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Old 03-04-2003, 08:05 AM   #1
Red Devil
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Question For the NA Pros.....

Okay, I may not have posted very much, but I have read enough over the years to be pretty much a scooby expert. I do have one question, as it has to do with a rarely addressed area of scooby tunning. I'm adding an exedy 13lb flywheel to my exhaust system, which consists of a magnaflow 3 inch inlet, 4 inch outlet race (straight flow) muffler. I also have a borla header. I am looking to pick up a catco high flow cat from summit racing and run either a 2.25 or 2.5 inch pipe from the header + Cat back. Considering the flywheel, should go with the 2.25 or 2.5 to prevent major losses from the low end, or will the flywheel make up for it? I've talked to a lot of experts about this, but not a lot of scooby experts, so I want professional opinions. With a 2.5 inch pipe and an open element intake will my low end be totally hosed? This is a little bit of a gray area for me, so I could use some solid advice before I go and screw stuff up. Danke!
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Old 03-04-2003, 10:13 AM   #2
DanzBorin
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2.25"...
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Old 03-04-2003, 11:27 AM   #3
Pat L.I.
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i had always heard stay with the 2.25 in order to keep that extra back pressure.

what are you doing with your car? daily driver? then you want the torque..right? or are you racing on a track where you will need the high end?

Pat
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Old 03-04-2003, 04:26 PM   #4
RawCode
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It is not back pressure that is giving benefit.


RawCode
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Old 03-04-2003, 04:28 PM   #5
DanzBorin
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Quote:
Originally posted by RawCode
It is not back pressure that is giving benefit.

ANY backpressure is is a problem for horse power.

RawCode
He is right, it's exhaust velocity... When you go too wide, you lose too much...
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Old 03-04-2003, 05:14 PM   #6
Red Devil
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Question

Well, it sounds like 2.25 is the concensus view. Should I be concerned that the muffler is kinda large (3 inch inlet, 4 out)? Also, if I run 2.25 pipes, will it be super critical that the high flow cat also be 2.25? The car is a daily driver, though when I'm done with the power and handling mods I may take it out for some auto-X, etc. Nothing too crazy though. I'm also thinking about putting new motor, tranny and pitch stop mounts as part of the next stage of work, but I'm a little concerned about rattling the car to peices. Off topic I guess...
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Old 03-05-2003, 02:29 AM   #7
Kostamojen
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Try out 2.5", midpipes are cheap.
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Old 03-05-2003, 01:13 PM   #8
IggDawg
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Quote:
Originally posted by DanzBorin

He is right, it's exhaust velocity... When you go too wide, you lose too much...
exactly. backpressure is just an unfortunate co-effect of having a system that supports good velocity.

-IggDawg
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Old 03-05-2003, 06:07 PM   #9
jasona
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i have 2.25 on mine, and it's still strong on the low end. i couldn't be happier with it.

well actually, i would be happier if it was quiet
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Old 03-05-2003, 10:50 PM   #10
imprezive
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what do you mean by "open element" intake. Because the cobb intake and the ganzflow lose virtually no torque down low. If you are planning on getting cams or anyother engine mods I would go with the 2.5. Personally i feel the RS has enough power down low, but needs help up top. I would sacrifice a little in the lower RPMs to maek up for it up top.
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Old 03-05-2003, 10:56 PM   #11
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Oh yeah another pet peeve of mine that you guys touched on is back pressure. You don't want backpressure. You don't need backpressure. Why would you want your exhaust to have pressure in the opposite direction. What you DO want is the air to move as fast as possible. And many times piping that is too big slows down the exhaust.
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Old 03-05-2003, 11:02 PM   #12
jasona
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2.25 is plenty large enough for top end power. my car pulls straight to redline now instead of feeling like its dying out at ~5500 RPM
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Old 03-05-2003, 11:15 PM   #13
Tattoo Collector
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Quote:
Originally posted by Kostamojen
Try out 2.5", midpipes are cheap.
There are so many factors to figure out. Your exhaust has to be made to count into effect your exhaust pulses. I agree 100% with exhaust velocity but the companies that make straight shot exhausts will make better power with smaller diameters b/c they don't have many restrictive bends. Anyway, enough of the babbling, leave the huge diameters to the turbo guys, just worry more about have a nice clean flow from the headers to the tailpipe.
Now, onto the flywheel. You are going to rev up quicker with a lighter flywheel, but you won't have as much rotational mass with a lighter clutch. This means that you will require more power to launch than you did before because there won't be as much rotational mass to get the car moving. A light flywheel will make your car rev up much quicker and allow you to use the horsepower higher up in the rev band. If you are drag racing, a little bit heavier clutch might be better; but, if you are doing spirited backroads, SOLO II or track driving, then you would want the lighter clutch.
I think that's about all I have to say. Does anyone agree/disagree???
I will have more feedback when I get my (9.6lbs) Fidanza in my car sometime next month when the motor is back together.
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Old 03-06-2003, 01:28 AM   #14
Kostamojen
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I dont recommend 2.5" for N/A, but a custom one only costs like $60-80, so why not try and then get rid of it if you dont like it and get another? I'm tired of everyone asking "why is it worse" when they havent tried it themselves.
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