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The liquid-propane injectors are fairly new technology and differ substantially from a typical gasoline injector. They must maintain the propane in a liquid state-that is, under pressure-until it reaches the injector tip, at which point it changes phase to vapor, expanding and cooling rapidly in the process. Note that the Sullivan intake was substantially modified to accept a pair of injectors for each cylinder. Because propane holds fewer BTUs per pound than gasoline, a higher volume of fuel flow is required to support a given horsepower level, though this is partially offset by the leaner air/fuel mixture required by the more efficient combustion of propane.