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03-02-2001, 09:50 AM | #1 |
Scooby Specialist
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Join Date: Jan 2001
Location: Seattle, WA
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SPO says that 2" exhaust is best!?!?!
I talked to Scott at SPO motorsports yesterday adn he asked me what size exhaust piping I had. I have a 5Zigen 2.25" Fireball catback. He said..." how thats aweful big"... I said " that's what she said" no wait I said " really I've heard that 2.25" is best" and he said that I was wrong. Am I?
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03-02-2001, 02:10 PM | #2 |
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im no pro, but 2 is aweful small. i think the scooby comes w/ 2.25. dont know for sure though.
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03-02-2001, 02:12 PM | #3 |
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i have a 2.5" on my RS in N1 shape
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03-02-2001, 02:27 PM | #4 |
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This is a very good question. A 2.25" exhaust has 65% more area than the stock RS exhaust. A 2.5" exhaust has more than TWICE the area of the stock exhaust. Do you think Subaru undersized the exhaust by that much?
I bet a smoothly mandrel bent exhaust that's 2" from the collector to the tailpipe, including a free flowing 2" cat, would make just as much power as a larger exhaust, assuming a near-stock motor. Subaru puts a 2.25" exhaust on their 280 HP 8000 RPM STI models. Do ya' think it might be overkill for your 165 HP RS motor? The RS has such a mild cam, I can't imagine it's all that choked up even at high RPM. |
03-02-2001, 02:32 PM | #5 |
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In Maximum Boost by Corky Bell, there's a graph of optimal exhaust size vs. hp. For turbo cars, 230hp requires a 2.25" exhaust, so for a NA car with even less power, an even smaller system would be required!
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03-02-2001, 02:39 PM | #6 |
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install header mufflers with the same size inlet as the header flange. true you could still do better with a fancy tuned-length exhaust, but short of that is the *one* sure way to know you have no exhaust gas velocity problems.
sure it's a little noisy, and sure you get some fumes sitting in traffic. and yes, there is a bumper cutout for a muffler that you won't be using. but at least we could stop worrying about 2" vs. 2.25" vs. 2.5". |
03-02-2001, 10:31 PM | #7 |
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2" is WAY to small. The exaust PORTS on the old 1.8L motor are 2" The crossover is 2" where it was not crushbent into shape.
The downpipe I think is 2.25" but necks down to effectively 1" in a place or two. This is on a 115HP TURBO car. Take corky's chart to be a minimum. |
03-03-2001, 05:58 AM | #8 |
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XT6...
Corky Bell knows his stuff... its wishful thinking to say a stock RS would need much more than 2" -Rich L -6psi -13.7 1/4 -2.25" exhaust (w/ hi flow cat) |
03-03-2001, 08:00 AM | #9 |
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I don't know much about the subject, so I'm just going to relay what I saw in Corky's book. That chart (which seems to have disappeared from my site & I'm too lazy to rescan it) illustrates the optimum exhaust diameter. Too little and you'll have too much backpressure. Too much and the pipe diameter is too big, slowing the velocity of the exhaust gases. He also mentions that a good rule of thumb is 10% larger than the turbo exhaust outlet.
This is for turbo cars, so again, a NA car will require less. |
03-03-2001, 03:42 PM | #10 |
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Sorry but that chart is WRONG for optimum. Why else is would you pick up HP w/o loosing low end torque by running a larger diamiter pipe than that chart claims.
The old pushrod 1.8L motors do NOT loose low end torque when running 1 7/8" true duals, and they put out all of 95 HP or so. Put a 2.5" downpipe and exaust on a 200HP Turbocharged car and see the power really wake up. |
03-03-2001, 04:07 PM | #11 |
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so what about the wisdom of going from a 2.5 down pipe/cat-back to a 3"? (on a turbo'd car) is which is the choice i am looking at right now. is there much to be gained?
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03-03-2001, 08:17 PM | #12 |
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Maybe, what kind of setup are you running.
A 3" downpipe and exaust would be useless on a otherwise stock XT, but would be perfect if you are running a big turbo with alot of HP. |
03-04-2001, 12:17 AM | #13 |
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scc did a report on this a while back i think. 2.25" was the best from their results, 2.5 is to big for na use.
keith |
03-04-2001, 12:32 AM | #14 |
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Corky can't be wrong, he's too damn succesfull. Those old 1.8L engines only had one exhaust port per head, right? Making that 2" hole you spoke of actually appropriately sized for the two cylinders on that side of the engine. On another note, the best exhaust for a turbo engine is ... no exhaust. That is definitely not the case for a N/A engine, which benefits from comparitively smaller diameter exhaust pipes. I'd guess that some people see HP gains upsizing from a 2" to a 2.5"+ exhaust system because they also increased the size of the cat inlet/outlets or removed them alltogether. Just a guess though.
Edit: I forgot to mention the point of all this: From what I have read, including Maximum Boost, I would also use a 2" system on my N/A RS if the money ever fell from the heavens. [This message has been edited by Greg Sharpe (edited March 04, 2001).] |
03-04-2001, 06:59 AM | #15 |
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Well here we go:
We must determine what are we looking for when we talk about which exhaust is best. Most of us reffer to which diameter gives more power but do not clarify about at which RPM and for how long. Let explain, Borla conducted some test with their headers and 2.0" mandrel bent catback and found to be the perfect match that would give the best torque/hp combination for our cars. Of course a 2.25" would have yield more HP at a higher RPM and RalliSpec and JC Sports have proven that a 2.5" would yield ever more HP but, what nobody is telling us is that those extras ponies come with a price. The avarage motor will see on a regular basis 2K-5K RPM and an occasional redline here and there so; Where do you really want the power band to be? Between 4500 and 6000 Rpm where it only happens once every so offten or between 1500 and 55000 RPM where it is more usefull. Remember Lots of HP will sell Lots of cars but, Lots of torque will win Lots races. If all the racing that your car would see is an occasional Rallycross (torque), Autocross (torque) or a Driving school (early shift torque)and the rest of the month you want good gas millage (torque). Probably your best bet would be the 2.0". On the other hand if you care less about torque and you believe that all you want is a high reving engine (4k-6200RPM) Drag racer then the 2.25" or 2.5" is for you. Remember ther isn't a perfect exhaust catback, just the perfect one for your driving style and needs. Thanks, Efrain N Alers www.NativoPerformance.com |
03-04-2001, 03:00 PM | #16 |
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2 1/4" pipe works well on my RS
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03-04-2001, 03:11 PM | #17 |
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HP increases are tricky. Most manufacturers quote the max HP increase and not the average HP increase because big numbers are more impressive.
I have the Borla setup. It's very nice, but I know that at least the catback portion was designed for the 2.2l Outback Sport and not the 2.5l RS. That's fine with me. I traded a little power for the peace of mind of buying from a company I can contact. I think that a full 2.25" setup would combine both significant top-end gains with minimal low-end loss, but it's impossible to say without testing...testing which should be done this summer. |
03-05-2001, 12:24 AM | #18 |
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Keith, SCC only tested assorted 2.25" exhausts and a 2.5" JC Sports exhaust. They didn't look at anything in between 1.75" & 2.25".
Corky Bell states that for a turbo car, no exhaust is the best exhaust because small diameter piping restricts flow. However, too big of a pipe will actually slow the velocity of the exhaust gases leaving the engine. However he doesn't say how he came up with his hp to exhaust diameter graph or what it's biased towards, such as maximum high end hp or low end torque and driveability. |
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