 3" exhaust
• ???

Drawbacks

• More restrictive than 3"
• ???

OK, so what I understand Paul is telling Steve is that 3" exhaust is overkill unless you're making low-mid 400s at least. But then I see from Bob who hit respectable numbers with his and enjoyed quicker spool times and was a lot gruntier in the low end.

So post up folks. Personal experience, bench racing, **** talking, whatever. Let's hear it!

 
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3" is huge, I think I'll be sticking with 2.5" but I'll probably have to restrict the hell out of it to keep it quiet.

 
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Probably not. My 3" setup is quieter than my 2.25" fart can catback.

 
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3"...

 
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I would focus on the manifold both exhaust and intake before I worry about 3 inch or 2.5 inch exhaust. Flow is everything especially with turbo. But if and when I'm turbo I'd just do 2.5 for practicallity. Less clearance problems especially in the US and our shitty roads and damn 3" is huge we don't have Evo's lol.

 
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If its a 350whp+ setup I'd pick 3" hands down as my personal opinion.

Your basic bolt on turbo kit with supporting mods can do quite well on 2.5".

I know this much, with an open dump I made less midrange and only a lil more top end, 7500rpm+ felt good though. The 2.5 I had on there was crap after 6500rpm but felt good around town. My 3 inch gave me back so much more torque than an open dump without choking the top end up one bit. Mind you I am using a dynomax muffler so its not a straigh through design. There is some back pressure there which is what I think contributes to it working so well. Its quiet until I get on it, I love it.

 
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i like 2" for n/a, and 3 inch isnt that much bigger, with the right muffler for some added backpressure, i think 3" could be a really good option, its partially about preference, id stick with what u prefer, but my vote is 3"

 
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If its too large it can hinder performance by reducing the scavenging capability

 
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all I know is that going from 60mm piping to 3" gave me about 20whp at 13lbs of boost.

Also it wasnt SUPER hard to run the 3" exhaust but the clearances where tight. The old saying goes... "measure twice, cut once" My vote is 3"

 
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Ya but what will 3" give me at 9LBS! lol

 
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I'm with Gery to an extent. I ran an open downpipe for well over a year at over 300 whp and it pulled like a son of a bitch in the top end. It was a straight up top end car, in fact, I can't remember losing a single race when I was pushing over 350.

BUT, now I have a full 3" and can tell a huge difference in the midrange, there's a lot more power and a much quicker spool. I'm only seeing about 250-ish in the midrange and I could feel a significant difference there.

As far as I'm concerned, 2.5 shouldn't be an option, if you're going to do it at all, do it right the first time with 3" piping.

If you go to evans-tuning.com and take a peek at the pre-tuning checklist, he only recommends 3" piping for turbo Hondas and now that I've installed one, I fully understand why.

 
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Yeah, drove the car in this morning, and she's a lot gruntier down low now and spool does seem to be faster with the 3" setup. I'll take the highway home today to let her stretch her legs a little. Manifold and exhaust aren't a factor. I've got the dezod log mani, which is good enough for me, Steve's got the EL mani, and we'll both be running Gerry's intakes. You're right Bob, clearances were tight, but I didn't bottom out at all on the way into work.

So let's tally up here.

Rufus: wants 2.5" from a noise level standpoint
Don: Recommends 3"
Ron: wants 2.5" from a practicality standpoint, not sure what you mean by practicality
Gerry: 3" is the way to go if you're doing an all out setup, but 2.5" is fine for sub-300 builds
Adam: votes for 3" with a decent muffler
Tom: Advises not to go too big due to potential velocity reduction
Bob: proven gains with 3"
Andy: found that 3" is gruntier in the midrange and if you're going to do it, then do it right the 1st time
Foos: likes his 3" setup so far, seems to have more torque, quicker spool

That's one vote for caution, two votes for 2.5", and six votes for 3".

Keep it coming!

 
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Keep in mind, if you're doing a 3" setup, having a restrictive muffler that doesn't give a true straight through flow (like in vibrant performance), you aren't getting an accurate gauge. Mine is quieter with 3" and vibrant parts than 2.5" for the record but its only going to be so quiet with a stage 3 cam and open dump tube.

 
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what about the oval-shaped piping where you get the same inner volume with a more speedbump-friendly diameter for us with vertically-challenged civics? could you not then get the flow characteristics of a 3" exhaust in a pipe with the same clearance as say a 2 1/4 pipe?

just throwing it out there...

 
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well i guess 3 inch it is then huh..lol

 
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From a flow standpoint, the less turbulence, the better. With an ovular shape, you're going to have more turbulence hindering flow at bend points. But is it enough to hinder performance? Only side by side dyno pulls with the same car and an oval vs. completely round exhaust would tell you how much of a difference you would see and where. My guess would be you wouldn't see that much at any given point....

 
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Boiler's Plan: Do both
3" from the turbo through the cat, since crapalytic converters clog up flow anyways, then go 2.5" from there back to clear up the space constraint. I doubt I'll need a 3" at all.... but what the hell, I already have the tubing.

 
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So far I'm happy with the 3". Real grunty at the bottom and spool is much faster. Admittedly my setup a week ago was 3" thru the cat to a 2-1/4" fartback setup, but it's still nice!

 
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F@ck it... i'll go 2.75....lol... naw i was thinking 3" even tho mine is really low...just gotta wait and see i guess

 
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^^lol i was thinking the same thing, go in the middle of both..

 
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lol i know right... i wonder if u would really get a mix of both world... i know that u would get a quicker spool than 2.5 plus it would help with clearance issues a lil bit too.. hmmmm just might work...lol

 
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who the hell knows...ill be going with 3 inch thou...lol

 
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aiight cool! how low are you?

 
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don't go as low as me Jay...i got 2.25 and i scrape all over the place....tore my rear bumper off the other day on some train tracks too

 
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DAMN!!!!!!!! what size rims u got? i didn't think that u was lower... cuz i got 18s and u know i'm on tein ss coilovers... is ur bumper ok??

 
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I'm also dropped on Tein SS. No scrapey.

 
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My opinion is go with the 3in piping. I mean 3in is the best overall turbo exhaust size. Even on my old GTI bone stock-putting on a 3in Turbo back made a huge difference. and it was only rated to 180hp stock.