When building or rebuilding an engine, you may determine the RPM at which the engine will reach peak torque. Using RAM air theory, you may perform simple calculations to determine the length and diameter of the intake runners as they relate to the piston displacement volume and volumetric efficiency of your engine.
Decide at what RPM you want peak torque to occur. Example: 4,000 RPM.
Subtract the desired RPM from 10,000. Example: 10,000 minus 4,000 = 6,000.
Divide the answer from Step 2 by 1,000. Example: 6,000 RPM divided by 1,000 = 6.
Multiply the answer from Step 3 by 4.3 cm. Example: 6 times 4.3 cm = 25.8 cm
Add the answer from step 4 to 17.8 cm to determine intake runner length. Example: 25.8 cm plus 17.8 cm = 43.6 cm
Multiply desired RPM, displacement volume in liters and volumetric efficiency. Example: 4,000 RPM times 1.2 L times 0.95 = 4,560.
Divide the answer from Step 1 by 3,330. Example: 4,560 divided by 3,330 = 1.369
Take the square root of the answer from Step 2 to determine intake runner diameter in inches. Example: The square root of 1.369 is 1.170 in.
