Intake Runner Diameter



Pictured above is a fuel injection system. Each of the 8 cylinders has an injector and intake runner. The intake runners are the tubes on top. They have a certain length and diameter.

What you do:

Choose a diameter which causes the pressure wave to arrive at the intake valve at the rpm you want.

This is a process called resonant tuning. Resonant tuning is the timing of the pressure wave produced by the closing of the intake valve. As the piston goes down, it draws in air. The air gains velocity in the intake runner as it is drawn in and then suddenly stops when the intake valve closes. This creates a pressure wave which travels back up the intake runner and then back down to the valve. Since the velocity of the pressure wave is constant, it can be timed by the length and diameter of the intake runner to arrive back at the intake valve when the intake valve is opening. This helps force air into the cylinder. The effects of resonant tuning are strongest at one specific rpm, so choose which rpm you want the pressure wave to assist with.

For HiPerMath calculations, resonant tuning does not cause a gain in air pressure, but rather avoids a loss. Possible air pressure loss is 1 psi (pounds per square inch or lbs/in2).

The HiPerMath equations are:

Choose your own diameter or use the following equation to base your intake runner diameter on carburetor size: