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Old 06-07-2005, 08:39 PM   #1
CitySubie
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Default 2.25 or 2.5

I hear 2.25 piping is optimal for OUR NA cars. At what point, horsepower, do I need to move up to a 2.5" pipe?
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Last edited by CitySubie; 06-09-2005 at 10:28 AM.
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Old 06-07-2005, 10:15 PM   #2
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there is NO concensus on this issue. Doing a search will garner you the most info on the subject. However, I think that there is more "evidence" indicating that 2.25" will be the best until you start messing with the internals and making "real" power, at least for that engine.

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Old 06-07-2005, 10:33 PM   #3
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Id say anything northward of 210 could probably make some good use of a 2.5", anything less i would probably stick with 2.25"
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Old 06-07-2005, 11:05 PM   #4
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Quote:
Originally Posted by CitySubie
I hear 2.25 piping is optimal for our NA cars. At what point, horsepower, do I need to move up to a 2.5" pipe?
When you are beyond intake, exhaust mods and are stretching towards the cams, equal length header, throttle body enlarged and raised redline range. I'm talking about when you have start going inside the engine and dealing with engine management. Just intake, header and exhaust mods doesn't warrant a 2.5" exhaust.
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Old 06-08-2005, 12:39 AM   #5
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Agreed, although Im not even sure if pushing 210hp would see much benefit. Because arent you still taking in the same amount of air? So your really not changing the amount of exhaust. A turbo instal would see great benefit from a larger pipe, quite possibly all the way to 3inch. Since the turbo spools based on the back pressure between fore and aft the turbo, having a larger pipe makes a larger difference in that pressure which helps the turbo.
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Old 06-08-2005, 03:56 AM   #6
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I have now tried them both and i can say get 2.5in. The low end still had plenty of power, and the top end really pulls to redline. 2.25 is not much bigger then stock at all. BTW, the only mod on the 2.5RS, besides the cat back, was a 3in intake.
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Old 06-08-2005, 03:08 PM   #7
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Quote:
Originally Posted by RS_to_WRX_swap
I have now tried them both and i can say get 2.5in. The low end still had plenty of power, and the top end really pulls to redline. 2.25 is not much bigger then stock at all. BTW, the only mod on the 2.5RS, besides the cat back, was a 3in intake.
Is that loss of "low end torque" the only negative attribute?
I have a free 2.5" muffler and looking for more higher end power because we all have plenty of twist power down low.
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Old 06-08-2005, 04:44 PM   #8
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get a WRAPPED 2.5" catback and you'll be fine. You'll get pull all the way to redline and also good hot gases from the wrap.
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Old 06-08-2005, 05:00 PM   #9
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Does any one know the exhaust gas velocities at the axel back for the different sized cat-back piping?

I would be curious to know how the exhaust gas velocities differ between the following sizes:
1.875" (stock)
2.0"
2.25"
2.50"

Personally I think a 2.0" exhaust from the cat back will give you the most gains if the headers are left stock. The 2.0" will give a little more flow without dropping the exhaust velocity very much.

Remember the N/A engine relies on the scavenging effect of the exhaust pulses to help pull the intake charge into the cylinders. If the cat-back piping is too large you may get lots of exhaust flow but very little velocity (siphoning) and this will cause poor performance. IMO, 2.5" is way too large for our n/a engines and performance will be lost.
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Old 06-08-2005, 07:27 PM   #10
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The COBB cat-back for the GD's is 2.5.
I think they know what they are doing.
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Old 06-08-2005, 07:54 PM   #11
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Quote:
Originally Posted by Sober_One
Remember the N/A engine relies on the scavenging effect of the exhaust pulses to help pull the intake charge into the cylinders. If the cat-back piping is too large you may get lots of exhaust flow but very little velocity (siphoning) and this will cause poor performance. IMO, 2.5" is way too large for our n/a engines and performance will be lost.
Yes this is true, but the scavenging will act different at different rpms. So it's really just a personal preference of what you want. At a higher rpms there will be more air to help fill the exhaust and help with scavenging as where the smaller size would start making more back pressure and have the opposite effect. So if your going for a low end power go with the smaller size, or if you feel the car has enough low end and you want to improve more up top then go bigger.
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Old 06-08-2005, 08:53 PM   #12
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Quote:
Originally Posted by Sober_One
Does any one know the exhaust gas velocities at the axel back for the different sized cat-back piping?

I would be curious to know how the exhaust gas velocities differ between the following sizes:
1.875" (stock)
2.0"
2.25"
2.50"

Personally I think a 2.0" exhaust from the cat back will give you the most gains if the headers are left stock. The 2.0" will give a little more flow without dropping the exhaust velocity very much.

Remember the N/A engine relies on the scavenging effect of the exhaust pulses to help pull the intake charge into the cylinders. If the cat-back piping is too large you may get lots of exhaust flow but very little velocity (siphoning) and this will cause poor performance. IMO, 2.5" is way too large for our n/a engines and performance will be lost.

Did you see what i wrote? Have you ever put a 2.5" cat back on your car? Trust me my 2.5 is doing anything but losing performance.
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Old 06-08-2005, 09:26 PM   #13
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Old 06-08-2005, 09:40 PM   #14
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I have 2.5".

Yes you lose low end some...but you will feel more response on the freeway. I find myself at times wishing I got 2.25 because of being in other RS's with smaller piping and feeling the torque.

If the 2.5" is free get it. Otherwise I would go with the 2.25".
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Old 06-09-2005, 01:35 AM   #15
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There are many variables including engine size, matched intake, etc. If you are doing up a hot cam and "improved" intake then typically the bigger intake will yeild results. For an other wise stoke motor looking for a bit of extra zip, the 2.25 will perform as well, if not better than most 2.5".

Once again do a specific search and try to locate actual dynos for the parts you are looking for with your cars specifics.

Cheers
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Old 06-09-2005, 08:04 AM   #16
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The cheapest route is typically a WRX takeoff axleback and a custom 2.25" midpipe (or a used Stromung).

That's what I've got. Found a WRX axleback with ~200 miles on it, for free, and a Stromung mid for half price, brand new. The shop installed it poorly, which I still need to fix, but overall it's a nice inexpensive upgrade.
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