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Old 03-30-2001, 09:09 PM   #1
cmb
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Post Optimal exhaust piping diameter for the WRX?

Sorry if this has been asked before, forgive me...I'm a newbie.

What is the maximum exhaust piping diameter one should go with for the WRX. I'm looking for substantial gains.

I know some heavily modified all motor Honda/Acuras who've gone 2.5 to even 3" and seen gains.

From what I've learned, turbo cars are good with 3"??? Is this the recommended max size?

Also...I know one cat is located prior to the turbo...are the other 2 cats along the exhaust piping...so replace/delete them?

What will be the power gains from dumping the 2 latter cats and upping the exhaust piping to 3" or whatever max. you guys recommend?

(One week and counting!)
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Old 03-30-2001, 10:25 PM   #2
illmatic
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Yup 3" exhaust is pretty standard for turbo cars. IMHO I think 3" is too big for an NA car. Do a search. They have a topic where an Australian wagon got around 260 to the wheels with 3" piping and removing two cats.
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Old 03-31-2001, 09:22 AM   #3
Imprezer
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80mm.
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Old 03-31-2001, 09:32 AM   #4
cmb
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thanx for the replies

illmatic: 260 to the wheels? really? That must've been with a slew of other upgrades? 260 whp would mean a 60whp gain?

Anyone know approx. how much is gained with 3" piping and 2 cats bye bye.?
5 whp?
10 whp?
20 whp?

And exhaust is supposed to help lag?
I know in my GSR, when the exhaust was widened, there was less torque, but more high end...so actually on the N/A car, a little less throttle response (more laggy) with bigger piping.

So with turbo, the bigger exhaust I assume helps expel exhaust faster? what does that do? Help the turbo spool faster? really? Have people felt a turbo lag reduction with exhaust?
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Old 03-31-2001, 09:34 AM   #5
couldnthinkaone
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Exclamation

as big as they can fit under the car!!
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Old 03-31-2001, 05:25 PM   #6
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cmb,

On a turbo car, basically the bigger the better. In a N/A car, freer exhaust will hurt the bottom end at expense of the top end, but when you think about how a turbo car makes it's torque and power it makes sense.

The turbo compresses the intake air to cram more air (and fuel) to the engine. This is done by using speed of the exhaust gases to turn the turbo. So in the low RPM range, the exhaust can't move the turbo fast enough, thereby making it act more like a N/A engine, and giving the feeling of lag.

As you reduce the backpressure, the exhaust will spin the turbo up to speed earlier in the RPM curve, and therefore reduce the range that it's acting like a N/A engine, and basically allow the engine to react better after shifting. This is how exhause mods reduce turbo lag.
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Old 03-31-2001, 10:42 PM   #7
cmb
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Thanx Concillian, very informative answer!



So 3"? Or would it be better to try 3.5"?
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Old 03-31-2001, 10:50 PM   #8
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If you want a sweet 3" exhaust system for your new WRX, wait for the Brullen. They will be making it for us next month!

Here's some info for you :

http://www.i-club.com/ubb-files/Foru...ML/002067.html
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Old 03-31-2001, 11:21 PM   #9
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3" on a N/A Honda 4cyl?? Good Lord, that had better be DRAG only or they will never see more than 3mpg due to the utter lack of torque.

A well designed 2.5" pipe is far better than a 3" poorly designed one, bu tin the WRX's case I might go with 3" to start with if you can find a good one.

DO NOT RUN WITHOUT A CAT. Its only a several thousand dollar fine if caught without one, and less than $150 to get one if not included in the exaust.
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Old 03-31-2001, 11:30 PM   #10
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Okay folks, a catback opens up the pipe to whatever OD you choice from the cat back. Gues what, you still have lots of restrictions with the smaller OD of the factory exhaust manifold, downpipes, cats etc.

I only go turbo back, never mess with catback if I can help it. I would suggest you seek out a turbo back solution with high flow cats (if in CA with smog restrictions, heck do it anyways, better for the environment).

All this hoopla over a catback, it helps a bit, but the Downpipes will really give you that added midrange that your looking for.

Just a bit of info. my $.02 FYI etc.

Tory
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Old 03-31-2001, 11:45 PM   #11
BobRS
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what does turbo back mean?
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Old 04-01-2001, 12:21 AM   #12
drftsub
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Quote:
Posted by BobRS:
what does turbo back mean?
From the "exit" side of the turbo out , IE the flange that attaches to the outlet of the turbo all the way to the muff. hence "turbo back"

Nate
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Old 04-01-2001, 03:19 AM   #13
Adrian128
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That 260 whp figure is totally wrong. the actual figure is about 16-18 whp. Someone calculated that given that figure at the wheels, the estimated flywheel hp would have been around 260, up from 227 with the 3" turbo back exhaust.

Also, the consensus among WRX tuners here in Australia is a 3" diameter. I guess that's the best overall size. I'm sure for specific uses there would be different "optimum" sizes.

If you want some insight, try reading this. from the WRX club here in NSW.

Exhaust report

[This message has been edited by Adrian128 (edited April 01, 2001).]

[This message has been edited by Adrian128 (edited April 01, 2001).]
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Old 04-01-2001, 09:18 AM   #14
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Old 04-01-2001, 10:23 AM   #15
Adrian128
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I'll be sticking to the 3" exhausts when I get mine. I'm not getting a Stromung anyway.
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Old 04-01-2001, 10:39 AM   #16
titsataki
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tmblue I just got mine and I am looking into exchausts. I will be staying with smog cats etc since I am in California... Now Is any 3 inch from the turbo back CARB ceritfied?
If not what happpens if I install it? Smog problems with CHP if noticed? Void Subaru warranty?

cheers

T.

PS: 260 at the crank from 227 with only an exhaust upgrade? No chip not better air filter?? If no chip no air filter it is impressive.
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Old 04-01-2001, 11:03 AM   #17
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Well, it's a felony to remove your OEM cats before the car hits 50000 miles, so I can't imagine that it's legal.
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Old 04-01-2001, 11:25 PM   #18
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A felony... Well it's not a felony to have bottomed out and had a hole put thru your stock cat, therefore had to replace it.

But there are certain state laws about modifying the cat on the car. But a hiflow cat passes emissions and visual inspection. I would prefer for people to run hi-flow cats vs running straight pipe "testpipes". I would think the government and the environmental agencies would agree with me..

But yes there are laws, and modification you do to your car, you do at your own risk.

Someone post a time when you recieved a citation for replacing your stock cat with a high flow cat? I have for exhaust, but that is not quite the same.

Does the CHP and other state agencys have the model number and part number for each cat that the factory puts on a car? I highly doubt it, but heck anything is possible.

Tory
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Old 04-04-2001, 07:49 PM   #19
boostit
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CHP doesn't really police emissions equipment. Here is a link detailing "official" (as opposed to the dic# chp guy who pulls you over) line on aftermarket exhaust systems.
http://www.vfaq.com/BADSM/CHP-Exhaust.html
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