2.5" or 3" exhaust

So the Impreza that has become my life's work is almost ready for an exhaust.
I was all ready to go the route that I had been recommended (2.5" but made that fatal mistake of reading the rally construction forums and saw that many others go the 3" route.

Since I am lacking a little in the mechanical knowledge side I thought I'd ask here at the fountain of wisdom!

What size would be optimal for me? - the car is a 95 Impreza with an 02 WRX drivetrain, therefore 2.0 turbo. It is going to stay pretty stock engine-wise till I can learn how to drive it (hey, I coming from a 75hp RWD Corolla!).
It is open class so will have the 34mm restrictor, an Autronic SM4 ECU with antilag capability, cat-less up and downpipes, probably stock boost (since the engine is stock), stock injectors etc...

I was told 2.5" would be sufficient by a chap who does a lot of NASCAR exhaust systems, and I was planning on getting one of the cats that Road/Race Engineering sell.
Any reason I should go bigger? Apart from looking cool with a big ricer pipe!

Cheers,

---

Nick Taylor.
Knobhead.
Los Angeles, CA.



Edited 1 time(s). Last edit at 05/03/2006 05:34PM by nick the brit.

2.5". You'll need the torque. The 2.0l motor doesn't push enough air to necessitate the 3", despite what all the 'experts' (who sell them) say. If it was an STi, I would say go for it.

Eli

Perfect.
Thanks Eli!

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Nick Taylor.
Knobhead.
Los Angeles, CA.

I ran my 2.3 liter 8 valve volvo on both...the 2.5 it came with, the 3" that i built.

i say run three, for sure.

All the backpressure you need is the turbo itself. You are trying to create the largest possible pressure drop across the exhaust side of the turbocharger, to maximise the amount of energy you can extract. The less restriction you have after the turbo, the faster you will spool and the more efficiently your turbo can work. Almost all turbo cars are dogshit off boost anyway, so you want the fastest possible spool up.

Going from the previous owners 2.5 to 3 inch in my case allowed the car to spool 400 rpm sooner with no other changes.

Why do you think john v is always pushing the cosworth exhaust housings with the 3" outlet on people?



Edited 1 time(s). Last edit at 05/03/2006 08:49PM by Parry.

benefits to 3 inch: quicker spool and lower turbine housing temps= longer turbo life.

Turbos do not "need" a smaller id to keep gas velocities up DOWNSTREAM of the turbo charger. Indeed they would be happiest if they could blow straight out to atmosphere.

Think of a waterwheel, same principle on a loager scale and slower velocity but valid regardless of what keyboard engine builders might parrot.

UPSTREAM of the waterwheel you'll see that often water is run thru a narrow path approaching the waterwheel---thats to up its velocity.
DOWNSTREAM any gases lingering on the "wrong" side of the turbine wheel will act as restisdtance to it spooling up.

The little Japonaise tub\rbos are interded to spin at approx 180,000 rpm for the stock boost therefore ANYTHING which helps it spool is a good thing.

Ford Motorsport reccomended for the 2000cc YB motors 3" all the way thru, and I would trust them over the collectiove "wisdom" of the Inter-sNet.

Consider it a foundation for future things.
Do it right the first time.

Even with your plans to run stock boost.

2 1/2" is barely adequate for normal aspirated rally 2,0 motors.

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

ditto what John said. I run 3" on my street car.

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Scott Manley
Spokane, WA
86' XR4Ti
37

Yep, the turbo itself provides MORE than enough backpressure. I would at least start it out as 3".

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Mike Hymbaugh
"Converted" drag racer
Castle Rock, CO
http://www.four-banger.net/first.html

Thanks guys!

"DOWNSTREAM any gases lingering on the "wrong" side of the turbine wheel will act as restisdtance to it spooling up."

I think this sentence made it most obvious why a larger diameter pipe is good!

Learning all ze time.

---

Nick Taylor.
Knobhead.
Los Angeles, CA.



Edited 1 time(s). Last edit at 05/04/2006 11:09AM by nick the brit.

...shows you how much I know!

Also keep in mind that 3" Subaru exhausts hang pretty low and are susceptible to all those nasty rocks. Perhaps a 4" DP that tapers to a 2.5"? There's some backpressure for yeh!

Eli