CFM Airflow Formula - RE: Efficiency Number: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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Nov. 19, 2009 11:15 a.m. socalwrench New Reader

Most of us know about the basic formula to determine CFM demands, or carburetor sizing. I realize there are several factors that can affect the engine's efficiency and intake velocity, but don't understand how a naturally aspirated engine can achieve 100-100% efficiency.

If you look at the formulas below the 'race' equation have a 1.1 factor.

Formula for determining carburetor airflow requirements in Cubic Feet per Minute:

Street carburetor CFM = .85 x RPM x CID / 3456

Street/Strip carburetor CFM = RPM x CID / 3456

Racing carburetor CFM = 1.1 x RPM x CID / 3456

CFM = Cubic feet per minute RPM = Engine revolutions per minute 3456 = Constant for conversion factors

.85, 1, and 1.1 = Approximate volumetric efficiencies for each type of engine
Nov. 19, 2009 11:24 a.m. DukeOfUndersteer Dork

Huh?
Nov. 19, 2009 11:26 a.m. Dr. Hess PowerDork

The race and other extreme performance motors, such as found in the Toyota Corolla, can have a volumetric efficiency greater than 1 at certain points of the map because there is a sort of supercharging effect from the momentum of the air as it rushes through the intake and into the cylinder, past the 3 intake valves.
Nov. 19, 2009 11:28 a.m. socalwrench New Reader

Alright, so depending on the engine (built for race purposes), it CAN achieve 100%+ efficiency at a certain RPM.
Nov. 19, 2009 11:31 a.m. Dr. Hess PowerDork

As I said, the Toyota 4AGE production, factory stock street car motor does this in Papasan's Corolla, and has since 1991. I think there's a Honduh motor (cough, S2000) that does it as well.
Nov. 19, 2009 11:39 a.m. socalwrench New Reader

Wow, that must sting to say that a F20 or F22 engine can breath that well- .

Seriously, thanks for the clarification. I doubt my small block Ford, even in race trim, could achieve 90% efficiency.
Nov. 19, 2009 11:43 a.m. 93celicaGT2 SuperDork

Actually the F20/F22 has been known to go higher than 100% in certain RPMs i believe.

Not sure how that all works, but hey, it sounds cool.
Nov. 19, 2009 11:45 a.m. Dr. Hess PowerDork

It would not surprise me to find that a Nascar SBF was over 100% VE. Aren't they like 7-800 HP without the restrictor plates?
Nov. 19, 2009 11:50 a.m. socalwrench New Reader

Yes, they sure are, but I don't have near as much money in my engine. I would imagine that the cylinder heads and intake flow extremely high numbers.