Equations for runner length, runner size, tb size, etc, etc.....
 

is there a way to calculate the runner length, runner diameter, plenum volume, throttle body size, head intake port cfm, base plate runner diameters for a given engine displacement, with certain cam and head specifications?

We used this formula for making an upper intake for a friend's 5.0 stang on his GT40 base. I wish I could remember where I found it online so that I could give the author proper credit... The math holds true no matter who put it on the web though... and you can also use it to determine how well a specific duration camshaft will work with a particular intake as well.

Build your own intake:

Warning: Math!
L = ((ECD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D

To compute the intake runner length (L), we need to know a few things:
-Effective Cam Duration (ECD) = 720° (4 cycle engine) - Advertised Cam Duration (Factory ~248° ) - 20° (lifter slop)
-Runner Diameter (D) = 1.5 (or so)
-Speed of Sound (V) = 1250 to 1300 fps, depending on temperature (use 1300)
-Reflective Value (RV) = How many times is the air going to bounce before the intake valve opens? Use 2 for LONG runners with high RPM's or 4 for short runners with moderate RPM's.
-Revolutions Per Minute (rpm) = 2500 for a good street intake, low RPM, daily driver or tow.

ECD = 720° - 248° - 20°
The ECD of the factory cam would be: 452
The formula for "optimum" intake runner length (L), with are numbers would be:
L = ((452° × 0.25 × 1300 × 2)÷ (2500 × 4)) - 0.75
L = 28.63" from the Plenum to the valve face.
The lower intake is 5" long and the head is 3" to the valve face.
So, L = 28.63 - 5" - 3"
(Stump-pulling Torque Intakes)
Upper runner length = 20.63" for 1.5" intake tubes tuned at 2,500 RPM, 4th bounce.
Upper runner length = 50.01" for 1.5" intake tubes tuned at 2,500 RPM, 2nd bounce.
Upper runner length = 15.73" for 1.5" intake tubes tuned at 3,000 RPM, 4th bounce.
Upper runner length = 40.21" for 1.5" intake tubes tuned at 3,000 RPM, 2nd bounce.
(Screaming Banshee Intakes)
Upper runner length = 7.54" for 1.5" intake tubes tuned at 4,500 RPM, 4th bounce.
Upper runner length = 23.89" for 1.5" intake tubes tuned at 4,500 RPM, 2nd bounce.
Upper runner length = 5.94" for 1.5" intake tubes tuned at 5,000 RPM, 4th bounce.
Upper runner length = 20.63" for 1.5" intake tubes tuned at 5,000 RPM, 2nd bounce.

Notice something strange about the first and last line? Just a coincidence? No, a harmonic! This is why long tubes are better, you have two peaks one at 2,500 RPM's (4th bounce RAM) and one at 5,000 (2nd Bounce RAM).

Using 1.5" aluminum tube, and the flanges cut off an existing manifold (too easy!), weld up an intake similar in looks to the Mustang GT-40. The plenum should be 3" tube and displace 90in3 (or 1/2 you engines displacement), the runners are determined with the formula (above) based on your desired RPM.

Excellent info :)

very good info, when i get some time ill sit down with some pen and paper and see what i come up with. so how would u go about figuring the throttle body size?

http://www.grapeaperacing.com/GrapeA...ntaketuned.cfm
http://www.grapeaperacing.com/GrapeA...ehelmholtz.cfm


Surf around those tech articles....very interesting stuff.

You could make two seperate plenums too...

See this thread for TB sizing:

https://www.thirdgen.org/techbb2/sho...hreadid=104116

I would say that you will be fine with the stock TB until at least 450 hp...unless you already have a bigger one or want to spend the $$$.

bringing this back from the dead because its a VERY VERY interesting subject. that equation is absolutely great, and simple to equate. the ONLY question i have is why isnt engine CI a factor in determining the answer? is it because that the intake cam and heads work together as a package and it doesnt mater what engine its put on? please go indepth, im extremely interested in this stuff. thank you

i think i figured it out, what i didnt pay attention to was the RPM variable, i assume the amount used is half of the total rpms the intake will be used for? for example 2500/5000, 2750/5500, 3000/6000 etc etc.