As previously mentioned, engines actually require a certain mass of fuel, NOT a certain volume of fuel per hour per horsepower. This can offer a bit of confusion since most fuel pumps are rated by volume, and not by mass. To determine the proper fuel pump required, a few mathematical conversions will need to be performed using the following information. There are 3.785 liters in 1 U.S. gallon and 1 gallon of gasoline (0.72 specific gravity @ 65° F) weighs 6.009 lb.

An additional fact to consider regarding the BSFC is that the specific gravity of the fuel that you are using is very important. The fuel that you put in your car should only be obtained from a source which supplies fuel intended for an automobile. Some people make the mistake of using aviation fuel (sometimes referred to as “Av Gas”) thinking that the higher octane of this fuel may offer a performance gain. The problem is that TRUE aviation fuel has a much lower specific gravity (commonly as low as 0.62 to 0.65) than automotive grade fuel (0.72 to 0.76). Herein lies the problem: as previously stated, an engine requires a certain mass of fuel per hour per horsepower, and 1 gallon of aviation gasoline has a lower mass than 1 gallon of automotive gasoline. Since the specific gravity of aviation gasoline is only about 90% that of automotive gasoline, all other things being equal, your engine will run approximately 10% lean by using aviation gasoline. Be sure to take the specific gravity and stoichiometric ratio of your desired fuel into consideration when sizing the fuel pump and injectors.

It is always a good idea to apply a safety factor to account for things such as pump-to-pump variability, voltage loss between the pump and the battery, etc., so we recommend you multiply the final output of the fuel pump by 0.90 to determine the capacity of the fuel pump at 90% output to be on the safe side.

To determine the overall capacity of a fuel pump rated in liters per hour (L/hr), use the following additional conversions:

Do: -------------------------------->To Get:

(L/hr)/3.785 ------------------------>U.S. gallons/hr

Multiply above by 6.009 lb/gallon ----->lb/hr

Multiply above by 0.9 ---------------->Capacity in lb/hr at 90%

Divide above by BSFC --------------->“Horsepower Capacity” (flywheel)

So for a fuel pump rated at 110 L/hr for example, supplying a naturally aspirated engine:

110/3.785 = 29.06 U.S. gallons/hr

29.06 x 6.009 = 174.62 lb/hr

174.62 x 0.9 = 157 lb/hr @ 90% capacity

157/0.50 = 314 hp safe naturally aspirated “Horsepower Capacity”

Safe “Horsepower Capacity” @ 40 psi with 12 V assuming 0.5 lb/hp-hr BSFC

60 L/hr pump = 95 lb/hr X 0.90 = 86 lb/hr, safe for up to 170 naturally aspirated flywheel hp

88 L/hr pump = 140 lb/hr X 0.90 = 126 lb/hr, safe for up to 250 naturally aspirated flywheel hp

110 L/hr pump = 175 lb/hr X 0.90 = 157 lb/hr, safe for up to 310 naturally aspirated flywheel hp

155 L/hr pump = 246 lb/hr X 0.90 = 221 lb/hr, safe for up to 440 naturally aspirated flywheel hp

190 L/hr pump = 302 lb/hr X 0.90 = 271 lb/hr, safe for up to 540 naturally aspirated flywheel hp

255 L/hr pump = 405 lb/hr X 0.90 = 364 lb/hr, safe for up to 720 naturally aspirated flywheel hp