Calculating the Maximum Flow of Fuel/Air in CFM
A point of confusion in the antique tractor world today is what method to use to determine how big a carburetor is needed to allow maximum fuel/air flow. What we need to look at here is fuel/air flowing into the engine measured in cubic feet per minute (CFM.) The best way to estimate maximum fuel flow is the following formula:
Max CFM = RPM x CID / 3456 x VE
Maximum CFM is the absolute maximum volume of gas, i.e. the fuel/air mixture, an engine can draw into its cylinders. The value of RPM is the engine's rated revolutions per minute under a load. The value of CID is the cubic inch displacement of the engine. Finally, volumetric efficiency (VE) is the measure of how efficiently a tractor's engine draws in the fuel mixture. A value of 0.80 is a good estimate for a two-cylinder tractor. This can be adjusted to fit the condition of the tractor. Set the value around 0.95 if the engine has a new "pulling" head or adjust it down to 0.75 if the head is old and pitted. Use the calculator below to determine Maximum CFM.