Welcome to the North American Subaru Impreza Owners Club Saturday July 26, 2014
Home Forums WikiNASIOC Products Store Modifications Upgrade Garage
NASIOC
Here you can view your subscribed threads, work with private messages and edit your profile and preferences Home Registration is free! Visit the NASIOC Store NASIOC Rules Search Find other members Frequently Asked Questions Calendar Archive NASIOC Upgrade Garage Logout
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.
* Registered users of the site do not see these ads.
Reply
 
Thread Tools Display Modes
Old 03-28-2002, 02:35 PM   #1
BrysImpreza
Scooby Guru
 
Member#: 3102
Join Date: Nov 2000
Chapter/Region: NESIC
Location: Northern, CT
Vehicle:
2012 Sport Limited
Ice Silver

Question Does Cold Air Intake Pipe Diameter give different performance results?

Does anyone know if intake pipe diameter has any kind of effect on the performance of the car? I know exhaust diameter piping effects performance, so how about intake?

I've gone through 3 different intakes and had completely different results with them.

1. JC Sports MAF based intake, 3" pipe diameter Dyno'ed at 92 HP to the wheels 110 Tq. Liked the torque result, but that was only at 5,000 rpm
Had no low end power and the car felt like it took effort to even take off, high end power was great after 4,500 rpm

2. MAF adaptor and K&N Cone filter, replaced intake pipe, kept rear airbox on.
Kept good low end torque, better gas mileage, good throttle response, but not much extra in high end power

3. New Intake, Mandrel bent custom 2.5" piping diameter, replaces all stock intake piping.
Good low end torque, decent midrange, great high end power. Gas mileage improved slightly, not sure on DYNO results, I'm going Apr 27 to do it.
* Registered users of the site do not see these ads.
BrysImpreza is offline   Reply With Quote
Old 03-28-2002, 04:16 PM   #2
amorenite95
Scooby Newbie
 
Member#: 15174
Join Date: Feb 2002
Location: I am silent_rob!
Vehicle:
02 Impreza 2.5 RS
white

Default

yeah I think it does. I think the bigger the better. I remember reading something on www.imprezars.com when I was buying the GanzFlow CAI and it said that they have a a 3" version and they say it does have some gains.
amorenite95 is offline   Reply With Quote
Old 03-28-2002, 07:14 PM   #3
Reciprocity
Scooby Specialist
 
Member#: 1165
Join Date: Mar 2000
Chapter/Region: BAIC
Location: Alameda, CA
Vehicle:
2012 dcrawford died
for your sins

Default

Ohhh...this gets tricky actually. Fluid (and air) dynamics tell us there is more to it than just making the pipe as huge as possible.

Think about it in exhaust flow terms...you don't put a huge diameter pipe on your N/A exhaust because the air won't flow as fast through a huge pipe compared to a pipe that has the right sized diameter. Too small and you start to restrict flow. It's all about tuning to a specific powerband...

Have you ever seen an Indy car...or F1...with those cones on their throttle bodies? Trumpets? Those are velocity stacks and they are tuned to give each cylinder it's own specific powerband...different sizes and diameters to create different hp/torque curves...

I am just barely breaching the dynamics involved here...but the point I am trying to make is that it is a lot more complex than just having it be as big as possible.

Here is the basic idea...the longer the runner the greater the high end power gain. The shorter the runner the greater the low end power gain (is that right? I believe it is but I could be wrong). Just like exhaust it's all about velocity. A great intake design would be a medium length pipe with the filter element as far away from the throttle body as possible. That way the air has time to gather speed through the intake track and won't be as turbulent (smooth and fast is the way to go). It would also be benificial to have the pipe start out with a large diameter...and get smaller and smaller until it reaches the throttle body. That will help promote velocity (venturi effect). On top of that it would be best to draw from air outside of the engine compartement...the hotter the air, the less dense it is, the lower the O2 content, the lower the explosive power available per unit of air. And along those lines you want to treat the intake in a manner to keep heat from entering in from the engine bay (along the lines of...but in a way opposite of the exhaust...where you want to keep the heat inside the track). AND if you can force the air in there somehow...you gain even more (either through a supercharger or turbocharger...or better yet take advantage of the energy in the air as the car moves through it...a la cowl induction).

How long should the intake be? How wide should it start? How small should it get? Those are all questions that require a lot of experience and/or education.

I am just barely touching the subject here. I could be wrong on a couple points...but honestly I am just trying to cover the basics. It's a very complicated subject.

But that's great that you tried out those different intakes...great information!

NC

PS: Imagine 4 individual throttle bodies...one for each cylinder. A huge backward hood scoop opened up where the windshield meets the hood....mmmmmmmm....power and insane response!

Last edited by Reciprocity; 03-28-2002 at 07:21 PM.
Reciprocity is offline   Reply With Quote
Old 03-28-2002, 07:25 PM   #4
BrysImpreza
Scooby Guru
 
Member#: 3102
Join Date: Nov 2000
Chapter/Region: NESIC
Location: Northern, CT
Vehicle:
2012 Sport Limited
Ice Silver

Default

I don't know much about physics, just how my car feels under acceleration, how it responds under deceleration (aka popping, backfiring) How easily my tires spin when I floor it in 1st gear, etc.

The newest one with the 2.5" piping has breathed some fire into my car, so that's why I was wondering if pipe diameter made a big difference

Bry.
BrysImpreza is offline   Reply With Quote
Old 03-28-2002, 07:32 PM   #5
Reciprocity
Scooby Specialist
 
Member#: 1165
Join Date: Mar 2000
Chapter/Region: BAIC
Location: Alameda, CA
Vehicle:
2012 dcrawford died
for your sins

Default

That's probably because 2.5 in. is the right diameter for the powerband inherit to the engine. I believe that 3 inches might be too big...and 2 is too small (guessing). If you want to find out the perfect length, diameter, and trumpet shape...that'll take some serious reseach.

If you are using a bare metal tube...insulate that baby. It'll make a difference.

NC
Reciprocity is offline   Reply With Quote
Old 03-28-2002, 07:42 PM   #6
BrysImpreza
Scooby Guru
 
Member#: 3102
Join Date: Nov 2000
Chapter/Region: NESIC
Location: Northern, CT
Vehicle:
2012 Sport Limited
Ice Silver

Default

I think the 3" piping is more functional for the 2.5 liter engine, I believe I read something about 2.75" being the perfect size for the 2.5 liter, but 2.75" is hard to come by, so the 3" is the better choice for price.

I have the 2.2 liter engine, I know for a fact Cobb Tuning recommends not to use some of their parts from the 2.5 on the 2.2 because they do not function properly (Cams and other engine internals)
So just doing some simple math on airflow velocity it seems that the 2.5" is the better choice for my car based on the recommended air to fuel ratio for the 2nd gen 2.2 liter engine (I'm hoping I have the right figures )

Bry.
BrysImpreza is offline   Reply With Quote
Old 03-28-2002, 08:34 PM   #7
Kevin Thomas
Street Racing Instructor
Moderator
 
Member#: 110
Join Date: Jul 1999
Location: 1997 OBS, 1996 SVX, 1988 RX
Vehicle:
1989 1989 XT6

Default Answers

Brys,

You pretty much summed it all up without theories and guesses. You showed through your experience how each intake setup affects the 2.5ltr engine's powerband by how it felt to you.

I would 'guess' that the best intake diameter would be the diameter of your throttle body. As far as length goes, I would follow dcrawford's advice on that.
Kevin Thomas is offline   Reply With Quote
Old 03-29-2002, 12:11 AM   #8
Patrick Olsen
NASIOC Supporter
 
Member#: 120
Join Date: Jul 1999
Chapter/Region: AKIC
Location: Where the Navy sends me...
Vehicle:
1997 Legacy 2.5GT
QuickSilver Metallic

Default

Quote:
Originally posted by dcrawford
Here is the basic idea...the longer the runner the greater the high end power gain. The shorter the runner the greater the low end power gain (is that right? I believe it is but I could be wrong).
Nope, you got it backwards. Long runners are good for low end torque, short runners are good for top end power. Other than that mix-up, I think everything else you said makes good sense.

Pat Olsen
'97 Legacy 2.5GT sedan
Patrick Olsen is offline   Reply With Quote
Old 03-29-2002, 02:53 AM   #9
Kostamojen
Scooby Guru
 
Member#: 2272
Join Date: Sep 2000
Chapter/Region: BAIC
Location: haha XD
Vehicle:
2013 Subaru 599 :P
Galaxy Blue Sexy

Default

The type of material the pipe is composed of also is a major issue... Along with the specific type of filter... (which are two possible reasons why the Cobb intake seems to be the best intake IMHO)
Kostamojen is offline   Reply With Quote
Old 03-29-2002, 02:58 AM   #10
Josico
Scooby Specialist
 
Member#: 8219
Join Date: Jul 2001
Location: Burnaby, BC, Canada
Vehicle:
2002 Impreza RS
Blue Ridge Pearl

Default

Quote:
Originally posted by Patrick Olsen

Nope, you got it backwards. Long runners are good for low end torque, short runners are good for top end power. Other than that mix-up, I think everything else you said makes good sense.

Pat Olsen
'97 Legacy 2.5GT sedan
when you speak of long and short runners, does that mean the intake piping?

if so, then how come a short runner intake w/ cone filter like a weapon r intake for example will result in a loss in power in the low end? where as it has power in the high end? (when compared to the 'long runner' of a stock intake which will give a better low end?)
Josico is offline   Reply With Quote
Old 03-29-2002, 11:03 AM   #11
BrysImpreza
Scooby Guru
 
Member#: 3102
Join Date: Nov 2000
Chapter/Region: NESIC
Location: Northern, CT
Vehicle:
2012 Sport Limited
Ice Silver

Default

The new intake I made is composed of Polished Aluminum, except the extension that I put from the MAF adaptor into the fender to make the filter suck nothing but cold air.

The filter is an RS Akimoto, supposedly one of the best on the market, it's performing better then my K&N did, but I think that's just because the top has an opening and something that looks like silk screening.

So I can see what Pat is saying about lenth, my car pulls great with the longer intake, but I had the same set-up with the 3" intake and it didn't perform as well as the 2.5" piping does.

Maybe it's just the filter?

Bry.
BrysImpreza is offline   Reply With Quote
Old 03-24-2013, 09:02 PM   #12
Kevin Thomas
Street Racing Instructor
Moderator
 
Member#: 110
Join Date: Jul 1999
Location: 1997 OBS, 1996 SVX, 1988 RX
Vehicle:
1989 1989 XT6

Default

Quote:
Originally Posted by Reciprocity View Post
Ohhh...this gets tricky actually. Fluid (and air) dynamics tell us there is more to it than just making the pipe as huge as possible.

Think about it in exhaust flow terms...you don't put a huge diameter pipe on your N/A exhaust because the air won't flow as fast through a huge pipe compared to a pipe that has the right sized diameter. Too small and you start to restrict flow. It's all about tuning to a specific powerband...

Have you ever seen an Indy car...or F1...with those cones on their throttle bodies? Trumpets? Those are velocity stacks and they are tuned to give each cylinder it's own specific powerband...different sizes and diameters to create different hp/torque curves...

I am just barely breaching the dynamics involved here...but the point I am trying to make is that it is a lot more complex than just having it be as big as possible.

Here is the basic idea...the longer the runner the greater the high end power gain. The shorter the runner the greater the low end power gain (is that right? I believe it is but I could be wrong). Just like exhaust it's all about velocity. A great intake design would be a medium length pipe with the filter element as far away from the throttle body as possible. That way the air has time to gather speed through the intake track and won't be as turbulent (smooth and fast is the way to go). It would also be benificial to have the pipe start out with a large diameter...and get smaller and smaller until it reaches the throttle body. That will help promote velocity (venturi effect). On top of that it would be best to draw from air outside of the engine compartement...the hotter the air, the less dense it is, the lower the O2 content, the lower the explosive power available per unit of air. And along those lines you want to treat the intake in a manner to keep heat from entering in from the engine bay (along the lines of...but in a way opposite of the exhaust...where you want to keep the heat inside the track). AND if you can force the air in there somehow...you gain even more (either through a supercharger or turbocharger...or better yet take advantage of the energy in the air as the car moves through it...a la cowl induction).

How long should the intake be? How wide should it start? How small should it get? Those are all questions that require a lot of experience and/or education.

I am just barely touching the subject here. I could be wrong on a couple points...but honestly I am just trying to cover the basics. It's a very complicated subject.

But that's great that you tried out those different intakes...great information!

NC

PS: Imagine 4 individual throttle bodies...one for each cylinder. A huge backward hood scoop opened up where the windshield meets the hood....mmmmmmmm....power and insane response!
11 years later and I'm JUST following up on your advice in another vehicle. I didn't want to lose this info so I'm posting in it for easy find.

I just happen to be searching through the internet on intake pipe sizing and somehow came across this. I'm working on testing a 3.5" to 3" pipe vs straight 3" pipe at the dragstrip next week. Thanks!
Kevin Thomas is offline   Reply With Quote
Old 03-25-2013, 02:08 PM   #13
Patrick Olsen
NASIOC Supporter
 
Member#: 120
Join Date: Jul 1999
Chapter/Region: AKIC
Location: Where the Navy sends me...
Vehicle:
1997 Legacy 2.5GT
QuickSilver Metallic

Default

Awww, man, I was gonna berate some stupid noob for needlessly bumping an 11yo thread, then I get to the bottom and see it's some old dude that bumped it.
Patrick Olsen is offline   Reply With Quote
Old 03-25-2013, 03:58 PM   #14
T Wrex--OH
Scooby Specialist
 
Member#: 179478
Join Date: May 2008
Chapter/Region: MWSOC
Location: Cleveland<OH>
Vehicle:
2000 Impreza 2.5 RS
Sedona Red Pearl

Default

Quote:
Originally Posted by Kevin Thomas View Post
11 years later and I'm JUST following up on your advice in another vehicle. I didn't want to lose this info so I'm posting in it for easy find.

I just happen to be searching through the internet on intake pipe sizing and somehow came across this. I'm working on testing a 3.5" to 3" pipe vs straight 3" pipe at the dragstrip next week. Thanks!

I'm GUESSING that the air velocity is going to go to **** on the 3.5 inch tubing. From my road dyno runs, I have found the best combination on my setup (2.5 liters) is quite near the diameter of the stock throttle body. I used 2.25" ID tubing, about 13 inches long. Best intake I've ever run.
T Wrex--OH is offline   Reply With Quote
Old 03-25-2013, 04:06 PM   #15
T Wrex--OH
Scooby Specialist
 
Member#: 179478
Join Date: May 2008
Chapter/Region: MWSOC
Location: Cleveland<OH>
Vehicle:
2000 Impreza 2.5 RS
Sedona Red Pearl

Default

I used this informative article:

http://www.team-integra.net/forum/bl...culations.html

...To calculate my 'optimal' intake tube. Part III of that article deals with how to calculate that.

It's fun, try it out!
T Wrex--OH is offline   Reply With Quote
Old 03-25-2013, 10:28 PM   #16
Kevin Thomas
Street Racing Instructor
Moderator
 
Member#: 110
Join Date: Jul 1999
Location: 1997 OBS, 1996 SVX, 1988 RX
Vehicle:
1989 1989 XT6

Default

Quote:
Originally Posted by Patrick Olsen View Post
Nope, you got it backwards. Long runners are good for low end torque, short runners are good for top end power. Other than that mix-up, I think everything else you said makes good sense.

Pat Olsen
'97 Legacy 2.5GT sedan
I was kicking myself for doing it. I'm always digging through this site and all it's tons of info. Too busy reading to post anymore. 'Sup old head..?
Kevin Thomas is offline   Reply With Quote
Old 03-29-2013, 02:25 PM   #17
Preludicrous
Scooby Newbie
 
Member#: 334356
Join Date: Oct 2012
Chapter/Region: SWIC
Location: Tucson AZ
Vehicle:
2006 2.5i Wagon
SRR

Default

Quote:
Originally Posted by Reciprocity View Post
...A great intake design would be a medium length pipe with the filter element as far away from the throttle body as possible. That way the air has time to gather speed through the intake track and won't be as turbulent (smooth and fast is the way to go). It would also be benificial to have the pipe start out with a large diameter...and get smaller and smaller until it reaches the throttle body.
Is this why we have that "torque box" mounted on the tb? I have always wanted to crack that thing open, but I would guess from Reciprocity's statement that it is simply chambered to constrict airflow as is passes through.

I have been thinking about redoing my custom intake and after reading this thread have realized that I may have been thinking in reverse. I currently have a 3" pipe running from maf to torque box, maybe replace that with a 2.5" and extend a 3" pipe from before maf into the fender?

heres a pic
Preludicrous is offline   Reply With Quote
Old 04-01-2013, 11:21 AM   #18
Charlie-III
Scooby Specialist
 
Member#: 30669
Join Date: Dec 2002
Chapter/Region: Tri-State
Location: 07456, North NJ
Vehicle:
1998 Legacy 2.5GT
Silver Sleeper Wagon

Default

Pat, I had the same thought.....LOL.....

Pre, the torque box is partly there to allow a mass of air for rapid throttle opening. Since air has weight to it, it takes a little bit to accelerate and the box helps out initially. The little "arms" on the older OEM intakes also helped with resonant tuning at different RPM's. Just running a straight pipe loses a lot of this tuning and can make for a very narrow power band, not what you want unless you're drag racing.

Dealing with a MAF can make things even worse if the flow through the MAF is not close to what OEM was. If it's turbulent, the MAF may not account for all the air which can lead to running lean. A little lean can make power, a lot lean can kill engines.
Charlie-III is offline   Reply With Quote
Old 04-01-2013, 11:29 AM   #19
Kevin Thomas
Street Racing Instructor
Moderator
 
Member#: 110
Join Date: Jul 1999
Location: 1997 OBS, 1996 SVX, 1988 RX
Vehicle:
1989 1989 XT6

Default Follow UP

Hey guys,

I followed up on this and posted my findings in a RAV4 forum. http://www.rav4world.com/forums/99-4...dragstrip.html

In a nutshell, I kept opening up the diameter of the cold-air piping by .5" every time it encountered a bend. This was to help keep exhaust velocity high and to try not to loose any high end horsepower. In the end, what I found out is that you can gain some extra hp by opening up your cold air intake piping slightly. It increased my 1/4 mile mph by 1.




DO NOT take my findings as pure fact. As I always have said over the years, please test these theories for yourself. I was never one to argue over ideas. I simply show the results and keep moving.



Preludicrous/Charlie,

As for the torque-box above, here is something you could try. From the maf sensor to the torque box, go with piping that is .5 to .75" wider than your maf sensor. This way, you will retain some of that off-idle air that the torque box provides but you don't compromise intake air velocity at higher rpms. With the torque box, when air goes through the maf and into the torque box, it slows down before speeding back up as it goes through the throttle body. What does that all mean? Hard to say without a dyno or some other measurement to show. I do read that you always want air velocity to be smooth and high (Be it on the intake side or on the exhaust side).

Last edited by Kevin Thomas; 04-01-2013 at 11:50 AM.
Kevin Thomas is offline   Reply With Quote
Old 04-01-2013, 12:02 PM   #20
Charlie-III
Scooby Specialist
 
Member#: 30669
Join Date: Dec 2002
Chapter/Region: Tri-State
Location: 07456, North NJ
Vehicle:
1998 Legacy 2.5GT
Silver Sleeper Wagon

Default

Kevin, I agree you want high velocity air. Part of why I "think" a light sanding of the inner wall of the metal intake tube is better. Something like 100-200 grit, just to kill the "smooth inner wall" a bit to reduce surface drag. Since the wall is now a little rough, it's a thin layer of turbulent flow which should actually reduce frictional losses. This can also help keep the airflow attached during bends in the pipe.

Yes, testing is the way to go to get a better idea.

I just get leery changing flow/direction pre-MAF unless you can really watch what comes out of the engine (is it lean or not).
I know my car is on the rich side, so a "little" leaning is OK, as long as it's consistent.

Frankly, except for trying to get colder air, I don't think a stock Subaru intake is much of a restriction (for the full powerband) unless it's really built.

[I actually want to make a small velocity stack just before the MAF in the stock airfilter box......just to help at higher RPM's.]
Charlie-III 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

Similar Threads
Thread Thread Starter Forum Replies Last Post
perrin cold air intake pipe and filter ( ebay link ) Evo killerz Engine/Power/Exhaust 0 10-09-2008 04:34 PM
Short ram intake with cold air intake pipe? fastwrx07 Newbies & FAQs 16 05-22-2008 12:20 AM
FS: APS Cold air intake pipe deathinacan Engine/Power/Exhaust 35 01-21-2008 02:02 PM
AEM cold air intake or Injen cold air intake? energy57 Factory 2.0L Turbo Powertrain 31 05-20-2002 12:27 AM
Results of the cold air intake test Reciprocity Normally Aspirated Powertrain 31 04-05-2002 11:58 PM


All times are GMT -4. The time now is 12:19 AM.


Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2014 Axivo Inc.
Copyright ©1999 - 2014, North American Subaru Impreza Owners Club, Inc.