3" 0r 2.5" for 91 SS

macipusy · Post by **macipusy** » Sat Aug 25, 2007 12:58 am

I am planning on building up my 91 turbo and wanted to know some input on weather to go with a 3" or a 2.5 exhaust.

What are the limitations of the 2.5"?
Does the 3" kill all the low end torque.

This is for a daily driver but just barely

know1 · Post by **know1** » Sat Aug 25, 2007 3:01 am

I always thought loosing torque with a big exhaust only happened to N/A cars. I know that with a bigger exhaust you will flow more air through your turbo. I hit fuel cut after boost creeping at the top of fourth (4eat) with my setup. De-snorkled, coffee mug, 3" catless dp to electric cutout.

wildacex187 · Post by **wildacex187** » Sat Aug 25, 2007 4:09 am

wat is the stock pipping size? i put it up on the lift today and it looked pretty wide to me. seems like the restriction is really the downpipe

92ss satinsvoice · Post by **92ss satinsvoice** » Sun Aug 26, 2007 8:48 am

the stock exhaust is, 2.25 inch. witch is a good sizs for stock.

greg donovan · Post by **greg donovan** » Sun Aug 26, 2007 3:34 pm

nevermind.

boro · Post by **boro** » Sun Aug 26, 2007 4:54 pm

I just picked up an HKS 3" down pipe. (not a bell mouth

) If I dont mate it to the stock exhaust I'll go 2.5" out. I hear that is a good combo.

macipusy · Post by **macipusy** » Wed Aug 29, 2007 4:01 am

So a 3" downpipe with a 2.5" turbo back is good up to what HP range?

I just want to get an idea of where the limitations will be later on.

dscoobydoo · Post by **dscoobydoo** » Wed Aug 29, 2007 6:52 pm

JUST GO FULL 3 INCHES!!!!!!

If you are worried about the limitations later on, then why bother thinking about the 2 1/2?

I went full 3- from downpipe to the inlet at the muffler. ( Dual 2.5 outlets at the muffler)

No regrets.
No restrictions.

BXSS · Post by **BXSS** » Tue Sep 04, 2007 3:37 pm

+1 - Go 3"anything else is a waste of time (unless it is 3.5-4")!

livestrong14 · Post by **livestrong14** » Wed Sep 19, 2007 4:41 am

I am running 3" from the turbo back, I will never do anything less on a turbo car than 3".

macipusy · Post by **macipusy** » Thu Sep 20, 2007 6:00 am

Sweet!!!! Thanks for the conclusive input

How is the sound on a 3" CBE....how does it change when upgraded to TBE?

I know that it varies with the brand and type...but generally

I am looking for a fairly quiet system, but not that quiet.

I was thinkink maybe a COBB?

AWD_addict · Post by **AWD_addict** » Thu Sep 20, 2007 6:36 am

know1 wrote:I always thought loosing torque with a big exhaust only happened to N/A cars.

True, turbos hate backpressure!

beatersubi · Post by **beatersubi** » Fri Sep 21, 2007 2:16 am

Does anyone have any idea as to how much more backpressure a 2.5" system produces over a 3" system? Or how much more power a 3" makes over a 2.5", all else being equall?

legacy92ej22t · Post by **legacy92ej22t** » Fri Sep 21, 2007 1:51 pm

An exhaust is only as big as it's smallest point so, going 3" dp to 2.5" CBE is a waste.

That being said it really depends on what your goals are. If you are going to lightly mod the car (stay with stock turbo, maybe a TMIC and a small increase in boost) then I'd go with the 2.5". If you are going to mod the car from medium to heavy then I'd go 3".

When I put on my 1st CES TBE system my SS was otherwise stock and and the low end suffered slightly but the mid and high rpm gains were noticeable. Once I started modding (Monster 16G, WRX TMIC ect...) that's when the 3" started to really pay off.

CES actually made a DP, the DP01, that was designed for smaller turbos (like the TD04) and it had a taper design. The main dump pipe went from 2.5" to 3" over about a 3' distance. They said it was to designed this way to retain the low end torque characteristics of the smaller turbo. Their DP03 DP (this is the DP that all the people here on the site with CES systems have on their cars) has a 3" main dump all the way and was designed for larger turbos.

macipusy · Post by **macipusy** » Fri Sep 21, 2007 8:28 pm

How far in a 400 hp build would a 3" catless DP with 2.5 CBE be the limiting factor.....I am just starting my build and have a good offer to get the TBE exhaust I listed above...I was thinking of running it for up to 2 years when I can go full 3" and not suffer the low end.

Opinions on the new plan...

TheSubaruJunkie · Post by **TheSubaruJunkie** » Sat Sep 22, 2007 2:21 am

If your car is a turbo, then going with a larger downpipe will not effect your low end power. The turbo creates all the backpressure, so your engine will not notice a difference between 2.5" and 3".

The turbo on the other hand will notice a difference. And thats where you will feel it. The turbo will preffer a 3" downpipe and exhaust. The motor on the other hand will never know, as all it see's is the backpressure created by the turbo.

As stated before. If you want to make real power, go with a 3".

-Brian

createnew · Post by **createnew** » Sat Sep 22, 2007 11:16 pm

6" Diesel Exhaust

legacy92ej22t · Post by **legacy92ej22t** » Sun Sep 23, 2007 3:38 am

TheSubaruJunkie wrote:If your car is a turbo, then going with a larger downpipe will not effect your low end power. The turbo creates all the backpressure, so your engine will not notice a difference between 2.5" and 3".

The turbo on the other hand will notice a difference. And thats where you will feel it. The turbo will preffer a 3" downpipe and exhaust. The motor on the other hand will never know, as all it see's is the backpressure created by the turbo.

As stated before. If you want to make real power, go with a 3".

-Brian

When you have a bigger DP, like a 3", you have a bigger pressure drop across the turbo. Because of this larger difference in pressure the turbo will spool quicker and air flows faster through the turbo. This causes a difference in back pressure pre-turbo and the low end can be effected. This is what people are talking about. It's not a myth.

Once on boost and running through the rpm's, this increased pressure drop becomes a benefit and you see an increase in performance.

When running a larger turbo like the 16G, 18G,20G,VF22,23,29,30,34,39 ect.. a larger then OEM TBE becomes a must because the OEM setup actually become a restriction. This is where a 2.5" setup can hurt you.

If you're not going to upgrade your turbo then a 2.5" setup is actually very attractive because you increase flow when on boost without effecting the off boost, low-end as much. Plus, you don't need the added .5" as much because the VF11 isn't capable of pushing much air even at increased boost levels.

beatersubi · Post by **beatersubi** » Sun Sep 23, 2007 10:06 pm

Very informative. Thanks.
So, my stock exh will not be a restriction for a td04? Anyone know what size stock WRX exh is?

legacy92ej22t · Post by **legacy92ej22t** » Mon Sep 24, 2007 12:14 am

Well it depends really. If you're OEM exhaust is old and has a lot of miles on it then yes, it can be very restrictive because one, it's only 2.25" and two, it has two cats that are restrictive and they could be clogged up too.

An OEM WRX exhaust is supposed to be 2.25" IIRC but I've seen in reviews that it's like 2.047 ID. That's probably what the OEM SS exhaust is too. If you get an OEM WRX exhaust then gut the DP cat at least.

A 2.5" setup would be a good fit for the TD04.

beatersubi · Post by **beatersubi** » Mon Sep 24, 2007 2:00 am

I'm currently running a td04 w/ the stock exh, and am planning on upgrading soon.
I was wondering about the benefits of 3" over 2.5" versus the popularity. I seemed that everyone was running 3" just because everyone else is.
I am planning on a turbo upgrade in the future. And if 2.5" is going to be a restriction, then I'll just go 3" and save myself the hassle and cost of re-doing it later.

macipusy · Post by **macipusy** » Tue Sep 25, 2007 5:46 am

Finally decided on the exhaust....basically got a good deal and turned out to be the right decision. It is a TBE with a 3" bellmouth Invidia catless DP mated to a 2.5 Borla CBE with a 4" singletip.

I made the decision to go split 3"/2.5" because I will not be upgrading my turbo until I instal a FMAI.....and my new TMIC has not even been installed.....so this is not happening for 2years.

This is a very good setup....great power at lower to mid range and a little at top....

Very happy with the gains and the sound....

legacy92ej22t · Post by **legacy92ej22t** » Tue Sep 25, 2007 9:04 pm

Sweet.

Arctic Assassian · Post by **Arctic Assassian** » Wed Sep 26, 2007 3:33 am

I'm glad yuor decided to go with the mix. A 3" dp is a great addition to any subie. It opens up the sounds of the turbo, and makes very intimidating noises. I know when I first ran my td04 with a topmount and 11 psi, I decatted my stock DP and got a three inch CBE. My car was louder than most sti's, an every wrx built here in alaska. I also know that my car was faster than any wrx runing the stock turbo; Because I ran a 12.8 on some torco. which is a little faster than I thought.

In my opinion, You should be able to hook up that front mount, before two years. That seems like a long time for a car that is driven every day, and is already sixteen years old. I think that with the 3" DP and 2.5 inch turboback (which gets switched like new shoes here in AK) you'll have great response with deeper boost coming on in the late 2000's. peace.
-Adam

dropdfocus · Post by **dropdfocus** » Wed Sep 26, 2007 3:52 am

macipusy wrote:Finally decided on the exhaust....basically got a good deal and turned out to be the right decision. It is a TBE with a 3" bellmouth Invidia catless DP mated to a 2.5 Borla CBE with a 4" singletip.

What ended up being your total price tally with everything purchased for the new exhaust?

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3" 0r 2.5" for 91 SS

3" 0r 2.5" for 91 SS