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  #1  
Old 05-29-2004, 07:38 PM
TorG0d
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exhaust diameter

Just for the record for all those who ask or have asked (and because I'm piecing together the parts for my exhaust right now): Is the exhaust diameter 2.25" or 2.5"? Is the engine really that dependant upon backpressure to make 3" too big?
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  #2  
Old 05-30-2004, 03:32 AM
SpeedyWRX
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Re: exhaust diameter

Quote:
Originally posted by TorG0d
Just for the record for all those who ask or have asked (and because I'm piecing together the parts for my exhaust right now): Is the exhaust diameter 2.25" or 2.5"? Is the engine really that dependant upon backpressure to make 3" too big?
Not sure what the stock diameter is if that's what you are asking, but to upgrade to 3" piping you have to be pushing some serious HP, but it's doable. An important thing to keep in mind is to make sure you have a high flow cat somewhere... my friend's SVX was catless and he felt a loss in power from 3k to 4k rpm due to no backpressure.
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  #3  
Old 05-30-2004, 02:23 PM
deruvian
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The stock piping size is 60mm (about 2.36 inches). I would be hesitant to change the size of the piping to anything larger than 2.5 inches. Something tells me that Subaru used such a strange size for a reason...
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  #4  
Old 05-30-2004, 03:03 PM
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Quote:
Originally posted by deruvian
Something tells me that Subaru used such a strange size for a reason...
because they use the metric system



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  #5  
Old 05-30-2004, 05:28 PM
deruvian
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Quote:
Originally posted by wawazat??


because they use the metric system



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lol. Jerk.

You know what I meant!
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  #6  
Old 05-30-2004, 06:14 PM
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Exhaust diameter

I am now on my sixth exhaust system experiment, the first was stock with no resonater, the second was true duals, 2", (from the joint after the cat outlet pipes) with just very free flowing Hooker Aero Chamber mufflers at the very back, way too loud below 3000rpm, but sweet like a porsche above 3000. Then, the same system with two 31" long Cherry Bombs, in the middle, very quiet, good flow, lots of power from 4500-6500rpm, next that system without the aft Hookers, again way too loud, the same results as system two. Then I hand-made headers with long primary tubes for torque, they again helped at higher revs, but cost power below 3000. Now my system has been altered/morphed into a six into one header with a 3" collecter, a 35" long Cherry Bomb, more tubing then an After Burner muffler with a crome straight pipe exiting thru the lower rear bumper. The newest system is too loud for my tastes below 2500 rpm, but at 3000 it's quiet enough to talk to my kids at normal levels.
When I drive Green-Eyed-Lady's compleatly stock SVX, hers is quicker off the line, and wonderfully quiet.
If I were not planning something more under the hood, this would not only be WAY too big, but it costs power at the lower rpms. Why spend money to go slower?

Last edited by svxfiles; 05-30-2004 at 06:17 PM.
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  #7  
Old 05-30-2004, 08:09 PM
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SVXtypeR dyno'd his car a while back with a K&N drop in filter and 3" cat back exhaust system (+ Level 10 Stage 3 tranny): SVXtypeR's Dyno Run



The 3" system seems to rob the engine of low-end torque which is essential in off the line acceleration (as the 4EAT has a 2500rpm stall). I've heard that 2.5" systems are much better for our cars - of course, if you're planning a big ball-bearing turbo install, then the 3" system will be better suited for that application (Evo's and STI's get gains from aftermarket 3" systems).

-Chike
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  #8  
Old 06-01-2004, 09:39 AM
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Quote:
Originally posted by TorG0d
Is the exhaust diameter 2.25" or 2.5"? Is the engine really that dependant upon backpressure to make 3" too big?
No engine is dependant on backpressure. This is a very common myth. Engines depend on exhaust gas velocity for the scavenging effect, which helps suck exhaust out of hte cylinders. backpressure is a byproduct of an exhaust with the proper diameter. It isn't in itself what the engine needs. in fact, backpressure is bad. but an exhaust that is too open is worse, since it reduces exhaust velocity. a little backpressure is a neccecary evil.
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  #9  
Old 06-01-2004, 10:03 AM
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Good explanation!

-Chike
Quote:
Originally posted by IggDawg
No engine is dependant on backpressure. This is a very common myth. Engines depend on exhaust gas velocity for the scavenging effect, which helps suck exhaust out of hte cylinders. backpressure is a byproduct of an exhaust with the proper diameter. It isn't in itself what the engine needs. in fact, backpressure is bad. but an exhaust that is too open is worse, since it reduces exhaust velocity. a little backpressure is a neccecary evil.
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  #10  
Old 06-01-2004, 10:05 AM
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exhaust

Yea, what he said.
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  #11  
Old 06-01-2004, 10:39 AM
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Quote:
Originally posted by Chiketkd
Good explanation!

-Chike
Thanks . The topic of backpressure got dealt with *a lot* in EJ25-land back when I had my Impreza. it got proven over and over again by a seemingly infinite number of people (who all started out getting exhausts too big for hteir engines) that exhaust diameter is a very important thing, and shouldn't be too big or too small. stock is a little narrow on the EJ25s, and people would start with 2.5" thinking that "a little bigger than what's reccommended will be fine". They get tempted by the giant exhausts on WRXs. They take their rumbly new car out only to find their low end missing (keep in mind that "high end" is a foreign term with EJ25s). so they spend more money going back down to 2.25" piping. Yes, 1/4" makes that much difference. It's called tuning becuase you're doing just that... fine tuning the system. small adjustments. you don't tune a piano with a baseball bat.

it's too bad our pool of people is so small, but the consensus seems to be that 2.5" is the preferred diameter. That's what I'll be going for when I get mine.
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  #12  
Old 06-01-2004, 11:06 AM
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Yup, from what I've heard, 2.5" is the ideal size for NA EG33 engines... If you add a turbo or S/C the exhaust diameter can be increased without having a -ve effect on backpressure.

-Chike
Quote:
Originally posted by IggDawg
it's too bad our pool of people is so small, but the consensus seems to be that 2.5" is the preferred diameter. That's what I'll be going for when I get mine.
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