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Old 03-23-2005, 09:12 PM   #1
wgb113
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Default Custom Exhaust Diameter?

I'm thinking of going with a custom cat-back for my '04 RS using a Borla universal muffler. I've read that you can go up to 2.5" but that 2.25" is better, is this true?

Thanks,
Bill
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Old 03-23-2005, 09:45 PM   #2
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Yes and no. Some the ppl u ask will say 2.5" is better. The others will say 2.25" is better. Others will say it depends on how modified ur car is. I still don't know who is right... good luck!

Last edited by bongchild; 03-23-2005 at 10:02 PM. Reason: in light of new information....
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Old 03-23-2005, 09:52 PM   #3
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The people that say 2.5" is better are hallucinating. You simply will not spend enough time at redline to need pipe this big, and you'll be sacrificing low-end torque, which is the backbone to street drivability. Mine is 2.25", pulls (hard) all the way to redline, has "meaty" bottom end, the entire powerband was improved. (I still want a header, though).
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Old 03-23-2005, 11:06 PM   #4
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the only way 2.5" could be better, is if you plan on turbo'ing the car. but 2.25 is much better for n/a.
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Old 03-23-2005, 11:48 PM   #5
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^^^ fully agree
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Old 03-24-2005, 01:18 AM   #6
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2.25 here and am loven it, glad i didnt go 2.5 would have lost some low end and gained high end, get the 2.5 if you are going to turbo the car, if no dont do it
Just my .02
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Old 03-24-2005, 02:15 AM   #7
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For your application I would NOT go over 2.25
2.5 would just be too big....plain and simple. It is unneaded and you will sacrifice a significant amount of low end torque. Also, the amount gained up top wouldn't be much anyways.
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Old 03-24-2005, 04:22 AM   #8
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don't go for 2.5"
i bought a WRX N1 w/o knowing it, and now i'm suffering...
no power below 3000 rpm...
so if anyone who has an RS N1 and wonna go turbo, let me know...
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Old 03-24-2005, 07:51 AM   #9
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2.25" is fine, unless you have done alot of headwork or higher compression pistons.
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Old 03-24-2005, 07:52 AM   #10
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Great replies guys! I'll stick with 2.25". The only other mod I plan on is adding Cobb's CAI at some point.

Any thoughts on my proposed setup using one of Borla's universal mufflers? It'd definitely be cheaper but I'm not sure how it will sound (too loud/barely noticeable). If I were to go with a Prodrive or Stromung, at least I know what to expect.

Bill
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Old 03-24-2005, 10:34 AM   #11
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Just ot rock the boat....I did these on a stock 2.5L n/a block. You've got 2.4 inches worth of exhaust valve area per cylinder! That's 9.6 inches worth of exhaust valve area allowing gasses out that just want to leave. 2.25 total exhaust diameter? I'm sorry, I've just never believed in it for the 2.5L block specifically. On 1.8's and 2.2's, 2.25 might work fine. But, here's my results:

2.5in exhaust, stock headers and cat, intake, chip, pulley.



3 inch exhaust, headers, cat, and other bolt-ons.



This ain't a 1.6L Honda. The engine wants to breathe. Let it, and you shall be greatly rewarded .
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Old 03-24-2005, 10:59 AM   #12
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Isn't the stock piping something like 1.875"? Wouldn't 2.25" be a pretty big step up from that? Wouldn't adding a CAI allow it to breathe (inhale) even better?

Bill
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Old 03-24-2005, 11:25 AM   #13
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Correct me if I am reading the plots wrong, but you are making peak torque at 3000rpm. That makes that torque pretty unused for most driving conditions. I don't think anyone will argue about bigger piping paying off at the higher rpms, but most people are not looking to run their cars to redline every day or on most shifts-thus the recomendation for 2.25" for most people's applications. Especially for auto-cross that kind of torque won't help too much.

.02

Jared

Quote:
Originally Posted by HamFist
Just ot rock the boat....I did these on a stock 2.5L n/a block. You've got 2.4 inches worth of exhaust valve area per cylinder! That's 9.6 inches worth of exhaust valve area allowing gasses out that just want to leave. 2.25 total exhaust diameter? I'm sorry, I've just never believed in it for the 2.5L block specifically. On 1.8's and 2.2's, 2.25 might work fine. But, here's my results:

2.5in exhaust, stock headers and cat, intake, chip, pulley.



3 inch exhaust, headers, cat, and other bolt-ons.



This ain't a 1.6L Honda. The engine wants to breathe. Let it, and you shall be greatly rewarded .

Last edited by gtwagon941; 03-24-2005 at 12:30 PM. Reason: looked at wrong curve
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Old 03-24-2005, 12:09 PM   #14
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I want to make amid pipe custom too, what would be the best material for it?
Thanx
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Old 03-24-2005, 12:21 PM   #15
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Those are nice gains....however, before peak torque (around 3000 rpm) the torque level is very similar to the 2.5" piping PLUS you have header and highflow cat. This is in no way an accurate comparison. Further more we are not discussing 2.5" to 3".....your dynos do not show that the 2.5 OR the 3" exhaust have better lowend torqe than the 2.25". I am not saying that they wouldn't but your post realy does nothing to help this debate.

The choice for exhaust diameter largely depends on what you want. As GT wagon said (although the peak torqe is around 3000 not 5500...you are looking at the hp) most people are looking for a good lowend gain. They want their idle to 3000 rpm range stronger because that is where most of these people will be while driving around town and while 2.5 or 3" exhaust will give them more power/torque above that area, I am fairly certain that the 2.25" exhaust will out perform the the 2.5 and certainly the 3" exhaust in this rpm range. If it were my car I would go with a 2.5" exhaust but that is not what some people are looking for. I am used to my Celica GTS where I spend most of my time from 6000-8400rpm so I am biased to the higher rev range (I'm sure this may change after I finish my turbo install though )
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Old 03-24-2005, 12:50 PM   #16
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I just got a quote from the place I planned on having it done at and it seems pretty high:

$750 for complete 2.25" stainless steel cat-back using Borla muffler and Flowmaster tip. $550 for same setup with aluminized piping.

I can get the damn Cobb SS catback for $650!

Bill
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Old 03-24-2005, 01:07 PM   #17
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haha stay far and away from that place
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Old 03-24-2005, 01:11 PM   #18
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Look again at the torque curve. My peak torque is at 3,000rpm...right in the middle. That's with 3 inch exhaust. Compare the 2 curves and concentrate on the middle. Sure, peak numbers ar higher, but look how much flatter the curve is in the second run with the other mods. That area under the curve, from 2200 to about 5000rpm is very fat, level, and happened with 3 inch exhaust. These engines breathe a lot better than people give them credit for. I never liked the 2.25 exhaust "rule", purely because each cylinder has more exhaust valve area than that! I'm not done breaking down the myths of this car by a long shot .
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Old 03-24-2005, 01:23 PM   #19
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Quote:
Originally Posted by wgb113
I just got a quote from the place I planned on having it done at and it seems pretty high:

$750 for complete 2.25" stainless steel cat-back using Borla muffler and Flowmaster tip. $550 for same setup with aluminized piping.

I can get the damn Cobb SS catback for $650!

Bill
I dont know what place quoted that but that is absolutely rediculous! You should pay $300 max for a custom job.
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Old 03-24-2005, 01:39 PM   #20
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That's exactly what I was thinking...best part is they quoted someone here at work the same exact price for a true dual cat-back for their Mustang. HTF can a single run the same $$$ as a dual???

Bill
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Old 03-24-2005, 01:42 PM   #21
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I too had this dillema, then I went to talk to Ron at Axis Power Racing (an engine builder who specializes in Subarus, also just happens to be one of the best tuners in the country), and Ron dodn't even think twice about telling me to go ahead and go with the 2.5" the low end loss is minimal to non existant, and the the high end gain is better than the 2.25". For some reason I trusted what he said and went with 2.5" couldn't have been happier, I still beat WRXs off the line and into the middle of second gear.
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Old 03-24-2005, 04:40 PM   #22
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Quote:
Originally Posted by ruskiyab
I still beat WRXs off the line and into the middle of second gear.
Do you have any videos of this? I find it pretty hard to believe unless the WRX drivers can't drive....
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Old 03-24-2005, 06:02 PM   #23
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Quote:
Originally Posted by mtskibum16
Do you have any videos of this? I find it pretty hard to believe unless the WRX drivers can't drive....
It is all about the fact that the 2.5 is bigger and better than the 2.0 in the WRX until boost comes on, then you're usually toast.

Jared
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Old 03-24-2005, 06:04 PM   #24
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Quote:
Originally Posted by Nicklbrso
I dont know what place quoted that but that is absolutely rediculous! You should pay $300 max for a custom job.
Exactly. I have a custom cat-back section made, ended up just over 230 for an awesome job.
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Old 03-24-2005, 06:13 PM   #25
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heres a question why is custom exhaust cheaper than commercial exhaust??
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