Intake manifold physics

rakkasan

rakkasan

Currahee!!
Mar 31, 2005
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j3pz;906811 said:
i started this thread hoping someone could tell me the physics of the different intakes and so everyone could benefit from my question. i learned some from the recent ffim threads about short runners and long runners. im mostly wondering about the plenums volume and shape. thanks in advance.
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Let me ask this first if you will. What are your intentions/what are you trying to do?
 
M

MKIIINA

Destroyer of Turbos
Mar 30, 2005
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i would actually like a full indepth discussion on this topic as i have a 10k foot view of how the manifold and air volume works but i would like something a bit more detailed.... in!
 
I

IJ.

Grumpy Old Man
Mar 30, 2005
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I come from a land down under
I've made 5+ of my own each time quite different in design to get a feel for what each change actually does as I could find any solid info on this after searching for hours and hours. (it's all a bit "black art")

Rough rule of the thumb

Shorter runners move the tq peak up

Longer runners move it back down

Smaller Plenum makes the boost hit hard

Larger Plenum softens the hit

This is the gumby version and each combination will be different.
 
Tire Shredder

Tire Shredder

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Sep 15, 2005
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to add to IJ's post...

large plenum = more torque but you can only make it so big....throttle responsoe goes down with a larger plenum aswell. At least for an NA....I would imagine it would have similar effects on a turbo car.

the less turns and sharp edges the air has to navigate the better....but it's not as critical as you may think in a turbo motor. The high pressure (boost) always finds the low pressure point. often when people say "that manifold looks like junk" because it's a box or whatever...actually aren't that bad.
 
Quin

Quin

Trans killer
Dec 5, 2006
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I always thought shape didn't make a whole lot of difference in a boosted app, but made all the difference in the world n/a.

Everything else I know I learned from IJ and he already posted.
 
T

themadhatter

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Jul 5, 2006
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Let me sugest a good book i think everyone should read its called "How to tune and modify engine management systems" and its by Jeff Hartman it has a short but good chapter on manifold design in the book but the book covers a lot of interesting subjects about EFI and is a very good read.
 
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funky_monkey58

Closing in on 200+MPH
Apr 3, 2006
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themadhatter;907349 said:
Let me sugest a good book i think everyone should read its called "How to tune and modify engine management systems" and its by Jeff Hartman it has a short but good chapter on manifold design in the book but the book covers a lot of interesting subjects about EFI and is a very good read.
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I have a copy of that book if anyone would like info from it.

-Sean
 
T

themadhatter

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Was it me or is the book a really well writen book and very interesting? Also on page 13 theres an awsome pic of a twinturbo 7m lol
 
Z

ZFast300Z

Trouble
Dec 15, 2007
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Small plenum's averaged around 1/2 engine CID, in displacement. Large plenum's displaced in the range of 1-1.5times engine CID in displacement.

For a force-fed application, runner design is less of a factor than for a N/A design. Plenum volume is the key---the rule of thumb is plenum volume >= engine volume when force-fed. A large plenum has a few draw backs though so it is not the magic bullit either. A large plenum breathing through a single butterfly will have a lag between throttle opening and throttle response will be off key.

Basically, there is no easy recipe for intake design. There are some rules of thumb though:
long thin runners=low end torque
short fat runners=high RPM power
plenum volume=engine volume for superchared applications but there is no perfect solution nor is there an easy formula to determine plenum volume

Then you have selection of difference designs for the plenum. Simple examples are log, tapered log, box, and tapered box. I suggest to stay away from any for of fancy intake manifold design (Like this ebay one, http://cgi.ebay.com/ebaymotors/NEW-...024QQitemZ370018550129QQrdZ1QQsspagenameZWDVW
unless you have access to a flow bench and can verify that your cfm per runner is the same.
 
D

Doward

Banned
Jan 11, 2006
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j3pz;908357 said:
thanks for the info guys. does anyone else wanna add anything?
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Meh, what the hell.

Intake design is governed by fluid dynamics. (Air is a fluid)

Take into consideration, these things:

Pressure does not just build everywhere, at once. A large plenum softens the hit when you go into boost, because it takes a little longer to fill that plenum.

Short runners build hi-rpm power, because every time the intake valve shuts, it sends a pulse wave back behind it, back up the runners. I just woke up, so I'm not going to get into pulse harmonics, but you can look it up and ask questions if you want ;) This is the basis of the ACIS system, btw. Time it so that the pulse is reflected back down the runner, just in time for the valve to be opening, giving you that 'supercharging' effect.

Long runners build low-rpm power, because you maintain get that pulse reflection at a much slower speed.

One thing I've haven't seen hit, is runner volume:

Small volume runners maintain a higher initial fluid velocity, letting you better fill the combustion chamber at lower rpms. As rpms increase, however, the smaller runners become a restriction.

Large volume runners maintain a slower initial fluid velocity, (reducing VE, and power, down low) but are less of a restriction in the higher rpms.

Remember that 'down low', 'higher rpms', 'lower rpms', etc are all very much RELATIVE. Optimally, you would build an intake that would maximize what your engine has.

That's what I'll be doing down the road, anyway.
 
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