For gasoline or alcohol fuels, that value represents only the amount of air and fuel that burns. The stoichiometric value is the ratio where all of the fuel burns with all of the oxygen from the air. The smaller the number on the left of the ratio, the richer the ratio. Typical AFR values for normally aspirated and blown engines are shown in the table. The table below shows recommended values for mechanical fuel injection common in racing. More engine speed makes more pounds of air and more pounds of fuel, but the ratio is not changed by the engine speed. It is simple since it is independent of engine speed. This is the approach that we use throughout our technical manuals and our fuel injection jetting calculator. These can be converted to pounds of fuel.Īn easy way for computing air-to-fuel ratio for tuning is to determine the air weight and fuel weight per revolution. The fuel can be measured in gallons per minute (GPM), cubic centimeters per second (cc’s per sec.) or other units of measure. In racing mechanical fuel injection, the fuel pump and jetting determine how much fuel weight goes into the engine. That is assuming both have similar efficiencies and air densities. In comparison, this blower at one-to-one drive speed displaces twice the amount of air as this engine. In contrast, a normally aspirated 4-cycle engine with 550 cubic inches of displacement is 225 cubic inches per revolution or one half of the displacement. AFR is extremely fat at about 0.35 to 1 in this oxygen-rich mixture of mostly nitro.įor example, a typical 14-71 Roots blower displaces 550 cubic inches per revolution. NHRA A-Fuel Dragster engine runs a high percentage of nitro at 90% that provides about 75% of the burn power from monopropellant combustion. In a supercharged engine, the size of the blower, blower overdrive, blower efficiency, and air density determine the pounds of air through the engine. In a normally aspirated engine, the size, volumetric efficiency, and air density determine the pounds of air through the engine. It is not the volume of air such as cubic feet per minute (CFM), nor is it the rate of fuel consumption such as gallons per minute (GPM). AFR is the ratio of the weight of air to the weight of fuel.
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