exhaust 2.5inch vs. 3.0 inch tubing? - Subaru WRX Forum
 
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#1 Old 07-13-2012, 12:38 AM
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exhaust 2.5inch vs. 3.0 inch tubing?

The place I got my exhaust done said I shouldn't do the 3inch as I would lose too much back pressure or something along those lines? What does that mean? The 3inch I think sounds better, what do you guys reccommend? Also he said slap an intake and it will sound better. After reading some of the FAQ and threads it seems this is something done later after a tune. I dont plan on doing that or going to that level, I really just want to improve the sound. As I take it, I could add a cobb intake and not run in to issues. So in a nutshell would I be okay to run 3inch tubing and not get back pressure issues (would help if someone could explain this some), or would it be smarter to stick with what I have, and add an intake? Thanks for all the help.
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#2 Old 07-13-2012, 12:51 AM
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I have the full Cobb turbo back exhaust and its all off 3 inches. Cobb is a premier company and they build 3inch systems for many cars. With a turbo car its important to be able to get the exhaust out efficiently along with any car for that matter. I would go with 3inch as pretty much all major exhaust companies design aftermarket exhaust that way. As far as the intake goes, I run stock as it is good up to 400hp, however if you want more of a turbo sound and a throatier intake note then get the Cobb short ram. Stay clear of cold air intakes as they are trouble with subarus unless you get the car tuned for them.
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#3 Old 07-13-2012, 10:07 AM
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Ugh.

Cobb's turboback is not 3". Most of the piping is but the downpipe exit is 2.5", meaning it's not a full 3" exhaust. I'm at 400 whp on a full (not just at the end of the downpipe) 2.5" exhaust, abiet I have an EWG. For most setups 2.5" is fine if that's what you get. Buy whichever you want.

And no, the stock intake is not good for 400 hp. And no, CAI are not automatically trouble on Subarus. Nor are all SRI's automatically good for them. It's intake specific.
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#4 Old 07-13-2012, 11:46 AM
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Ugh.

Cobb's turboback is not 3". Most of the piping is but the downpipe exit is 2.5", meaning it's not a full 3" exhaust. I'm at 400 whp on a full (not just at the end of the downpipe) 2.5" exhaust, abiet I have an EWG. For most setups 2.5" is fine if that's what you get. Buy whichever you want.

And no, the stock intake is not good for 400 hp. And no, CAI are not automatically trouble on Subarus. Nor are all SRI's automatically good for them. It's intake specific.

thanks for taking the time for clarification. do you reccomend the cobb short ram considering i wont be doing a tune in the near future, and its mainly for purposes of sound? So 3" catback would not present issues without a tune? grrr, now im regretting not just buying the stage 2 cobb kit, i should have just done that for everything you get and seems to allow for more mods in the future. but im in the situation im in now and dont have money for it haha so ill just stick to the original plan.
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#5 Old 07-13-2012, 02:54 PM
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Quote:
Originally Posted by brfatal View Post
Ugh.

Cobb's turboback is not 3". Most of the piping is but the downpipe exit is 2.5", meaning it's not a full 3" exhaust. I'm at 400 whp on a full (not just at the end of the downpipe) 2.5" exhaust, abiet I have an EWG. For most setups 2.5" is fine if that's what you get. Buy whichever you want.

And no, the stock intake is not good for 400 hp. And no, CAI are not automatically trouble on Subarus. Nor are all SRI's automatically good for them. It's intake specific.
So I was a little off on the Cobb, although on the website it said 3". Eh, well, thanks for clarifiying brfatal.

I guess I must have been miss informed about the capability of the stock intake capable of supporting up to 400ish HP. On the note of the CAI, I haven't seen any non tuned wrx's not have lean issues or problems running a CAI. I always thought this was due to the 90 degree bend in the piping. I haven't seen any issues with SRI's as they pretty much don't change a whole lot over factory box, other than maybe a little more air flow or bling factor
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#6 Old 07-13-2012, 05:06 PM
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It's tapered. So yes and no. I had a Perrin CAI on my STi and WRX in the past and it performed well. Then again they were tuned and we don't get much rain here.

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#7 Old 07-13-2012, 09:28 PM
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Quote:
Originally Posted by dustin10 View Post
thanks for taking the time for clarification. do you reccomend the cobb short ram considering i wont be doing a tune in the near future, and its mainly for purposes of sound? So 3" catback would not present issues without a tune? grrr, now im regretting not just buying the stage 2 cobb kit, i should have just done that for everything you get and seems to allow for more mods in the future. but im in the situation im in now and dont have money for it haha so ill just stick to the original plan.
You don't need a tune for any catback. The Cobb SRI might not be the best money spent at this point in your mod path.


Quote:
Originally Posted by wrxsnowman View Post
So I was a little off on the Cobb, although on the website it said 3". Eh, well, thanks for clarifiying brfatal.

I guess I must have been miss informed about the capability of the stock intake capable of supporting up to 400ish HP. On the note of the CAI, I haven't seen any non tuned wrx's not have lean issues or problems running a CAI. I always thought this was due to the 90 degree bend in the piping. I haven't seen any issues with SRI's as they pretty much don't change a whole lot over factory box, other than maybe a little more air flow or bling factor
Check the taper at the end to see what I meant. It's there because the downpipe is designed to mate to the stock catback.



K&N Typhoon and AEM V2 are examples of CAIs that have been shown to be fine. It's not a the 1st mod you should do if power is your goal, but at stage 2 it's a worthwhile expenditure, esp if you're getting custom tuned.

Perrin, SPT, Injen, and the original AEM intake are SRI's that require tuning in order to use.
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#8 Old 07-15-2012, 08:12 PM
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Originally Posted by dustin10 View Post
The place I got my exhaust done said I shouldn't do the 3inch as I would lose too much back pressure or something along those lines? What does that mean? The 3inch I think sounds better, what do you guys reccommend? Also he said slap an intake and it will sound better. After reading some of the FAQ and threads it seems this is something done later after a tune. I dont plan on doing that or going to that level, I really just want to improve the sound. As I take it, I could add a cobb intake and not run in to issues. So in a nutshell would I be okay to run 3inch tubing and not get back pressure issues (would help if someone could explain this some), or would it be smarter to stick with what I have, and add an intake? Thanks for all the help.
A turbocharged car doesn't requires back pressure. As far as 2.5 vs 3, meh it could be debated, but 2.5 will work fine for most people means.
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#9 Old 07-15-2012, 09:40 PM
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^Who told you that Keith? !Laugh!

The 2.5 is a little better at holding the backpressue. Maybe not on the Subie platform but on others having to change idle is the difference between smoking and not when talking about catted and cattless

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#10 Old 07-15-2012, 10:30 PM
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^Who told you that Keith? !Laugh!

The 2.5 is a little better at holding the backpressue. Maybe not on the Subie platform but on others having to change idle is the difference between smoking and not when talking about catted and cattless
so if i run a cattless dp would the 2.5 or 3inch be optimal, what about a catted then?
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#11 Old 07-16-2012, 12:43 AM
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#12 Old 07-16-2012, 05:09 AM
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^Who told you that Keith? !Laugh!

The 2.5 is a little better at holding the backpressue. Maybe not on the Subie platform but on others having to change idle is the difference between smoking and not when talking about catted and cattless
Which were you referring to?

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