I've noticed that alot of companies are producing exhaust systems for the WRX with 3" piping. www.spdusa.com and scoobysport both recommend 2.5" piping because it supposedly allows the turbo to build boost quicker than the 3" and the 3" doesn't provide any advantage at higher rpms either (assuming you're using the stock turbo, which I will be doing). The exhaust shootout results in Turbo magazine didn't seem to agree with this opinion as some of the 3" designs outperformed the sole 2.5" at lower rpms. Does anyone have any experience with this? Should I stick with 2.5" or go with a 3" design?

me general rule is "go with the largest you can stand to hear"

I installed a 3" catback on my friends car last week and it feels like it gained power. It sounds really nice. It came with a silencer too just in case you get tired of the rumble.

2.5 is good for what you really need. 3 inch is nice but unless you have 300+ hp then it is not necessary.

when we had a dyno day at pruven performance arranged by "AllfriedEggs" ;)
approx 30 scoobies the dyno results showed that the
2.5 exhausts retained more torque but lesser hp and the 3" exhausts lost torque but got more hp ...

Andy...

Originally posted by Weasel 555
when we had a dyno day at pruven performance arranged by "AllfriedEggs" ;)
approx 30 scoobies the dyno results showed that the
2.5 exhausts retained more torque but lesser hp and the 3" exhausts lost torque but got more hp ...

Andy...

With the WRX, I'd want the torque.

Like Mr Lobster said, if you are doing lots of mods and want bhp numbers go for 3" plus but for low down torque and drivability the 2.5" will show like for like gains certainly until higher up the rev band.

I was lucky enough to run this turbo back system on my car last year it was great...low end top end really made a differnce the butt dyno liked :)

areg

torque is what you need ;)

http://www.turbomagazine.com/tech/0212tur_exhaust/
Here ya go. Look at the specs of the Blitz exhause and compare them to the rest and see which on maks more torque. ;)

I thought that the bigger the better for turbo's? Anyway, isn't maintaining a good, steady flow of exhaust gases at a constant velocity supposed to be the key to good power? Just wondering.

Originally posted by Weasel 555
2.5 exhausts retained more torque but lesser hp and the 3" exhausts lost torque but got more hp ...

Andy...

But where? If the 3" exhaust only yields higher HP numbers at like 6000 (for instance) with torque loss until then (vs. 2.5") then I would think that, just like the AWD advantage of moving forward first, by the time the 3" exhaust started to gain the 2.5" would be too far ahead to catch in most races. I wanna know which design has more area under the curve - that's more useable than peak power at least for street driving.

at the dyno day we were all looking and comparing the dyno results etc with our mods and noticed this :)

if i knew the exact reasoning behind this i would let you know !!

but i don't i'm just going back what we saw etc

my dyno sheet is posted in this link
http://www.i-club.com/forums/showthread.php?s=&threadid=1080&pagenumber=2

maybe its cause we had modded wrx's with similar mods like vf23's hks boost controller set the same but just some of us had 2.5" exhaust and some had 3" systems...

Andy

IMO- You'd want to run the largest exhaust piping within reason. for example 3" tubing is popular. What you DONT want to do is run with no type of downpiping as this could lead to turbine over run and be potentionally fatal for the turbo. Turbos love a none restrictive exhaust system but an exhaust system nothelest. Smaller exhaust piping will lead to back pressure and that could lead to the dreaded boost lag.

What is tend to see here on the forums is that a lot of people here think of upgrading the turbo right away for a larger unit when I feel one should be thinking about freeing up all restriction in the exhaust system from the exhaust ports to the muffler tip. What about an length header? I know it's expensive but it shouldn't be over looked. Imagine what your car would run like with absolutely no restrictions on your car exhaust and intake, the engine would be zinging to the redline. Gadzooks!

My 2-scu

Okay, it looks like 2.5" is the winner for my setup. I'd rather have torque at the low end than hp at the top. Thanks.

Smaller exhaust
piping will lead to back pressure and that could lead to the dreaded
boost lag.

That's what I would think too. But, www.spdusa.com claims that a certain amount of backpressure is required for the turbo to spool quickly and 3" does not produce the "ideal" backpressure.

If you want to retain some low end torque, go w/ 3" downpipe and w/ a 2.5" catback to gain some mid/top hp.

Originally posted by 1fastGC
If you want to retain some low end torque, go w/ 3" downpipe and w/ a 2.5" catback to gain some mid/top hp.

Yup, good idea.

specialiest say that all you need is to got 10 percent larger then the turbo exit hole. technically you could you 2 3/8 - 2.5 but thats has no cool factor

Originally posted by bone324
Okay, it looks like 2.5" is the winner for my setup. I'd rather have torque at the low end than hp at the top. Thanks.



That's what I would think too. But, www.spdusa.com claims that a certain amount of backpressure is required for the turbo to spool quickly and 3" does not produce the "ideal" backpressure.

I read that too. you would think that NO restriction woould allow the turbo to spool faster. Hmm...

-scu

hey all

on my old rex wagon, i originally used a apexi old skool turbo back. the dp was non-bell mouth. the entire system went from ~73mm to 80mm as it exited into the muffler(if you could call it a muffler). i found a huge increase in torque and a appreciable increase in hp. alot less lag, and faster spool.

i am repeating others, but if you want lots of top end and huge hp #'s go big (like even 4"_. i prefer the torque over hp, especially down low, and therefore would stay at 2.5-2.75 or taper it in either direction from 3"-2.5 or like the apex i had.

but someone really hit paydirt when they said to free up restrictions. that alone on my car made all the difference, in both drivability and performance. vishnu uppipe and catless exhaust (a;though shiv would argue a cat makes just as much or more power) makes the turbo work alot less and allows air to move through faster. and in the end, that is the bottom line. squish and squeeze more air through the motor.

josh