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Old 11-26-2003, 10:28 AM   #1
koolaid
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Default 3 in.

Why isnt a 3 in. CB good for high way cruising and lookin nice? I dont usually drive aggressivly
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Old 11-26-2003, 10:40 AM   #2
Subaru IMP VR
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This is because there is not enough back pressure having too big of an exhaust is almost as bad as having too small of an exhaust... and back pressure is what makes your turbos work properly But I don't know specificly what exhaust would be bad for you I am an Impreza guy not an outback guy My reccomendation DO A SEARCH .... and I bet it would look Killer ...... but not perform to well Not to Sure tho....
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Old 11-26-2003, 11:12 AM   #3
Matt Monson
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Actually backpressure kills the performance of a turbo and that is why when you do have boost you want a 3" exhaust. But in an NA build you are trying to keep a really high exhaust velocity, and haveing some backpressure helps with that. If the exhaust is too big the exhaust velocity goes down and you don't get the proper scavenging of the cylinders that a well designed exhaust provides. 2.25" tends to be about right for the EJ25. If you have headwork and cams, then 2.5" is a good upgrade. But for any NA EJ25 3' is just way to big for performance...
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Old 11-26-2003, 11:20 AM   #4
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Back pressure has nothing to do with velocity.

Phil =)
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Old 11-26-2003, 04:00 PM   #5
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In fact, when people says that there is no enough backpressure, they're wrong. back pressure is never an advantage. however, it's better to have optimal exhaust velocity and back pressure, than no backpressure and too low gas velocity. large exhaust piping usualy mean no backpressure, but low velocity, wich is bad. the exhaust gases need to travel at a certain speed in order to create a vaccum effect for the next exhaust step. 2.25" piping is the best of both world on the 2.5 engine. bigger is overkill. smaller would need to be too much efficiently designed to be cost effective. You should always avoid sharp bent and large velocity change on a short distance. going from big to small would increase rapidly exhaust velocity, creating some kind of aerodymanic wall and causing backpressure.
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Old 11-27-2003, 01:36 AM   #6
2.5RSMatt
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I have 2.5" piping and well I'm thinking of changing it to 2.25". Think it's worth it?
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Old 11-27-2003, 01:53 AM   #7
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Quote:
Originally posted by NicEJ25
In fact, when people says that there is no enough backpressure, they're wrong. back pressure is never an advantage. however, it's better to have optimal exhaust velocity and back pressure, than no backpressure and too low gas velocity. large exhaust piping usualy mean no backpressure, but low velocity, wich is bad. the exhaust gases need to travel at a certain speed in order to create a vaccum effect for the next exhaust step. 2.25" piping is the best of both world on the 2.5 engine. bigger is overkill. smaller would need to be too much efficiently designed to be cost effective. You should always avoid sharp bent and large velocity change on a short distance. going from big to small would increase rapidly exhaust velocity, creating some kind of aerodymanic wall and causing backpressure.
w3rd
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Old 11-27-2003, 08:22 AM   #8
NicEJ25
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what's weird in that, Jasona? i was only supporting Matt Monson's message, and explaining the link between velocity and back pressure. have I lost some people ?
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Old 11-27-2003, 12:10 PM   #9
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Quote:
Originally posted by 2.5RSMatt
I have 2.5" piping and well I'm thinking of changing it to 2.25". Think it's worth it?
ehh, i don't know how much power you'd really gain by swapping down to 2.25", it would probably be a little quieter, but i think you'd be better off spending that money on cams or something to take advantage of the increased exhaust flow you could handle
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Old 11-27-2003, 12:36 PM   #10
Charlie-III
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First off, the original post is for an Impreza Outback, which came in a 2.2L only AFAIK.

Thus even a 2.25 may be big. The biggest issue is finding the parts for a 2.2L.

What a few are trying to say is, the diameter of the pipe (for a NA build) is critical to the length of the pipe as well. What you want is to have the exhaust pulses exit the tailpipe and create a suction that will draw in the next cylinders charge.
If the timing is off, you can end up with flat spots in the power.

BTW, hi Matt.
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Old 11-27-2003, 02:33 PM   #11
jasona
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Quote:
Originally posted by NicEJ25
what's weird in that, Jasona? i was only supporting Matt Monson's message, and explaining the link between velocity and back pressure. have I lost some people ?
I was agreeing with you. I always see people saying you need backpressure, but never feel like typing out all that for an expanation.
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Old 11-29-2003, 04:03 AM   #12
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well...for once I have no reason to hack on koolaid.

a 3 inch exhaust will be fine for your car...regardless of mods...good on ya for starting a thread with some merit...
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Old 11-29-2003, 05:58 AM   #13
Paul Sakai
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Quote:
Originally posted by gonzosquirrel
well...for once I have no reason to hack on koolaid.

a 3 inch exhaust will be fine for your car...regardless of mods...good on ya for starting a thread with some merit...
Umm....no 3" would be good for yours not his.
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Old 11-29-2003, 09:28 AM   #14
koolaid
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Quote:
Originally posted by Charlie-III
First off, the original post is for an Impreza Outback, which came in a 2.2L only AFAIK.

Thus even a 2.25 may be big. The biggest issue is finding the parts for a 2.2L.

What a few are trying to say is, the diameter of the pipe (for a NA build) is critical to the length of the pipe as well. What you want is to have the exhaust pulses exit the tailpipe and create a suction that will draw in the next cylinders charge.
If the timing is off, you can end up with flat spots in the power.

BTW, hi Matt.
If it fits a 2.5 (RS, etc) it fits a 2.2
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Old 11-29-2003, 12:12 PM   #15
ttboy19
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I put a Borla catback(not the hush) on my RS. Didn't even bother checking the diameter of the piping. I think it's big. I was rushed into buying it because it was on sale. Anyone have an idea?
It feels a lot better than stock exhaust. It sounds BEEFY, but could I get more power with smaller piping?
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Old 11-29-2003, 01:15 PM   #16
Matt Monson
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Quote:
Originally posted by jasona
I was agreeing with you. I always see people saying you need backpressure, but never feel like typing out all that for an expanation.
NicEJ25,
I appreciate that you took the time to make the long explanation. My remarks about back pressure were misleading because I said you needed back pressure for exhaust velocity,when I know very well it is really that you just can't get rid of all the back pressure ans still have a good exhaust velocity.I just worded it lazily without the complete explanation and it has been restated properly And I think jasona was saying word not weird???

Koolaid,
Not true. Not every part for a 2.5l will work on a 2.2l. The EJ22 heads found on the OBS Imprezas only have one exhaust port per side, which is why Borla make different headers for them. But most everything else will work...

BTW, Hi Charlie
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Old 11-29-2003, 04:11 PM   #17
Charlie-III
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Hi Matt.

Btw, thanks for covering me.
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Old 11-29-2003, 05:00 PM   #18
jasona
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Quote:
Originally posted by Matt Monson
And I think jasona was saying word not weird???
You are correct
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