I have come across several references from different FSAE teams claiming that the velocity of the air flowing through the infamous 20mm restrictor will reach the speed of sound at some RPM in their operable range. From my research, 15,000 RPM seems to be the absolute maximum RPM for any FSAE engine (please correct me if I am wrong).

My goal was to find out, at what RPM will our 600cc engine reach choked flow. To my surprise, I found that flow will not be choked even at 15,000 RPM.

I am hoping someone can either confirm my calculations or explain what I did wrong here. I am attempting to calculate the average velocity of the air flowing through the 20mm intake air restrictor when a 600cc engine (4 stroke) is operating at 15,000 RPM.

Engine Displacement = 600 cm^{3}

Volumetric Flow Rate @ 15,000 RPM = (600 cm^{3})*(15,000/2) = 4.5e^{6}cm^{3}/min

Cross-sectional Area of Restrictor = 3.14159*1^{2}= 3.14159 cm^{2}

Velocity of Air Through Restrictor = (4.5e^{6}cm^{3}/min)/(3.14159 cm^{2}) = 1.432395e^{6}cm/min

Velocity converted to m/sec = (1.432395e^{6}cm/min)*(1/100)*(1/60) =238.7 m/sec

The calculated average velocity of 238.7 m/sec is much less than the average speed of sound (roughly 343 m/sec). Assuming the engine will never operate above 15,000 RPM, it can be concluded that: a naturally aspirated, 600cc, 4-stroke engine with a 20mm intake air restrictor will never reach choked flow.

Are there any variables I did not account for? I know that the pressure drop through the restrictor can be ignored for this calculation due to the relationship between the bulk modulus of elasticity of the air, density and the speed of sound. In simpler terms, the pressure drop will not decrease the relative speed of sound at the restrictor; which would make it easier to achieve choked flow.