Downloadable Excel sheet for Intake manifold runner length by tdogg74

5 minute read

Compliments of tdogg74 @ www.vwvortex.com

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