# TECH: Downloadable Excel sheet for Intake manifold runner length.



## tdogg74 (Mar 1, 2002)

*Caution:** **NERD-ALERT***

Download here

Feel free to do as you wish with it, just give credit where its due (me). Its completely self-populating with the exception of 3 elements: RPM, Cam, Intake runner length. You add those to come up with the ideal runner length for a custom intake manifold. The cells are labeled with what the number represent for clarity. Keep in mind though, that runner length is only one piece of the puzzle. Runner diameter plays a role as well, and as a general rule, shorter runners favor a fatter diameter over longer runners that favor a narrower diameter to keep velocity up. There is also plenum volume and ram pipe length to consider. I'll go into these in later updates. But for now, lets discuss runner length.

I'm going to do my best to walk you through the different equations to come up with the numbers you need. (keep in mind, I'm not a good teacher) Like mentioned earlier, all you need to input is the cam duration (advertised duration), rpm of desired valve opening, and total runner length. If you already have a manifold, and know the size, you can just add the cam duration and total runner length and then play with the rpm input to match the number of bounces until the valve cracks. General rule of thumb, you want the wave to arrive just after the valve cracks open. Too early, and it bounces back to the plenum and you lose the effect. And if you know what cam and rpm you want your wave to hit, you can play with the runner length until you get the harmonic bounce right. (dont worry, all this "bounce" mumbo-jumbo will make sense later  )

So we'll take a few specs, loading the ones I emboldened out into the spreadsheet:

Basing this off the *288** cam. (add to spreadsheet)
Basing this off of *3425rpms*. (add to spreadsheet)
Basing this off of the MKIV manifold length of 14" + 3.73" (ABA intake port length) for a total of *17.75"*. (add to spreadsheet)
Basing this off a stock ABA head.
There are 720* in crank rotations per one cam rotation.
We are going to use 1126ft/sec as the speed of sound in these calcs. (1126ft/sec = sea level @ 68*F)

Line 1- We take 720* and minus the 288* of "open" and that gives us 432* that the valve is closed per one cam rotation.

Line 2- Then we take the rpm, 3425. We want to convert that RPM (rotations per MINUTE) to RPS (rotations per SECOND)...so we multiply 3425 by 60 (60 seconds in a minute)

Line 3- We take the RPS and multiply it by 360* (in one rotation of the cam) and you get the amount of degrees the camshaft spins at the specified rpm. 20,550 rotations.

Line 4- Next we take the degrees the cam is closed, 432*, and divide it by the amount of degrees the cam is closed at 3425rpms and we get the amount of TIME the valve is closed.

**This amount of time the valve is closed, .021022 seconds, is the critical time factor. During this .021022 seconds that the intake valve is closed, the pressure wave is moving at 1,126 feet/second and travels 23.67 feet in that brief time.
At resonant conditions, the pressure wave has to travel 23.67 feet to arrive at the intake valve when it is open. Since the pressure wave spends this time going up the runner AND going back down the runner, the runner length is actually only half of 23.67 feet, or 11.835 feet, which is equal to 142.0239 inches.

So with that...

Line 5- We take the 1126FPS sound travels and multiply it by the time the valve is closed. We get the distance the wave travels during .021022 seconds which is from the first bounce off the closed valve to the plenum back to when the valve opens again. Halve that and we get the distance we need from the valve to the bell of the intake runner.

Line 6- We take the halved distance (valve to plenum) and convert it to inches by multiplying by 12 and we get 142.0239 inches.

So now you're wondering, how the hell does a 17.75" intake runner work, if our equation says we need 142.0239"?

Easy peasy....

Line 7- We enter in the total intake runner length of 17.75" (14" MKIV manifold runner length + 3.75" ABA intake port length) and divide it by 13,512 (speed of sound in inches per second)

We get .001314 seconds for the sound wave to go the length of our runner from valve to plenum. Now we take that and double it to establish our round trip time. (.002627 seconds)

So now we have the self populating chart. As you look at it, each bounce is the initial time doubled. And as you look down the chart, you are going to notice that it takes the sound wave EIGHT trips before it hits the open valve. The wave just keeps going back and forth 7 times until the 8th time when the valve finally cracks open.

So we have this:

3425rpms is the peak rpm for a stock MKIV manifold running a 288* cam. 

Play around with different variables and see how things change.

P.S. If anyone knows how to load this sheet onto a website so its still active, let me know. I would rather this on a website than have to download it...just easier that way. 

Thx -Trav


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## greyhare (Dec 24, 2003)

Cool, I have been too lazy to do this myself.

I will try a few combinations and let you know how it goes.


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## Eganx (Apr 30, 2004)

****en right.....just what I'm looking for.......I started calculating runner length today at work while running a lathe with a long cycle time.......I think I'll start over with this spread sheet


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## tdogg74 (Mar 1, 2002)

You guys got any questions, let me know. :thumbup:


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## O_Matt (May 7, 2007)

You are awesome t-dogg. :beer:


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## notso2slo (May 1, 2006)

Awesome. But I'm a little confused...

I understand the calculations and what they mean.

From what I see, you can input any RPM and use the table created to see what resonance hits closer.

In your example, you chose 3425rpm, and that hits at the 8th resonance. Why the 8th? (excuse me if I don't know something here, I know about the helmholtz resonances, but why the 8th?)

Do you have a spreadsheet for imputing the desired RPM to calculate the optimum runner length? Or imputing your current runner length to determine your optimum RPM?


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## tdogg74 (Mar 1, 2002)

If you changed the duration, it would change the amount of time the valve is closed. A smaller cam would hit at a different rpm and and at a different bouce. Its all about finding the rpm sweet spot. 

But for the most part, you're going to go about using this sheet in one of two ways: You are either going to be building a manifold from scratch and you know the duration and the rpm you want or you already have a manifold and want to find out what rpm it hits with the cam you're using.


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## tdogg74 (Mar 1, 2002)

notso2slo said:


> And do you have a spreadsheet for imputing the desired RPM to calculate the optimum runner length? Or imputing your current runner length to determine your optimum RPM?


Um...thats what that sheet is. You can change the rpm, cam, and runner length. Thats why the cells are different colors so you know what you can change.


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## notso2slo (May 1, 2006)

I see the changeable aspects. But what if I want to know what the optimum runner length is for a given RPM and duration?


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## VDUBIN (Jun 28, 2001)

this is a great write up tdogg. I have a small spreadsheet that I use that uses hemholtz's rpm of resonance formula to find runner length.

rpm=642*340*(sqrt(A/(L*V)))*(sqrt((C-1)/(C+1)))

A runner area (cm2) ex: .6
L runner length (cm) ex: 45.085
V displacement per cyl (cc) ex: 1985cc /4= 49.625cc
C compression ratio ex: 10.0

rpm=642*340*(sqrt(.6/(45.085*49.625)))*(sqrt((10-1)/(10+1)))= 3233

so a runner area of .6cm2 is ideal for the rpm and runner length you are designing for your intake. I have this in an excell sheet also. it is here intake formula

I hope this helps. thanks man.


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## tdogg74 (Mar 1, 2002)

WERD! :thumbup:


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## notso2slo (May 1, 2006)

Again, maybe I'm missing something, but why the 8th resonance?


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## VDUBIN (Jun 28, 2001)

I just thought of something that i am suprised has not been mentioned yet. All of these calculations are "ball park" calcs. You state in your example that these calcs are based on sea level numbers. If you are new to this stuff, air acts like a fluid and a sound wave inside an intake and the best you can do is design for a general setting. Since the density of air changes with temp and altitude and therefore changes how the intake will respond on any given day and in different regions. However, since we cannot as yet design an intake that can adjust for these changes, close is good enough. The OEMs are using a second throttle butterfly inside the intake runner to help with these changes by altering the flow and therfore the velocity, they can achieve optimal flow with a single runner lenth over a larger rpm band and air temp/pressure readings.


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## VDUBIN (Jun 28, 2001)

The 8th is not important. What is important is that you arrive at a numer very near the resonant lenght. You can have an intake with a 5 trip resonance and be fine. But if you are at 6.5 trips then at your desired rpm the wave is traveling away from the valve and therefore you are not getting the natural oscillation boost that you are trying to tune your intake for.

Think of it like this... Yoy are in a hallway going from end to end and there are two closed dooors. One door opens on a set schedule and you can only move at X speed. You need to shorten then hallway so that whenthe door opens you are there to slide though nice and easy. If you are walking away from the door when it opens you still get through but not as easilly and if there are millions of you is this hallway it just gets worse. So, 3 trips or 8 trips no worries... Just get to the door on time.

Hope this helps.


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## notso2slo (May 1, 2006)

Hah, my math skills are extremely rusty, but I'm trying to make an equation that would give the RPM at each resonance given the duration and runner length.


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## blubayou (Oct 3, 2001)

I played around with this a bit last night, thanks for putting it up!

I do have a question though. I ask as I am a bit confused on what we're trying to get from these calcs. It's been said that hitting on the 8th bounce isn't important, so why is 3425rpm the peak instead of 5481rpm?

I used your sample figures and played around with the rpms on line 2. I found that 5481rpms put the valve opening at the exact same time as the 5th bounce. I'm sure you could play with other rpms to find different valve opening times to correspond to different bounces, as I did.

Is there a bounce instance that is preferred over another or am I missing something altogether?


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## tdogg74 (Mar 1, 2002)

Its simple. Sound travels at a finite speed (as per your elevation/air temp). With that, we can closely guess how long a sound wave travels down the intake runner. Knowing the relative speed of the sound wave, and the distance it has to travel (our intake runner) you can see when the wave will hit the valve. The goal is to get that wave to hit the valve as it opens so it provides the harmonic wave supercharging effect we're trying for. 

I guess my sheet isnt a good a tool as I though, with so many questions coming. You definitely have to play around with the variables. If you know the cam and rpm you want that wave to hit the opened valve, you need to play around with different runner lengths until you find how many bounces the wave undergoes before it hits the valve opening. Its trial and error. To me, this way makes sense. But Im sure there are other sources on the web to figure this out. Chrysler engineer's has been doing it this way since the 1960's. 



> The 8th is not important. What is important is that you arrive at a numer very near the resonant lenght. You can have an intake with a 5 trip resonance and be fine. But if you are at 6.5 trips then at your desired rpm the wave is traveling away from the valve and therefore you are not getting the natural oscillation boost that you are trying to tune your intake for.


Yes and no. The 8th in my example is when the initial sound wave travels the distance it needs to travel relative to the variable to hit the open valve. Too soon, and it continues to bounce off the valve one more time and hits the open valve already open. WIth that you lose some of the wave momentum. Too late, and you achieve the same result as hitting too soon. The goal here is to have your wave hit as the valve cracks open. This is the whole principle behind wave harmonic tuning, is it not?


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## tdogg74 (Mar 1, 2002)

Using that sheet, here is an example of the difference between the MKIV and MKIII manifold on an ABA head using a 270 cam. 

The MKIII manifold has a total runner length of 13.825" (runner length + port length). With a 270* cam, the sonic wave travels 8 times before the wave hits the open valve. The rpm this happens at is 4580rpms. At that specific rpm, the valve is closed for .016376 seconds, but in that time of closure, the wave has to travel 110.6332" both to the plenum and back to the valve. So it takes 8 round trips before that sonic wave hits the open valve and charges the cylinder. And that happens at 4580rpms.

The MKIV manifold has a total runner length of 17.75" (runner length + port length). With a 270* cam, the sonic wave travels 6 times before the wave hits the open valve. The rpm this happens at is 4757rpms. At that specific rpm, the valve is closed for .015766 seconds, but in that time of closure, the wave has to travel 106.5167" both to the plenum and back to the valve. So it takes 6 round trips before that sonic wave hits the open valve and charges the cylinder. And that happens at 4757rpms. 

You can see the differences in how the runner length plays its part in how many times the wave travels up and down the runner before it hits the open valve and at which rpm this is achieved.


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## blubayou (Oct 3, 2001)

Thanks for answering the questions.

I guess my question was surrounding the relationship between the different rpm values where a wave hits an open valve (with cam and runner length constant). Since your first example shows this could happen at both 3425 rpm and 5481 rpm, would I be correct in assuming that at 5481 rpm there could be a second, possibly less significant gain to be had (i.e. - smaller peak in power)?


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## notso2slo (May 1, 2006)

tdogg74 said:


> I guess my sheet isnt a good a tool as I though, with so many questions coming.


I just want to say that this sheet IS awesome, and is very handy. And I thank you for making it.


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## BSD (Jul 8, 2008)

*hm*

Awesome post tdogg, although I have many calculations ahead of me for my custom supercharged awd 8v setup. :thumbup: :laugh:


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