2.5" vs 3" exhaust
 

What kind of power loss would I expect going from a 3" xpipe to a 2.5" xpipe?

I had an awesome custome exhaust made for me. My buddy also got one made by the same guy but in 2.5". I prefer the sound I his better.

I went 3" because I have alpha3076 and all the goodies. I should be around 600whp.

Woul I be choking it too much going to 2.5"?

His car is also a k16 car so should be quieter vs an A3076, i just added cats to my 3" x-pipe system, much improved idle sound but still loud. I dont think you'd lose much going to 2.5".

On 3076s you need a 3" exhaust...

on 3076's i wouldnt do 2.5 in

I have a different opinion on this - I don't care for the loudness of a 3" system on a daily basis, so I run a 2.5 by personal choice. My car dynos slightly higher in a very small RPM band with a 3" vs. 2.5" (same day/same dyno). So in answer to the OP's question, at 565 AWHP you will give up about 25 HP for literally a fraction of a second in a tiny spot in the RPM band. Early in the RPM band and later in the RPM band my car dynoed identically with both 2.5 and 3.0 and full supporting mods. The power difference between the 2 systems at 565 AWHP (K24/18G) was pretty insignificant.

Putting dynos aside, the other factor to consider and IMO more important is driving enjoyment. Personally, I have gone back and forth between 2.5 and 3.0 several times on my car. I can not feel the difference in power via seat of the pants, however, I prefer the sound of the 2.5, as it is more comfortable in the cabin and it makes for a more enjoyable driving experience for me. So, if I were doing 3076's, I would keep a 2.5" system on my car because I like it/prefer it for sound and I would not want to have a loud exhaust just because I put on a certain turbo. I would give up a little HP for sound comfort. IMO, the car will still have PLENTY of power and be much more enjoyable to drive (for my taste). What good is maximizinmg your power on a dyno by a few percent, if you don't enjoy the car as much any more? That's what it would be for me - everyone is different. Fortunately, I have the luxury of testing all the systems and choosing what I like best...which is what I have done. Everyone is different.

John, what about the greater restriction leading to increased temps?

I have never had occasion to measure one vs. the other, but I know that the EVOMS GT800 kit that uses the 3076 turbos has been run with the OEM exhaust, 2.5" exhausts, 2.75" exhausts and 3" exhausts. So I don't believe it's a deal breaking issue.

Hi John,Please correct me if I am wrong....But doesnt your 3'' and 2.5" use the same size piping in the X configeration[3"]and the rest of the system except of the muffler tips and the inner muffler core are the difference?...So can it really be called a 2.5 "system??
I know most will say that it goes by the final opening size on the tips,BUT IMO I tend to not fully agree with that..There are those other considerations of gas flow in the other areas of an exhaust system...Thanks..I am just sharing and taking info in..

Hi John,

No, the internal "X" is 2.5" on the 2.5 system and 3" on the 3.0 system. The tips on the 2.5 quiet are 2.5" and the tips on the 3.0 loud are 3". The only thing they share is the pipe out of the cats to the muffler is 3" on both systems.

Thanks for clarifying.
When you did your comparison on your car,didn't you have a 3" system and then put the 2.5" tips on?

I urge anyone in SE MI/Detroit area to listen to my SpeedTech 3" cat-less system. It is not obnoxiously loud as people may think IMO. Cold start-up is wicked but it is only a bit louder than stock at warm idle. WOT is wild, cruising around (2000-3500 rpm) drone is MINIMAL.

Hi,Yours has the standard muffler system?

Yes - 3.0 Loud X-cellerator Cross-Flow “Race” Exhaust System w/cat Bypass Pipes.

John,

I did that, too - we dynoed everything - both models of mufflers and then each muffler with the different model tips. We found that the exhaust dynoed almost identically based on the lowest common denominator of pipe diameter, even if just the tips. For instance, I have seen some 70mm systems that use the OEM tips - based on my experience, there is no sense in using the 70mm pipe, if it's not carried all the way to the end...there will be little to no benefit to the larger pipe before the tips.

Thanks, I appreciate your throwing this out for the 6speed community !