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!

 

[90tsisicko]

I am putting a 2.5in exhaust on my car...does it matter if it is crush or mandrel bent...i am running the 14b at 12lbs and think that this exhaust set up is sufficient enough...just wanted to see what other people thought

Adam

 

[K_Hill]

It will be more effiecent if it was mandrel bent. Why not go with 3" exhaust??

 

[falcone4g63]

On that turbo/psi it will more than adequate.

 

[K_Hill]

I figured 3" would be better if later he wanted to do more upgrades to his car. But that is just my 2 cents.

 

[90tsisicko]

i got the exhaust for $200. i mught get 3in down the road but i dont know. will i notice any gains with that set up or no??

Thanks for the info guys!

Adam

 

[wallyman]

I'm looking to get an aftermarket turboback exhaust system. I was just wondering if i should go with a 2.5 or 3 inch exhaust? My hp goal is around 250. I would like to keep my car fairly quiet. Can some of you guys please share some of your experiences?

 

[~laser_craver~]

if your only looking to make 250 hp 2.5'' is just fine, most people go with 3 because they plan on up grading to a 50 trim of some sort later on downt the line and having 3'' can hurt anything so....

 

[97EclipseGSTer]

yup....2.5 inches should be fine if your only going to be at 250 hp. But I will GUARENTEE that once your at 250 for awhile, you will want to go higher...Its not that expensive to go three inches....If i were you, I would go 3 for the fact you can get a nice system for just a few bucks more.

 

[Spaniel1287]

Yea I'm just about ready to put a full turbo back exhuast on my car as well. Its a 95 FWD Automatic with no turbo upgrades yet. Would a full 3" piping including the downpipe and turboback give me boost creep ya think?

 

[95blackGsTurbo]

Yes you will experience some boost creep. Depending on the mods you have it might or might not be that big of an issue.

 

[wallyman]

Can you please explain how boost creep is caused?

 

[punishercb]

Dude, before somebody grumpy yells at you(not that it ever happens on this site......), I highly recommend that you search. It's been covered alot and there are plenty of posts on it.

good luck

 

[eclipsegs-t92]

remember that you cant tell somone to search

 

[Pezking]

2 posts and no link... WTF

Boost Creep explained

That is btw something like the 10th or 15th thread in the faqs...

 

[Spaniel1287]

I'm about to order a full exhuast for my 95 FWD TSI. I am for sure getting a 3" Catback, my question is if i should get a 2.5" or 3" Downpipe. I do not want to have boost creep. I am gonna have the 14b on my 2g here soon after the exhaust is on which would create more power than the crappy T25. But the question is, should i get 2.5 or 3 inch downpipe. Thanks.

 

[Pure92TSi]

3 inch...you won't really see boost creep on the 14B, at least i don't on mine with the 3 inch dp.

 

[95biotch]

yea, definetely go wth the 3" down pipe. Besides boost creep isnt caused by the downpipe, is caused when your wastegate cant bleed out enough pressure and you get more boost than you want.

 

[dsm726]

hey guys i was just wondering what the real difference between a 2.5 inch and a 3 inch exhaust would be. I will not be running more than 20 lbs of boost in the future, so i didnt know if it would be better to get a 3 inch cat back with a 3 inch o2 eliminator dp, or if i should just get a 2.5 inch o2 eliminator dp with the 2.5 inch cat back i already have? i really want to save money, but if its going to make a big difference, i will get the three inch.

 

[Black95TSIawd]

Telling us you will be running 20lbs of boost means nothing. What kind of turbo will you be running? What kind of power are you expecting to make? From the looks of things, a 2.5in turboback will probably be enough for you. Just remember we have turbo cars and no matter what, a bigger exhaust or no exhaust will make more power than a smaller one. Whether it be a 5hp different with a stock turbo/16g or 30hp difference at 400whp levels.

 

[blcknspo0ln]

you'll never notice the difference.. in anything (like mentioned above, unless you're making over 450whp)

 

[tstkl]

yea, wouldnt the 2.5 make more power on a stock or near stock car over the 3 inch, since the exhaust volume will be lower and the stock compression is so low that the egts would drastically drop by the time it got to the end of the system? whatever, either way just tell us what time your aiming for, or how much power and well tell you what you need...

 

[The Wave]

If you were worried about that then you could always wrap it. I would say go 3" if you are going with a turbo that will flow any decent amount of air, and sell your 2.5" to help with the cost. A 3" exhaust has a cross sectional area of close to 7" squared, where as a 2.5" has a cross sectional area of only 5" squared.

 

[punishercb]

Honestly, just go with a 3". I have a 3" and my car's almost stock. It isn't a big deal at all. Get a good muffler and it's not even that loud either. If I could go back I'd do the exact same thing but just add a resonator to make it a little quieter. But like everyone else said, you're going to have to figure out what turbo you'll be running 20 lbs with. But a 3" will def. be required if ## running 20 lbs either way. Since you already have a 2.5 inch, I wouldn't blame you if you didn't. But 3" is great.

 

[Boosted98RS]

I would definately go with a 3" my friend has a 98 GS-T with still the t-25, 3" Apexi n1 turbo-back exhaust, hard intercooler piping, hks sqqv, dejon intake, and an 3" Electric QTP cutout right after the downpipe. I raced him 2 times with my old setup from a 60 roll and the first time with the cutout closed he could barely pull me just inched on me and got out maybe half a car at most. Then he wanted to see if the cutout actually helped his car so he opened it and we started at 60 again. He started nudging me alot harder this time and actually put 1 or 1 1/2 cars or so on me. I was suprised that the cutout actually helped with the t-25.

 

[blcknspo0ln]

i'm surprised your friend didn't boostcreep.

cutout's are retarded IMO, most people aren't not making enough power to outflow a 3" nevermind a cutout at the cat.