Welcome to the North American Subaru Impreza Owners Club Thursday March 28, 2024
Home Forums Images WikiNASIOC Products Store Modifications Upgrade Garage
NASIOC
Go Back   NASIOC > NASIOC Technical > Normally Aspirated Powertrain

Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!
Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.







* As an Amazon Associate I earn from qualifying purchases. 
* Registered users of the site do not see these ads. 
Reply
 
Thread Tools Display Modes
Old 03-02-2001, 09:50 AM   #1
adamsrs
Scooby Specialist
 
Member#: 3768
Join Date: Jan 2001
Location: Seattle, WA
Have a Nice Day? 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?
adamsrs www.unnatural.org
* Registered users of the site do not see these ads.
adamsrs is offline   Reply With Quote
Sponsored Links
* Registered users of the site do not see these ads.
Old 03-02-2001, 02:10 PM   #2
Damon
Scooby Specialist
 
Member#: 2902
Join Date: Nov 2000
Chapter/Region: TXIC
Location: S/E TX USA
Question

im no pro, but 2 is aweful small. i think the scooby comes w/ 2.25. dont know for sure though.
Damon is offline   Reply With Quote
Old 03-02-2001, 02:12 PM   #3
bratmantlz
Scooby Specialist
 
Member#: 3231
Join Date: Dec 2000
Chapter/Region: RMIC
Location: colorado springs colorado usa
Vehicle:
88 626 Turbo
white

Post

i have a 2.5" on my RS in N1 shape
bratmantlz is offline   Reply With Quote
Old 03-02-2001, 02:27 PM   #4
Jon Bogert
Scooby Specialist
 
Member#: 1133
Join Date: Mar 2000
Location: SE PA
Vehicle:
09 GTR, 02 996 C4S
95 993 C4, 71 911, 04 STI

Post

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.
Jon Bogert is offline   Reply With Quote
Old 03-02-2001, 02:32 PM   #5
RidinLow
Scooby Guru
 
Member#: 329
Join Date: Sep 1999
Chapter/Region: Tri-State
Location: NJ
Vehicle:
1995 325i
Arctic Gray

Post

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!
RidinLow is offline   Reply With Quote
Old 03-02-2001, 02:39 PM   #6
ColinL
Scooby Guru
 
Member#: 114
Join Date: Jul 1999
Chapter/Region: MWSOC
Location: Wichita, KS and Whoring, OT
Vehicle:
'03 Evo, Rice White
'01 Erion CBR 929

Post

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".
ColinL is offline   Reply With Quote
Old 03-02-2001, 10:31 PM   #7
XT6Wagon
Scooby Specialist
 
Member#: 524
Join Date: Nov 1999
Location: WA
Vehicle:
04 STi
White

Post

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.
XT6Wagon is offline   Reply With Quote
Old 03-03-2001, 05:58 AM   #8
Rich L
Scooby Specialist
 
Member#: 961
Join Date: Feb 2000
Chapter/Region: Tri-State
Location: near Atco(NJ) [email protected]
Vehicle:
1999 RS-Tw/ CobraMAF
Custom Rallispec kit/IC#1

Post

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)
Rich L is offline   Reply With Quote
Old 03-03-2001, 08:00 AM   #9
RidinLow
Scooby Guru
 
Member#: 329
Join Date: Sep 1999
Chapter/Region: Tri-State
Location: NJ
Vehicle:
1995 325i
Arctic Gray

Post

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.
RidinLow is offline   Reply With Quote
Old 03-03-2001, 03:42 PM   #10
XT6Wagon
Scooby Specialist
 
Member#: 524
Join Date: Nov 1999
Location: WA
Vehicle:
04 STi
White

Post

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.
XT6Wagon is offline   Reply With Quote
Old 03-03-2001, 04:07 PM   #11
M8
Scooby Specialist
 
Member#: 949
Join Date: Feb 2000
Chapter/Region: Tri-State
Location: NJ
Vehicle:
06 F2000
Speed Triple

Post

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?
M8 is offline   Reply With Quote
Old 03-03-2001, 08:17 PM   #12
XT6Wagon
Scooby Specialist
 
Member#: 524
Join Date: Nov 1999
Location: WA
Vehicle:
04 STi
White

Post

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.
XT6Wagon is offline   Reply With Quote
Old 03-04-2001, 12:17 AM   #13
Keith
Scooby Guru
 
Member#: 191
Join Date: Aug 1999
Chapter/Region: NESIC
Location: seattle
Vehicle:
2012 J train
Brown

Post

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
Keith is offline   Reply With Quote
Old 03-04-2001, 12:32 AM   #14
Greg Sharpe
Scooby Specialist
 
Member#: 741
Join Date: Jan 2000
Chapter/Region: MAIC
Location: DC
Vehicle:
2010 Outback
Dirt Green

Post

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).]
Greg Sharpe is offline   Reply With Quote
Old 03-04-2001, 06:59 AM   #15
Nativo Peformance
Scooby Newbie
 
Member#: 4610
Join Date: Mar 2001
Location: Coamo, PR
Vehicle:
1998 Impreza 2.5RS
Rally Blue Pearl

Post

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
Nativo Peformance is offline   Reply With Quote
Old 03-04-2001, 03:00 PM   #16
XTREME 2.5
Scooby Newbie
 
Member#: 3330
Join Date: Dec 2000
Location: Tucson, AZ
Post

2 1/4" pipe works well on my RS
XTREME 2.5 is offline   Reply With Quote
Old 03-04-2001, 03:11 PM   #17
Overtime
Scooby Specialist
 
Member#: 1346
Join Date: Apr 2000
Post

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.
Overtime is offline   Reply With Quote
Old 03-05-2001, 12:24 AM   #18
RidinLow
Scooby Guru
 
Member#: 329
Join Date: Sep 1999
Chapter/Region: Tri-State
Location: NJ
Vehicle:
1995 325i
Arctic Gray

Post

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.
RidinLow is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

All times are GMT -4. The time now is 06:22 AM.


Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2024 Axivo Inc.
Copyright ©1999 - 2019, North American Subaru Impreza Owners Club, Inc.

As an Amazon Associate I earn from qualifying purchases.

When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission
Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.