2.5 vs. 3.0 in.

Okay im still confused on what is the difference from 2.5 in. exhaust and 3 in. exhaust. I have read that 3 in. exhaust will give you turbo lag until you upgrade your turbo. and 2.5 is perfect for stock turbo. So will someone please tell me what my best bet would be. I have a 99 gst with 1g bov, dejontool uicp, aftermarket intake. I plan to put a boost controller on and a downpipe sometime within this year or next year.i wont be replacing the turbo anytime soon, unless it breaks on me. Also, what is the stock exhaust size?

Smallest partof the stock exhaust is reportedly 1 7/8". I went to 2.5" only to upgrade again later. And even with the 2.5" on the 14b I got a lot of creep (larger exhaust gives you LESS lag, but more creep). On the stock turbo it doesnt really matter. It all really depends on your goals for the car. If you think a 3" is in your future you might as well get it now and prepare to deal with creep when you upgrade the turbo. Or if a 2.5" will support your goals (runs 11s often enough) just go for that and creep problems will be less, but will still need to be dealt with later.

Thanks Kevin! but to get rid of the boost creep (too much boost or volatile boost) i read that you have to port the 02 sensor or something like that. Is that the only mod i would have to do when i get my exhaust? And with a 2.5 ill have less lag and less creep? If somehow dealt with the boost creep, would it be better to get the 3'' even if im not planning on upgrading the turbo soon? Please help....in dire need of it!

on top of what kevin said, 2.5 will give you better low end torque than 3" especially on the stock turbo.

yes, porting the O2 sensor is the way to fix the creep. and don't worry about porting it untill you have the exaust done. if it doesn't creep bad than there really is no need to port it. i have dual 2.5" and get both spike and creep when i drive. altho, it creeps just about enough @17psi so i don't plan on porting it untill later on when i upgrade. saves me the money for a BC now :)

yes, 2.5" will give you LESS lagg than 3" and LESS creep than 3".

depends on what your definition of SOON is. if soon means couple years later, i would just get the 2.5" now, personally i like having a good low end torque. and will plan on upgrade to 3" "after" i get a 16G.

Easy now.

2.5in will induce lag, but you are correct about it creeping less.

As for poritng the O2 sensor, is this some newfangled mod?

Bigger exhaust does NOT give more lag, it gives less. Ideally no exhaust would be best on a turbo, the bigger and shorter the exhaust system the better.

I'm sure he means the O2 HOUSING. The stock O2 housing wastegate passage is very restrictive. Get an EVO O2 housing. Much better.

I'm sure he means the O2 HOUSING. :uhoh: Ya think?

Sometimes my sarcasm isn't appreciated.

What's up with the whole "losing low-end with 3 inch exhaust" deal?

A.) When I'm racing, I don't spend much (if any) time below 4k rpm, so low-end isn't really that big of a deal! :P

B.) I have a hard time believing 3" exhaust will kill low-end torque, that just doesn't make much sense on a turbo car.

Someone feel free to help clear this up, I'm not exactly an expert at this :)

Folks take the N/A exhaust theory and try to apply it to turbos. WRONGO!

Easy now.

2.5in will induce lag, but you are correct about it creeping less.

As for poritng the O2 sensor, is this some newfangled mod?


good looking out dude!. typo. meant 3" LESS lagg. obviously more air, faster spool up.

as for the low end torque thinging, it had already been discussed before. our encyclopedia Kevin wrote up a long article about that. let me see if i can find it.

http://www.automotiveforums.com/vbulletin/showthread.php?t=233536&highlight=low+end+torque

http://www.automotiveforums.com/vbulletin/showthread.php?t=202335&highlight=backpressure

the best exhaust for a turbo is no exhaust at all.

remember that. and get a GReddy Profec B spec-2. i had creep issues until i got that little baby. now i got rock solid boost control ;)

3" does decrease low end torque. I had a ricey dual exhaust that killed low end. lol. dont think ill do that to my GST but u never know, that ricer had to come out sometimes....and it big and grREeEEN and..ARRGHHHH HULKK SMAASSSHHHHH...

Shpyder- did it decrease it on your old car (a NT)

If so, that is what everone is saying. A 3" exhaust will lower low-end on a NT but it wont effect anything on a turbo.

sadly, I can't say I've ever had problems with creep. I have been running the profec B since day one. and it seems to be dead on (minus the small problem with not holding boost but I really think thats the wastegate, anyway) I wouldn't do things twice, if you're gonna upgrade, do it to the farthest possible extent so you don't have to do it again. saves time and money. I doubt that the negative effects of a 3 inch exhaust will outweigh the positive advantages it possesses.Good luck with your choice

what do you guys mean about no exhaust for a turbo is the best exhaust?

Boost controller has NO effect on boost creep. Some poeple donthave trouble with it for some reason, but its independant of what boost controller you use.

Porting the turbine housing does far more than the o2 housing. I KNOW I have posted a novel or two with details and numbers from my own experience somewhere.

Also, larger flapper will reduce boost creep, but will INCREASE its tendency to blow open at higher boost and limit how much boost you can run. Again, I know a posted all the info on this somewhere here as well. I think the most comprehensive thread on it is at Tuners though, search for WG blowing open info, I believe that pulls up that thread (thats the title of it too, if I'm not mistaken). I put that up when all the numbers were still fresh in my head. But basically boost would drop from set boost of 25 psi to 19-20 psi at high rpm, on a 16g. Most poeple assume with themods I have you are out flowing the turbo and it cant keep up. I decided to do an experiment, rather than believe everyone else (of course :D) and found that it was the WG blowing open. I wired it shut (I was using the external on the manifold at this point) and it held boost to within 1 psi of 25 all the way to redline.

So my advice is port the hell out of the turbine housing (see the vfaq, some good points there, and then port the O2 housing to match the turbine hosuing) and avoid larger flappers if you plan to run over 20 psi someday. ;)

okay i talked to this guy i know who used to be a mechanic...i think hes bulshiting though...but anyway, he said if i am going to modify the exhaust, i shouldnt just put a cat back...he said im gonna need to instal the Downpipe...he claims that with just a 3" catback and stock downpipe, my engine can fuck up....the thing is i dont have monety for both...is he bullshitting??

You will be fine with just a cat-back

It is wise to replace the whole exhaust in search of maximum power, but no, you wont hurt antyhing doing it in stages. Thats how most people do it in fact, for the same reason as you. ;)

Thanks guys! Always a help! I appreciate it!

ahh... another one trying to scam for more money.

Yea no kidding man! I was telling him i never heard of such a thing, and he was telling me all this bullshit and i was like "okay buddy, ill ask the DSM experts"

is he the sales?.. cause all sales person at auto shops lie all the time, no matter where you go. i always talk to the mechanic or the owner.

nah it wasnt even at a shop.....well kind of it was a mechanic and this performance shop.

what exactly happens when u stick a 3in cat back on a stock Turbo DSM??

sound, a little more power

My current exhaust plan:

3 in. muffler shop DP followed by a cutout, welded to the rest of the stock system. Anyone know if all this will fit under my car, or will I have to ditch the cat? Call me a card-carrying greenpeace member, but I'd like to keep it on for daily driving :D.

I got a quote for $125 for a full 3 in. from a local exhaust shop...but I really want an electric cutout :iceslolan:!

how the hell do you find those local exhaust shops?

btw nice ass pic Joe

Hey joe, is that really a pic of you doing an AWD burnout, or did you just do some nice photoshopping? ;)

it looks like photoshop

Hint: Make the smoke darker closer to the wheels :wink:

at first glance you might not be able to tell, but if you at it closely and yes its in a driveway, you could....but nice shot anyway!