Welcome to the North American Subaru Impreza Owners Club Tuesday September 2, 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 03-27-2009, 03:50 AM   #1
Back Road Runner
Scooby Guru
 
Member#: 60082
Join Date: Apr 2004
Chapter/Region: MWSOC
Location: Minnesota
Vehicle:
2004 Forester STI
Silver

Default Looking at the intake manifold - by the numbers

This is just for debate, but it's also something to think about.

Basically the intake manifold is equivalent the the exhaust header. You have the same primary piping, pipe diameter, pipe length, and a secondary collector area, the plenum. It functions similarly in that it controls power band and flow.

Useful length covering basic design and equations:
http://mgcgti.com/Page35.html

Intake pipe diameter for a given engine displacement and rpm range:


The number we get for a 2.5L engine that revs to 6500 rpm is 2.04" (internal diameter). The stock intake uses 2.5" (outer diameter) piping and is sufficient for this flow capability, even with much higher revs (8k).


Intake length:
L = 7 + 1.7*10000/RPM

Shooting towards peak 6500 rpm, we get a short length of just 9.62"
Shooting for mid rpm of 3000 rpm, we get 12.67"
For reference, idle at 750 rpm we get a lengthy 29.67"

I'll note the stock intake is rather lengthy and tuned near idle. However, the torque box is a large air storage area, so I don't think there's much in the case of tuning outside of the box. A CAI setup on the other hand may gain influence. The length given seems to start at the plenum and ends wherever it ends, at the throttle body. In stock form, this seems to be rather short since the torque box is the end of this tuned section. Wave tuning is terminated by a change in pressure, generally a large chamber, like the plenum on the intake or a resonator or large cat in the exhaust after the collector. It would seem throttle body spacers would affect this tuned section. Personal experience with the use of a TB spacer suggests no change when using the stock torque box, at least not perceivable.


Plenum volume:
2500-3500 rpm : 80%-90% engine displacement
5000-6000 rpm : 50%-60% engine displacement
7000-7500 rpm : 35%-50% engine displacement

Engine displacement is 2.5L or 2500cc. The stock plenum is around 700-800cc, just 30%. A throttle body spacer adds around 80cc to this, not much. A big thing noticed here is the very lack of air storage for the intake runners.

In this sense, I'm kind of curious if the Subaru intake manifold plenum acts more like a runner extension, and sort of moves the plenum out to the air box in a sense. There is some fault in this though. If functional in this sense, one would assume similar changes to the torque curve from both intake manifold spacers and throttle body spacers. I've personally done both at separate times and had no effect with the throttle body spacer other then smoother power delivery which is appropriate with a bigger plenum size. The intake manifold spacer did however bump up low end torque as it should. This tends to indicate that the parts still act normally. Still, the small plenum may be an indicator of why the torque box is infamous for maintaining low end torque. There have been butt dyno and real dyno accusations leaning both ways, so it's tough to say. General observation points to a rather small intake plenum that is less than adequate, at least from a numbers stand point.

Intake runner length:


This comes down to which reflections you want to use. Earlier reflections have some usable energy, but the runner lengths get so long, it's not practical. Up to 10 psi intake boost pressure can be created.

Generally, more then one reflection will be used throughout the rev range. The stock intake is somewhere in the neighborhood of 20", a rough, rough guess. At 6500 rpm this puts us near the 2nd reflection. We hit the 3rd reflection at 5000 rpm, 4th reflection at 4000 rpm, and so on.

Pulse strength is 10% for the 2nd reflection, 7% for the 3rd, and 4% for the 4th.

It would be fun to use the primary pulse, but we'd need a runner length of 50" to do so, not practical at all.

When looking at the early reflections, a minor rpm focus change of just 500rpm, say 4k to 4.5k peak, requires a solid 4" of change at the 2nd reflection and still 2" at the 4th reflection. This tends to indicate that the runner length isn't terribly sensitive to length. For example, with a 8mm spacer, one wouldn't see a shift of even 100 rpm in any thing short of the 5th reflection. Interestingly, although the peak doesn't shift, the tilt of the curve seems to shift considerably, and this doesn't really seem to be represented by the calculations in any way.

Runner diameter:


In terms of runner diameter, a change of 0.1" does shift peak rpm from 4k to 4.5k, a good bit more sensitive to change then length. It is also said that this is the average diameter, so even flaring ends for flow will slightly increase the effective diameter.

Interestingly, runner diameter seems to equal intake pipe diameter. For full engine flow at 6500 rpm, you're looking at an intake pipe of around 2". Runner calculators will also indicate 2" too. I guess it assumes full system flow through one cylinder path.

In stock trim, peak torque sits at right around 3500 rpm. This would indicate an average runner diameter of 1.5". I wouldn't be surprised if this is what it is stock.
* Registered users of the site do not see these ads.
Back Road Runner is offline   Reply With Quote
Old 03-27-2009, 06:46 AM   #2
Storm
Scooby Guru
 
Member#: 5218
Join Date: Mar 2001
Chapter/Region: MWSOC
Location: SAUL'S Motorsports
Vehicle:
96L Most Over-
Developed Beater

Default

What is the generally accepted VE % that people are using for the EJ series motors?

Jay
Storm is offline   Reply With Quote
Old 03-27-2009, 10:05 AM   #3
Skidd
Scooby Specialist
 
Member#: 1853
Join Date: Jul 2000
Chapter/Region: TXIC
Location: SA-TX (Hozer expat)
Vehicle:
GC6 Supercharged
STM

Default

When I was crunching the numbers for my supercharger project, I did a bunch of searches back then on both here and rs25. Obviously #s were all over the map, but the average number I found was a VE of 90%. Which isn't too bad. When I used this VE in some of my various other calculations, it seemed to support them too. So IMO, 90% represented a reasonable guess.
Skidd is offline   Reply With Quote
Old 03-27-2009, 01:15 PM   #4
Back Road Runner
Scooby Guru
 
Member#: 60082
Join Date: Apr 2004
Chapter/Region: MWSOC
Location: Minnesota
Vehicle:
2004 Forester STI
Silver

Default

85% is the average normally used. It will depend upon engine and setup as to what the rear efficiency actually is. Highly tuned race engines will be in the 90s, consumer vehicles are down in the 80s. As well, the efficiency isn't constant. It will vary throughout the rpm range.
Back Road Runner is offline   Reply With Quote
Old 03-27-2009, 01:31 PM   #5
formula91
Scooby Specialist
 
Member#: 102984
Join Date: Dec 2005
Chapter/Region: MWSOC
Location: Northbrook, IL
Vehicle:
2005 2.5RS 4EAT
Platinum Silver

Default

What are reflections? Does that relate to the wave tuning you mentioned earlier? I'm not exactly an expert on fluid physics so I'm not sure what this means, but if you would enlighten me, that would be great.

The last part mentions that for full engine flow, 2" intake piping would be sufficient. What does this mean for most people who have intakes at 3 " diameter? Aside from the changes in the intake volume which may be good for low end, would this mean that peak torque has been shifted very high?

I'm not very good at this. lol
formula91 is offline   Reply With Quote
Old 03-27-2009, 02:54 PM   #6
Skidd
Scooby Specialist
 
Member#: 1853
Join Date: Jul 2000
Chapter/Region: TXIC
Location: SA-TX (Hozer expat)
Vehicle:
GC6 Supercharged
STM

Default

Quote:
Originally Posted by Back Road Runner View Post
Highly tuned race engines will be in the 90s
According to Corky Bell, this number can reach 100%.
Skidd is offline   Reply With Quote
Old 03-27-2009, 03:09 PM   #7
Zefy
Scooby Specialist
 
Member#: 105104
Join Date: Jan 2006
Chapter/Region: VIC
Location: Coquitlam, BC, Canada
Vehicle:
1979 BRAT / 01 RSTI
99 2.5TS (DEAD) / 87 GL

Default

good write up you got going. i'm currently researching plenum and intake design for my project. interested to see what everybody thinks.
Zefy is online now   Reply With Quote
Old 03-27-2009, 03:35 PM   #8
Storm
Scooby Guru
 
Member#: 5218
Join Date: Mar 2001
Chapter/Region: MWSOC
Location: SAUL'S Motorsports
Vehicle:
96L Most Over-
Developed Beater

Default

I know the VE can go up to and beyond 100% even in NA form, as prostock motors typically reach VE levels as high as 110-120% in a narrow rpm range, effectively supercharging itself. I was just curious what numbers most people use for a "run of the mill" EJ25 or even EJ22 with stock cams. 85% seems logical, if not conservative.

Good stuff Back Road Runner!!!
Jay
Storm is offline   Reply With Quote
Old 03-27-2009, 04:32 PM   #9
Calum W.
Scooby Newbie
 
Member#: 181932
Join Date: Jun 2008
Chapter/Region: E. Canada
Location: Middle Sackville, Nova Scotia
Vehicle:
2008 Impreza 2.5i

Default

This page is almost word for word taken from David Vizard's How to Build Horsepower books. http://www.grapeaperacing.com/
http://www.grapeaperacing.com/tech/inductionsystems.pdf

You have to take some of this with a grain of salt, and/or think broader spectrum. If you use the intake, upstream of the throttle body to help tune the motor, you're going to raise horsepower at certain RPM's, but lower it at others. If you make the intake big enough that it doesn't really act as a helmholtz resonator then you'll broaden your power band slightly. Your application and gearing will dicitate which is better. Similarly, going with individual throttle bodys over a single throttle & plenum will broaden your power band and lower the peak power.

Using runners tuned to the first wave is often counter productive. The length of the runner and the packaging required to fit it under a hood often means there's so much added restriction that it ends up lowering power.

The rest of your assumptions and research seem accurate, though I haven't confirmed the math.

Take a look at what the NA cars are running any form of 4cyl racing. Typically the runners are pretty short, way shorter than would be effective for a street driven car as they are tuned for peak power at a much higher RPM, but the proof of the theory is still there.

Here's a couple pics of the intake that's going on my street/strip car. Peak power should be around 7000 rpm. The intake running in the head is right around 3.5 inchs long.

Calum W. is offline   Reply With Quote
Old 03-27-2009, 04:57 PM   #10
Back Road Runner
Scooby Guru
 
Member#: 60082
Join Date: Apr 2004
Chapter/Region: MWSOC
Location: Minnesota
Vehicle:
2004 Forester STI
Silver

Default

That's a good read Calum. It's basically the same info I've come across.

I'm not specifically knowledgeable with the VE range. We're generally looking at an air pump. 100% VE means we get exactly the the displacement of the engine. We generally get less due to flow restrictions. +100% indicates we actually shove more in then the actually volume displaced, requiring compression of the air above ambient. This isn't easy to do and becomes very specific. I can see why it's achievable in racing because the band width generally is more narrow.

Pressure wave tuning is the same as you find in musical instruments. It's a resonance in the system. The engine runs in pulses, not constant. What you end up with is a vibration, kind of like what your lips would do playing a trumpet. The air flow isn't constant. In fact, if it was constant, there'd be no resonance and any pressure waves to work with. We tune the pipe length and size to resonant and reflect the waves back and forth. When these pressure waves line up with the opening of the intake port, we get this extra shove of higher pressure air into the cylinder. Do it right, and you effectively compress and supercharge the intake in a sense. You get more molecules into the combustion chamber then normal at a higher pressure level. VE effectively goes above 100%. You may see this in a highly tuned race engine, but it's not something you'll see in a normal consumer vehicle. It is something we do like to aim for though, because efficiency is always good. In the case of an engine, this may mean a narrower bandwidth of power though, kind of a jack of all trades thing versus a specialist.

None of this info is new or amazing. Many of the books on this subject are pretty old actually. There are a ton of web sites, threads, etc. that cover this math, theory, and design ideas. I bring it up because the intake is largely overlooked. We focus on the exhaust side a bit with aftermarket headers, but we don't do much on the opposite end. Partially, we just don't have aftermarket options and we only have a few choices to tune the intake some through the use of spacers and P&P work, and that's about it.

Now it's important to realize that the intake is only one part of the whole system. The heads, the cams, valves, pistons, and exhaust all still play roles in the system as a whole.

Last edited by Back Road Runner; 03-27-2009 at 05:27 PM.
Back Road Runner is offline   Reply With Quote
Old 03-27-2009, 06:36 PM   #11
Calum W.
Scooby Newbie
 
Member#: 181932
Join Date: Jun 2008
Chapter/Region: E. Canada
Location: Middle Sackville, Nova Scotia
Vehicle:
2008 Impreza 2.5i

Default

That intake in my pics is based off of a stock intake. The runners are ported and polished, cut to about 2/3 of the original length, and a new & much bigger plenum was added.

There's no reason something similar couldn't be done with a suby intake. You could either cut the runners down and creat two plenums and run two throttle bodies. Doing this on a MAF engine wouldn't work so well, but on a MAP based engine you can just run a vacumm hose from each plenum, tee them together and run them to the MAP sensor.

Or you could just cut the runners down, put them back on an engine as a jig, and make a larger plenum.

Of course for either option you'd need to do your math and plan ahead. Knowing the duration of the cams you'll be running will help alot.
Calum W. is offline   Reply With Quote
Old 03-27-2009, 07:44 PM   #12
Back Road Runner
Scooby Guru
 
Member#: 60082
Join Date: Apr 2004
Chapter/Region: MWSOC
Location: Minnesota
Vehicle:
2004 Forester STI
Silver

Default

I'd love to be able to weld and just build a custom one.

One thing I'm curious about the design is the intake pipe prior to the plenum. This is where the throttle body is located. This is supposed to be a tuned piece, but the TB sits smack dab in the middle. If the pipe is supposed to be tuned to 12", should the TB be placed 12" away from the plenum? I'm not really sure how much the butterfly affects the wave tuning or if the pressure waves mainly care about just the end points. I could see it not being a major issue at WOT, but at partial, the valve would be in the way and create it's own pressure change.
Back Road Runner is offline   Reply With Quote
Old 03-30-2009, 10:17 PM   #13
williaty
Scooby Guru
 
Member#: 71092
Join Date: Sep 2004
Chapter/Region: MWSOC
Location: Delaware County, Ohio
Vehicle:
2005 2.5RS Wagon
Regal Blue Pearl

Default

Mention was made of shortening the intake runners and bifurcating the plenum. Here's one from a GC8:

williaty is offline   Reply With Quote
Old 03-31-2009, 02:36 AM   #14
Back Road Runner
Scooby Guru
 
Member#: 60082
Join Date: Apr 2004
Chapter/Region: MWSOC
Location: Minnesota
Vehicle:
2004 Forester STI
Silver

Default

That's pretty neat. Pardom my ignorance, but what's the attached hardware do?
Back Road Runner is offline   Reply With Quote
Old 03-31-2009, 02:48 AM   #15
williaty
Scooby Guru
 
Member#: 71092
Join Date: Sep 2004
Chapter/Region: MWSOC
Location: Delaware County, Ohio
Vehicle:
2005 2.5RS Wagon
Regal Blue Pearl

Default

It's a wet NOS system. Well, it started as a wet spray, but apparently he's decided to run it dry as he has 3200cc/min per cylinder of injectors on it. It's an RSTi with a innertube-sized turbo, air-to-water intercooler, ~200 shot of NOS, pure meth fuel, and sequential tranny. Before they got the boost dialed in, it was running low-9s, IIRC. I don't think it's been back to the track since it was finished.

Last edited by williaty; 03-31-2009 at 02:57 AM.
williaty is offline   Reply With Quote
Old 03-31-2009, 03:21 AM   #16
HamFist
Scooby Specialist
 
Member#: 2112
Join Date: Aug 2000
Chapter/Region: MWSOC
Vehicle:
2000 Impreza 2.5RS
BRP

Default

Quote:
Originally Posted by Back Road Runner View Post
That's pretty neat. Pardom my ignorance, but what's the attached hardware do?
It makes people ask questions . Good thread BRR.

I purposely use stock airboxes because it maintains the lowend torque, dyno proven. There are mysteries in these stock parts, dude. I don't know why or how, but I get the best results still playing with modded stock stuff. That's not to say there's not better stuff out there, but stock parts get so overlooked its not even funny.

I think there is also a nasty resonance fight inside the plenum. Maybe that torque box gives the pulses a place to sort things out? The even spaced crank creates even pulses in the intake tract. However, the grouping looks like it kinda fights back and forth. Why did the stock airbox make flat torque on my dyno runs vs. my velocity stack that killed midrange power?

Last edited by HamFist; 03-31-2009 at 04:18 AM.
HamFist is offline   Reply With Quote
Old 03-31-2009, 02:56 PM   #17
Back Road Runner
Scooby Guru
 
Member#: 60082
Join Date: Apr 2004
Chapter/Region: MWSOC
Location: Minnesota
Vehicle:
2004 Forester STI
Silver

Default

That is my curiosity. Because the stock plenum is small, I'm curious of the stock airbox acts as a secondary point to do the work the plenum should do. It's just that it should be considerably larger, as in 50%-300% larger depending on rpm focus, especially for lower rpms. The stock airbox sort of becomes the secondary candidate. If that is actually what happens, I don't know.
Back Road Runner 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
wanted: anyone have the stock oem bolt that holds the red intake manifold on the head 949 SCIC Private Classifieds 3 07-30-2009 09:30 PM
The Sti intake manifold or the Legacy intake manifold Robson Texas Impreza Club Forum -- TXIC 3 07-12-2009 01:12 PM
blew the back end of a vacuum hose off the intake manifold not sure where it goes... Physics Junkie Factory 2.5L Turbo Powertrain 3 03-15-2005 09:12 PM
Intakes that don't go under the intake manifold scoobyimprza Factory 2.0L Turbo Powertrain 6 08-04-2003 11:11 PM
is the intake manifold hose the same imprezive one Factory 2.0L Turbo Powertrain 0 05-23-2002 09:39 AM


All times are GMT -4. The time now is 09:41 PM.


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.