note: This is a continuation of a previous article on Air Scoop Size.
In the article in our freebies section entitled Air Scoop Size, an example of a 3,400 CFM blown engine was analyzed. The discussion of that value is continued to illustrate the very large amount of air used by a race engine to make power.
Analysis continued: A drag race Pro-Mod engine with the 14-71 blower at 20% overdrive is typically revved to 9,900 RPM. The displacement of the supercharger is treated as the potential amount of air pumped into the engine. The displacement of a fresh 14-71 supercharger with tight seals is 550 cubic inches per blower revolution.
The displacement per eng. rev. is:
550 cubic inches x 1.20 blower OD = 660 cubic in. per eng. rev.
The engine airflow in cubic feet per engine rev. is:
660 cubic in. per eng. rev. / (12 x 12 x 12) = 0.382 cubic ft. per eng. rev.
The air flow through the blower is limited by the air flow efficiency of the blower. The actual amount of air that a blower takes in is limited by leakage and inlet flow turbulence from air and fuel flinging around the top of the blower from the rotating rotors. A value of 90% air flow efficiency is considered in this case. While it is not an accurate number, the same standard used in further studies becomes a repeatable number useful for relative analysis.
The air flow considering the inlet efficiency is:
0.382 cubic in. per eng. rev. x 90% = 0.343 cubic ft. per eng. rev.
To convert from cubic feet per rev. to cubic feet per minute (CFM):
0.343 cubic ft. per eng. rev. x 9,900 RPM = 3,400 CFM
This example further illustrates the CFM referenced in the article. It also provides an indication of the amount of CFM in a race engine of this size. That amount is quite large and responsible for the amazing performance from this type of racing.
More on Air Scoop Size For a Normally Aspirated Engine: An example of 1,800 CFM was done for a normally aspirated (NA) engine. The analysis is further illustrated. Consider a current mountain motor with 950 cubic in. at 7,900 RPM.
Tech Analysis: The engine displacement per revolution is the basis for the potential amount of air pumped into the engine as discussed before. The potential displacement per eng. rev. is:
950 cubic in. x 1/2 = 475 cubic in. per eng. rev.
Note: A common 4-cycle engine displaces only 1/2 of its cubic inch size per revolution. The other half of its displacement is on an exhaust and intake cycle.
A typical volumetric efficiency is 83% at the horsepower peak (volumetric efficiency may be over 100% at the torque peak).
475 cubic in. per eng. rev. x 0.83 = 394 cubic in. per eng. rev.
The engine airflow in cubic feet per rev. is:
394 cubic in. per eng. rev. / (12 x 12 x 12) = 0.228 cubic ft. per eng. rev.
To convert cubic feet per rev. to cubic feet per minute (CFM):
0.228 cubic ft. per eng. rev x 7,900 RPM = 1800 CFM
This is the amount of CFM in this high output, powerful NA race engine of today. That is also an amazing value from a viewpoint of CFM.