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Old 09-30-2007, 11:24 PM   #1
STiBottom
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Default Building a Custom Intake Manifold

So I got to reading and am interested in trying to build my own manifold. I have some questions for those who have built their own already.
-With the design of the throttle body being where it is... is it necessary to route the runners to be completely equal length?
-If they have to be equal length would it be possible to pitch the whole assembly toward the front of the car to make the rearward cylinder runners start "higher" to compensate for the natural unequal length of the design?

Thanks for the insight,
Michael
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Old 09-30-2007, 11:28 PM   #2
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Interesting - Subscribed.

- No input from me but I would like to see if people have done this.
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Old 10-01-2007, 12:05 AM   #3
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Scratch building an intake manifold is a bit more complicated than it looks.
Anyone can build one the question is will it perform as well as or better than the stock manifold.

Issues to consider:
Strength of the plenum -- custom manifolds for boosted applications need to give careful consideration to the pressures built up inside the plenum during high boost and on occasion you have a backfire into the manifold. If you are running high boost (say 24 psi) and have a roof area of 30 sq inches (5x6 inches) there will be 720 lbs of force trying to blow the top of the plenum off. Plus you have the constant flexing of the plenum due to pressure changes. Many of them have problems with cracking due to these two causes.

Runner length can have major effects on off boost performance due to ram and resonance effects. Longer runners move peak torque down, and shorter runners move it up the rpm band. Equal length runners give a sharper effect than runners that are slightly staggered in length. Length changes of less than 2 inches typically have very small effects, so that is all the accuracy you need regarding runner length.

The effect of resonance and ram effects under boost is widely debated so read up and make your own choice.

Runner diameter can also effect manifold behavior. Large diameter runners make for high rpm power but sluggish low speed response as it takes longer for air speeds to build up in the larger volume runners.

Last is even if it looks good the dyno and ET may not like it, so you will need to test test test to get the best out of it. Even very small changes to runner entry radius and location can have huge effects on cylinder balance etc. The difference in flow between a sharp cut off tube and a properly formed radius can be as much as 17%.

An EFI engine at least does not need to worry about wet flow like a carburated engine but if you start to use water injection then that also becomes an issue.

Good luck if you choose to try it, there are lots of things to read up on this topic so get friendly with google!!

Larry
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Old 10-01-2007, 12:20 AM   #4
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larry, I like the last bit, "get friendly with google"...
haha
I'll try and find the proper title to one of my manifold design books, as it was the best one of the several I've read. I'll post it up later
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Old 10-01-2007, 02:15 AM   #5
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Quote:
Originally Posted by hotrod View Post
Scratch building an intake manifold is a bit more complicated than it looks.
Anyone can build one the question is will it perform as well as or better than the stock manifold.

Issues to consider:
Strength of the plenum -- custom manifolds for boosted applications need to give careful consideration to the pressures built up inside the plenum during high boost and on occasion you have a backfire into the manifold. If you are running high boost (say 24 psi) and have a roof area of 30 sq inches (5x6 inches) there will be 720 lbs of force trying to blow the top of the plenum off. Plus you have the constant flexing of the plenum due to pressure changes. Many of them have problems with cracking due to these two causes.

Runner length can have major effects on off boost performance due to ram and resonance effects. Longer runners move peak torque down, and shorter runners move it up the rpm band. Equal length runners give a sharper effect than runners that are slightly staggered in length. Length changes of less than 2 inches typically have very small effects, so that is all the accuracy you need regarding runner length.

The effect of resonance and ram effects under boost is widely debated so read up and make your own choice.

Runner diameter can also effect manifold behavior. Large diameter runners make for high rpm power but sluggish low speed response as it takes longer for air speeds to build up in the larger volume runners.

Last is even if it looks good the dyno and ET may not like it, so you will need to test test test to get the best out of it. Even very small changes to runner entry radius and location can have huge effects on cylinder balance etc. The difference in flow between a sharp cut off tube and a properly formed radius can be as much as 17%.

An EFI engine at least does not need to worry about wet flow like a carburated engine but if you start to use water injection then that also becomes an issue.

Good luck if you choose to try it, there are lots of things to read up on this topic so get friendly with google!!

Larry
So would you suggest that stock is better than many of the one-off jobs we're seeing floating around the forums?
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Old 10-01-2007, 06:53 AM   #6
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Quote:
So would you suggest that stock is better than many of the one-off jobs we're seeing floating around the forums?
No but the likelyhood of building a one off with limited testing and significantly improving over a lightly ported stock manifold is not great.
It all depends on your budget and willingness to experiment. Modern cylinder heads and intake manifolds are good enough that the huge gains from a couple hours work are a thing of the past.

A short runner large plenum manifold is almost sure to out perform the stock manifold in the upper rpm range, but that could range from only a few percent more power to a significant improvement. There are proven manifold designs that could be easily copied (or at least their principles of construction since many of them come from other engine design) and you will most likely have a better manifold but I just want to remind folks that even the manufactures occasionally turn out a manifold that sucks even after dropping a bunch of dollars on R&D. That is why folks still port and improve even after market performance manifolds after doing flow testing.

My engine might love your new manifold and your engine might hate it due to other parts of the package (cams, tuning, turbo, etc. etc.)

You certainly can't argue with the results of Andy's custom manifold for example.
http://forums.nasioc.com/forums/showthread.php?t=831023

But you can be sure he spent some time planning the details of the design.

Larry
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Old 10-01-2007, 08:23 AM   #7
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Thanks for the info man and the name of that book would be awesome.

I was not going to rush into this job without speaking with numerous people. In my formula sae team at school all parts are custom and hand made (including the manifold) using a GSXR 600 motor. The one used last year worked really well and it just got me thinking that it would be a nice addition to my car.
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Old 10-01-2007, 08:58 AM   #8
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ah, another FSAE guy...did you do the engine team and manifold designs? what team?...if you engine guy actually did his homework and design, he should be able to give you at least one spreadsheet for runner length and diameter. If you are one of the wealthier schools you probably have recardo wave program and virtual 4-stroke.

where are you looking to make your main power gains? imagining mid to top end...
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Old 10-01-2007, 01:55 PM   #9
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Quote:
Originally Posted by hotrod View Post
No but the likelyhood of building a one off with limited testing and significantly improving over a lightly ported stock manifold is not great.
It all depends on your budget and willingness to experiment. Modern cylinder heads and intake manifolds are good enough that the huge gains from a couple hours work are a thing of the past.

A short runner large plenum manifold is almost sure to out perform the stock manifold in the upper rpm range, but that could range from only a few percent more power to a significant improvement. There are proven manifold designs that could be easily copied (or at least their principles of construction since many of them come from other engine design) and you will most likely have a better manifold but I just want to remind folks that even the manufactures occasionally turn out a manifold that sucks even after dropping a bunch of dollars on R&D. That is why folks still port and improve even after market performance manifolds after doing flow testing.

My engine might love your new manifold and your engine might hate it due to other parts of the package (cams, tuning, turbo, etc. etc.)

You certainly can't argue with the results of Andy's custom manifold for example.
http://forums.nasioc.com/forums/showthread.php?t=831023

But you can be sure he spent some time planning the details of the design.

Larry
Hmm... definitely food for thought! Thanks
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Old 10-01-2007, 02:09 PM   #10
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Good ole Virtual 4 stroke. I accidentally kepts a copy of it from my FSAE days (Oregon State). It really does open your eyes to how changing engine parameters affects the dyno curve. There's cheaper programs out there that will give similar results. I have a bunch of spreadsheets that will help with runner design. I'll see if I can dig them up.
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Old 10-01-2007, 04:53 PM   #11
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I am at southern polytechnic down in Ga. We build everything. I am sure i can talk to the guy that made the one from last year and see what he has to say. As for moving the power curve I would be interested in moving it toward having more top end and less down low. I am thinking about doing a mock up on autocad that way it will be easier to deal with in the build. Oh and on another note. GT (Georgia Tech) does not build their car haha.
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Old 10-01-2007, 06:27 PM   #12
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A good place for you guys to start if you don't have access to advanced automotive engineering text books is:

Scientific Design of Exhaust & Intake Systems (Engineering and Performance) (Paperback)
by Philip H. Smith (Author), John C. Morrison (Author)

It is a book you will read and re-read many times, and it discusses the intake exaust cycle and what is going on without cluttering the discussion up with a bunch of advanced math. There are several virtual engine dyno simulation programs that are also nice to play with.

Another general readership book is:

Practical Gas Flow: Techniques for Low Budget Performance Tuning
ISBN: 1855205645

Larry
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Old 10-01-2007, 07:33 PM   #13
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Thanks for the info man and the name of that book would be awesome.

I was not going to rush into this job without speaking with numerous people. In my formula sae team at school all parts are custom and hand made (including the manifold) using a GSXR 600 motor. The one used last year worked really well and it just got me thinking that it would be a nice addition to my car.
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Old 10-01-2007, 10:18 PM   #14
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I just ordered those books on Amazon Larry haha. I also saw another one that caught my eye in the "People who bought this also bought this" section on Engine Management so I have my readings for the next couple of weeks I guess.

Thanks
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Old 10-01-2007, 10:49 PM   #15
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I just noticed that my above post could be missread.

"There are several virtual engine dyno simulation programs that are also nice to play with."

There are no virtual engine dyno programs in that book! That was a generic comment that on the web you can find several virtual engine programs that are fun to play with and for some engines at least are very very good at predicting engine performance.

If I ever get off my butt I will probably pick up a copy of Dynomation Advanced With ProTools to play with but keep putting it off as it is a $500 package.

Larry

Last edited by hotrod; 10-01-2007 at 10:58 PM.
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Old 10-04-2007, 01:00 PM   #16
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Search!

Almost every answer is in this thread with tons of pics.

http://forums.nasioc.com/forums/show....php?t=1227220

Bill


For a graph of all of the resonance frequencies on an STI with stock cams:
http://forums.nasioc.com/forums/show...&postcount=143
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Old 10-04-2007, 01:07 PM   #17
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If you are looking for some practical experience on performance/problems with a custom intake, contact www.DentSport.com as they have custom fabricated several for their racing STIs.
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