Does Size Matter? 2.5" vs. 3" Exhaust discussions. [merged]

[Boostin18]

2.5" vs. 3" exhaust discussions are merged here.

I have a 90 GSX and when i bought it the guy said it was full 3" turbo back with a EVO3 O2 housing (ported), now here is my question. When I took the exhaust down, it had a 2.5" exhaust flange at the top of the down pipe and then flares for about a half inch out to 3", if that makes any sense. This is my first 3" exhaust and I was wondering if this is right? I was thinking of porting the o2 housing because it is 2.5", making it 3". Then bringing the exhaust to a shop and having them weld a 3" flange to about a half inch long peice of 3" ehxaust pipe and then welding it on to the whole exhaust. So basically getting rid of the 2.5" flange and the half inch long flare and just welding on the 3" flange with the new piece. Would this make a diffrence if i do this or not. I don't know if this would make any diffrence or not, but when i put a boost controller on, i would try and get it to set at 15psi, but never would it would just spike and go down to like 12 or 13, but never above 15, but never at 15. I also have a ported 2G manifold (havent had it off to make sure it really is ported). Any help would be great!

 

[Bmxr152]

I know this is an old thread, but what if i use the stock with a round 4" body magnaflow 2.5 inch straight through muffler on the end of the downpipe. Its only going to see the strip and maybie the sreets sometimes. im only going to be running the 14b my new project is all stock at the moment

 

[antiheroX]

Put the catback on.

 

[project_tsi]

I run open 02 housing and I love it.

 

[xveganxcowboyx]

I run open 02 housing and I love it.

I did that for a month or so. Loved it and it wasn't as loud as you'd think. I am happy to not draw so much attention now, but if it wasn't a DD...

 

[dsm-onster]

I ran open o2 housing for kicks. I recorded good numbers vs. 3" turbo back exhaust and test pipe w/ a pretend muffler. The exhaust fumes made me sick and every one knew when I was coming on account of the exhaust smell that permeated my clothing.

 

[TermTrky]

So, I hear that exhaust that has no backpressure can sacrifice low-end torque. I would get an exhaust, but I'd match it up @ 2 1/2", depending on mods. Now, don't run open-head, cause you may be a victim of valve float. I don't know if these DSM's do it but my Chevy sure would have

 

[Bmxr152]

On an NT, but just think a turbo runs boost, the faster it gets out the faster it spools right?

 

[1gDSM4g63]

I've been running my downpipe open for over a week now. I don't really have problems with boost creep. It sounds both ricey and in some way like a v8 if you ask me(depending on the RPMs).

By ricey do you mean it sounds like a weed whacker like those Honduh guys?

And all cats will cause power loss. You should go test pipe. That will tone it down a little going to the muffler.

 

[xveganxcowboyx]

So, I hear that exhaust that has no backpressure can sacrifice low-end torque. I would get an exhaust, but I'd match it up @ 2 1/2", depending on mods. Now, don't run open-head, cause you may be a victim of valve float. I don't know if these DSM's do it but my Chevy sure would have

Bad boy. Read the FAQ and other collected info. This myth gets destroyed almost daily on these forums.

 

[TermTrky]

Sorry, don't understand these cars! I come from Pontiac 400 ville

So, turbo 4g63t can rock a 3" exhaust.
What about an NT?

 

[Bmxr152]

Well an N/T is much diffrent, it wouldnt go bigger than 2.5in catback on a nt

 

[xveganxcowboyx]

Sorry, don't understand these cars! I come from Pontiac 400 ville

So, turbo 4g63t can rock a 3" exhaust.
What about an NT?

Ugh, I guess I'll elaborate, though I was hoping you'd search some. It's all about exhaust velocity, pressure, etc. It doesn't matter what engine (excluding 2-stroke stuff). Back pressure is always a power killer. On a naturally aspirated engine you can achieve better velocity in the low end with smaller diameter piping due to a scavenging effect in exhaust pulses (where a vacuum is created behind the pulse which helps suck more exhaust out). You still do not want any back-pressure, but going too large on the piping will wreck the scavenging effect and hurt torque.

On any turbocharged engine this is a non-issue. The turbo breaks up any exhaust pulses so no gains will be seen with smaller piping. Here you want to go as big as is reasonable for the space and budget. Depending on the setup you may not see large gains between 2.5, 3, 4 or 8 inches, but it will not hurt performance. For these cars most people find 3" to be a good size, though for smaller goals 2.5" may be adequate. I couldn't see going more than 2 1/4 on a N/A but I also couldn't see using one for anything other than a commuter so....


*edit* In case it isn't clear, whether you're doing it on a turbo or not you want it sized appropriately, but as free flowing as possible. You'd have to check out the N/T area to see what has given them the best gains.

 

[dsm-onster]

Just to add. Look at backpressure as a negative side-effect when properly sizing pipe for scavenging. For naturally-asperated applications, this backpressure is a neccesary devil worth contending to get the proper scavenging at the proper rpm level.

But, as mentioned, scavenging (post turbo) cannot exist in a turbo application.

 

[Needsoles]

I am debating with myself if i should get a three inch exhaust or a two and a half inch for my car. i already have a ported o2 and a down pipe.

 

[hypnoticpimp]

It depends on your goals. A 2.5 exhaust becomes a restriction when u reach around 400 hp, so if you're shooting for higher than 400 hp a 3 inch will do.

 

[Needsoles]

i don't plan on running over 400hp so i can run with 2.5 inch exhaust. One more question how much better is it to spend the extra money on a high flow cat?

 

[hypnoticpimp]

Honestly if you're running an o2 dump why would you bother getting a high flow cat? Just get a downpipe that deletes the cat or a test pipe.

 

[crazy_skier]

because he wants to be legal for 90% of the time, oh and the high flow will not affect horsepower more than 1-2 but your spool up will be a little higher, my buddy with a hahn super 16g had a 300 rpm drop on spool up to 20 psi with the take out of a hahn cat. Also, I like how the 3" sounds and it will help get the boost higher because of creep on a factory turbo nevermind I see that you have a 16g, I would say 3", you could go 2.5 but the 3" is used alot with 16g's

 

[asd_gst]

i have a 91 gst, im about to buy a 3" turbo back exhaust for my car, with the stock 14b turbo and all stock car will this have a negative effect on my car, i know if you put a big exhaust on like a n/a car all stock it will make the hp go down ### no back pressure will build, but i plan on building my car to the 300-400 whp range, i just want to know if this would have a negative effect on my car until it is built

 

[colonel_odem]

If you do, get stainless steel.

 

[fwdmaster]

No negative effects getting a 3 inch exhaust, when I upgraded the exhaust on my gst I gained more top-end what kind of exhaust are you getting?

 

[kmoore]

If you do, get stainless steel.

Ehh, not really necessary. But if you want to fork out the extra cash go for it.

 

[erald01]

You don't need back pressure on turbo cars. The 3'' exhaust will help flow exhaust freely, but it will flow as much as the turbo flows.

 

[asd_gst]

its a custom made exhaust with a magnaflow muffler. full 3 inch turbo back with downpipe
its used im getting it for like $350

 

[tkelly27]

While I have heard of people noticing a slight drop in low/mid range torque (although I don't think it's even possible, if you search, it's out there), I've never noticed such a thing with any turbo car I've owned. Generally speaking you'll have more top end, better spool, and an exhaust note that makes the car more fun to drive without loosing anything.