RSR Inlet Valve Mach Index Calculator

Typical Values:

Harley Stock Evo Big Twin: Piston Diam 3.498; Valve Diam 1.840; RPM 5200; Stroke 4.250;

Harley Stock Twin Cam 88: Piston Diam 3.75; Valve Diam 1.840; RPM 5200; Stroke 4.000;

ORCA 113/126/132/139": Piston Diam 113: 4.000; 126/139: 4.375; 132: 4.250; Inlet Valve Diam 113: 2.200; 126/132/139: 2.220; RPM 6500; Stroke 113: 4.500; 126: 4.1875; 132/139: 4.625;

Mean Valve Flow Coefficient: There is a poor correlation between piston bore diameter, piston velocity and the area of the valve opening. Many years ago experimental testing on a wide variety of engines was undertaken to come up with a flow coefficient at various lifts and crankshaft angles.These correction factors gave better predictions of the actual Mach Index. For simplicity sake use .336 for one inlet valve and .490 for two inlet valves. You will quickly see Harley engines are not high rpm designs. You can, however, plan your engine setup around the Mach and rpm numbers you desire.

Valve Sizes and Valve Lifts: Increasing valve size and lifts has the effect of lowering the Mach Index. It all gets too complex for a simple calculator.We left valve lifts out of the equation.

Inlet Valve Mach Index Calculator
Piston Diameter : (Inches)
Inlet Valve Diameter : (Inches)
Engine RPM : (typically peak)
Engine Stroke : (Inches)
Mean Valve Flow Coefficient : (.336 for one inlet valve; .490 for two inlet valves)
Number of Inlet Valves : (Per Cylinder)


Chuck Yeager Was There First

The Mach Index is a mathematical expression of the speed, in this case, of the inlet velocity, relative to the speed of sound. Chuck Yeager was the first man to "break" the sound barrier, but this is not something we wish to do in flow through our inlet valves. Volumetric efficiency will start to fall off sharply past six tenths the speed of sound (Mach .6) so the engine designer will not want to exceed this .6 mach value at peak rpms. In as much as the speed of sound is temperature dependent and the exhaust valve has different requirements, these calculations do not apply to the exhaust valve. Your cylinder head is where all the power resides and you should choose your cylinder head carefully and not be seduced by some polished billet piece that has no science beyond the highly polished finish. You may not reach Mach 1 on two wheels but your exhaust port can make sonic "barks" and your inlet port should be more than half way there.

Inlet Valve Closing Versus Mach Index

At high Mach Index Values the cylinder pressure may be well below inlet pressure at Bottom Dead Center. In this case, delaying the inlet valve closing allows more flow into the cylinder up to the time when the inlet and cylinder pressures reach equality. Thus, the higher the value of the Mach Index, the later the inlet valve should be closed to increase, or maintain volumetric efficiency. As you design your motor you may be involved in Mach Index values in the .4 to .75 range. Keep in mind that to prevent drops in volumetric efficiency you need to keep the camshafts in the picture. Check our our RSR Dynamic Compression Calculator.

Exhaust Valve Considerations

The exhaust valve lives in a different world, a much hotter world, a world of sonic waves and lower than sonic gas flow. When the exhaust valve opens there are much higher pressures involved than was the case on the inlet event. The speed of sound is temperature dependent so don't use the above calculator for exhaust valve sizing. Your exhaust valve flow capacity should be in the range of 65 to 80 percent of the inlet valve capacity. Increasing the exhaust valve capacity beyond this will not give performance increases and will in fact kill your power. As the inlet valve goes up, as in our 113/126/132/139 ORCA motors, the exhaust size has to follow.