1. / One Formula: David Vizard's Rule for IM Runner Length
The general rule is that you should begin with a runner length of 17.8 cm for a 10,000 rpm peak torque location, from the intake opening to the plenum chamber. You add 4.3 cm to the runner length for every 1000 rpm that you want the peak torque to occur before the 10,000 rpm.
So, for instance, if peak torque should occur at 4,000 rpm the total runner length should be 17.8 cm + (6 x 4.3 cm) = 43.6 cm.
Vizard also suggests that you can calculate the ideal runner diameter by the equation :
SQRT [ (target rpm for peak torque x Displacement x VE)/ 3330 ]
SQRT = square root
VE = Volumetric Efficiency in %
Displacement in Liters
So if we want peak torque at 5800 rpm at 95% VE in a teg, VE = 0.95
SQRT [ (5800x 1.8 L x 0.95)/3330]
= 1.73 in. or 43.8 mm (1,73 x 25.4 mm/in.) is the ideal runner diameter.
2./ Another Formula to Calculate Runner Length for a Specific Peak Torque RPM: from Steve Magnante at Hot Rod magazine
N x L = 84,000
where N represents the desired engine rpm for peak torque and L is the length in inches from the opening of the runner tube to the valve head.
3./ Website Calculator
Or you can forget the formulas and just plug in the numbers and this calculator will crunch out the numbers for you: