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hvychvy81 (Automotive) (OP)
18 Sep 06 9:46
Hello all,

I'm currently working on an Individual Throttle body intake for my Car. I was wondering what to use to calculate the optimum runner length for the intake, before the throttle bodies. Better if there was a way to choose which Rpm the power would be coming on full. If anyone has any advice let me know... Thanks in advance, Stephen
Fabrico (Automotive)
18 Sep 06 14:51

Every individual body setup I've seen for racing on-road or off, has runners just long enough to get the bodies nicely situated. In other words, very short.

hvychvy81 (Automotive) (OP)
19 Sep 06 10:20
This link has the best information I've found do far... Let me know what you guys think.
ramblerman (Automotive)
19 Sep 06 15:19
Actually, if I understand hvychvy81's question correctly, this link might be the best one on team integra's website:
Make sure you go to page two of that post, as well.
One source I've come across as I work on modifying my Kawasaki GPz1100 is the MegaSquirt forum. There is an interesting thread on there with good technical input.
The discussion about which pulse you are tuning for is interesting. I'm going to have to research this more.
hvychvy81 (Automotive) (OP)
19 Sep 06 18:19
Well, actually the integra page you posted didnt really point me in the direction i was looking for, but it was some very interesting information. In the page it said that when running ITB's, that you didnt tune it for maximum resonant pulse... I guess what i'm looking for is similar, but different in the same respect. Really looking for how to calculate best performance of runner lengths and velocity stack lengths/sizes.
Greenlight (Mechanical)
20 Sep 06 0:36
Here's a basic formula:

84,000 / tuned rpm = runner length

The runner length is measured from the back of the intake valve to the start of the radiused entry of the runner. Most people use the max. torque rpm as the "tuned rpm".

Also, as a general rule of thumb, the port cross section area should be ~83% of the valve area in the intake port and taper out at a 3.5 degree angle as it goes toward the entry of the runner.

There are more exact formulas that use the compression ratio, engine displacement, and cam timing, but this one is close.
hvychvy81 (Automotive) (OP)
20 Sep 06 8:39
I believe I read that in another thread that was going here not too long ago. Is this equation effective for both full intake manifolds and individual throttle bodies setups? I would think there would be a difference in tuning ability since ITB's do not make use of pulse resonance.
patprimmer (Publican)
20 Sep 06 9:58
ITBs do not make use of pulse tuning? What do you mean by individual throttle bodies. By my understanding they most certainly make use of tuned length effect. Cam timing will have a significant effect on the required length to optimise a specified rpm.


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hvychvy81 (Automotive) (OP)
20 Sep 06 12:44
"Notice that with invididual throttle bodies (ITB's) you lose this resonance effect because the reflected wave escapes out into the engine bay (or the atmosphere) and is not stored and returned by a plenum/acoustic chamber. ITB's do NOT use ram theory to get that extra kick at peak torque because they usually in race form do not have a plenum. In some street ITB's, a plenum is attached for practical reasons (sound deadening and filtering). They rely on very very large amounts of passive cylinder filling based on the piston's effects and use tuned air horn height and tapered diameter (with an S-shaped velocity stack opening) to get the N/A pressure boost effect"

This is the qoute from the team-integra site... that was the info that i was taking into consideration.

I understand that lenth of the runners as well as the lenth, diameter, and volume of the velocity stacks, will be used for tuning the setup. Thats the information that I'm trying to find. What factors do I need to take into consideration when "tuning" the ITB setup? What length of the runners will provide maximum velocity and power, and at which rpm band? How do I calculate the optimum velocity stck length and taper? This is basically what i ment. My apologies if I filed to state my question correctly.

Thank you for the responses. This is great information.
patprimmer (Publican)
20 Sep 06 19:55
I guess you are talking about what American aftermarket calls Stack Injection as is very commonly used on Sprint Cars and NA Injected class drag racing.

Have you looked on sites like Kinsler, Crower, Enderley and Hilbourne (Fuel Injection Enterprises I think).

I can find the URLs and correct spellings later if you can't find them from this.


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Helpful Member!  GregLocock (Automotive)
21 Sep 06 2:18
That quote is utter rubbish. The amount of reflection back from a change in area is proportional to the ratio of the area change. As such, a pipe that terminates into the open air will have a very strong reflection.

This incidentally is why you are not deafened by tailpipe noise.

Incidentally they banned variable length manifolds on F1 cars, which have ITBs, I hardly think they'd bother if they didn't work.


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rpmag (Automotive)
21 Sep 06 8:01
I might suggest that you have a play here:
however also try performance trends engine analyser pro free download, it will allow you to model the cam.
gijim (Electrical)
21 Sep 06 22:50
If I recall correctly, longer intake runners use lower order harmonics to create the "forced induction" effect.  Because there are less reflections, the pulses are stronger, and you get a higher VE at that RPM the runner is tuned at.

For a first order harmonic, the length of the runner is something like 12 feet...  

Now if I understand this correctly, the longer runners are very particular about the rpm range over which they resonate.  That 12 foot runner for instance might only provide that "forced induction" for +/-50 rpm until the second harmonic (rpm) is reached

If that's true, then short runners don't provide a whole lot of the compression effect, but do it over a much broader range.

Anyone care to comment on that conjecture?

I know that saying "short runners are good for high RPM use and long runners are good for low end torque use" is too broad a description...  Just like how you can hit 2 or more notes on a recorder with the same fingering if you blow hard enough.  winky smile
mburgess (Mechanical)
21 Sep 06 23:26
For individual runner length & diameters you can either test away on the dyno or use a resonable simulation, simple formula based of rpm & cam duration are not really relevant. A good wave analysis program that takes into account cam profile, flow data, valve dia, throat dia & throttle body size & type should give you an accurate analysis. I have found dynomation to be good for inlet analysis but crap on exhausts, but for accurate results you need cam maps & flow bench data. The end of the day you will still need to test your results but a good simulation can help get you there.
kitabel (Automotive)
30 Sep 06 11:44
Re: "That quote is utter rubbish. The amount of reflection back from a change in area is proportional to the ratio of the area change. As such, a pipe that terminates into the open air will have a very strong reflection."

I think the author of the quote is confusing recovered energy from a Helmholtz device (pressurized plenum) with simple wave resonance, which, of course, relies for the strength of the pulse the difference in X-sectional area between the tube and the atmosphere (~). IIRC the expension ratio should be at least 625% to use all available energy and avoid any secondary wave causing superposition etc.
SomptingGuy (Automotive)
30 Sep 06 20:07
"the author of the quote" is correct and not confused.

Pressure waves reflect from open ends just as strongly as from closed ends, except that they flip their sign at open ends.
GregLocock (Automotive)
30 Sep 06 20:39
"you lose this resonance effect because the reflected wave escapes out into the engine bay "

No it doesn't. It is rubbish. The reflected wave goes back down the pipe, otherwise it wouldn't be a reflection.


Greg Locock

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358t (Automotive)
17 Nov 06 17:02
Larry Meaux of Meaux Racing Heads has written a program that will calculate the numbers you are seeking and much more.  The name of the Program is Pipemax.  You can get the program and Larry's contact info at

P.S> I'm not trying to sell or promote his software.  I'm just a satisfied user of this program.

MichaelPal (Mechanical)
20 Nov 06 14:29
ok bear with me for a second...


L=runner length in mm
C=speed of sound (around 334m/s)

and you want to set the runner length as to get a degree value of 80-90 degrees of crankshaft revolution.

all of this info is out of a book im reading for my FSAE class written by Hienz Hessler (sp? i'll check it later ).

where the degree value is when the pulse 1st returns to the intake valve. as it continues to return to flow/velocity/amount of charge ( or whatever u want to maximize) this value reaches a max just as the valve closes.

my question is, if anyone followed me through that, is WHY 80-90 DEGREES?!?!?1

i've been trying to figure this out all damn day and i can't figure out why...the only reason the book states is 80-90 degrees is through trial and error

but i was wondering if anyone has any hard fact as to why this is...

thanks, sorry for the sloppy post, i kinda got excited!!!

~Michael Pal
facty (Automotive)
20 Nov 06 16:40
Greg, I'm surprised you haven't CUT this post as there seems to have been an awful lot of "rubbish" posted (your words, not mine - I would have used something much stronger).

For anyone out there genuinely interested in truly understanding the "unsteady gas dynamics" effects that explain the tuning of intake and exhaust ducts then read Blair's two books on the subject. They contain the definitive explanation of this subject - heavy reading in parts, but worth it!
patprimmer (Publican)
20 Nov 06 20:36
The 80 to 90 deg is to do with valve timing starting the process and where the piston os when the reflected pulse arrives. You want it to arrive as the inlet flow is falling off, but the valve is still open, so you get that extra shoy of air before the valve closes.


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MichaelPal (Mechanical)
21 Nov 06 8:34
an obvious answer pat...but that's what i needed


~Michael Pal

Pal's the name, FSAE race cars are my game.

Terps Racing

oab (Automotive)
25 Nov 06 17:26
The 80/90* depends on cam timings, and is used on engines at higher rpms. As the inertia of the shorter gas column can move faster..
The sonic wave starts at the inlet valve opening, travels to the end of the track, to reflect back as a positive wave, meets the piston 80/90* down the stroke. The closed pipe, reflects the positive wave back to the open end of the tract, a reversal takes place to reflect a negative wave back into the cylinder as the piston is slowing at BDC. The negative wave is reflected back from the piston, through the cylinder dropping the pressure to increase filling. The wave finally reflects back from the open end of the tract as a positive pressure, to increase the external pressure on the inlet gas to assist filling and prevent charge reversal.


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