How often does a mechanical fuel pump squirt?

March 1, 2016 | By Staff

Question:

Regarding mechanical fuel pumps… my fuzzy math thinks it takes two revolutions of the crank to get one squirt. Right or wrong?

Answer:

You are quite right, Frank. In a four-cycle engine the cam turns half as fast as the crankshaft, so the fuel pump would only make one squirt for two revolutions of the crank. It is also interesting that if the engine uses timing gears, the cam and crankshaft will turn in opposite directions, but if the system uses a chain or belt, the crank and cam will spin the same direction. Back in the early ’60s, one cam grinder made cams for Chevrolet small-blocks with two lobes opposite each other instead of just one for each valve, and the cam turned at ¼ crank speed. It was done as an attempt to ease wear on the camshaft and valve train.

Gears
The cam spins at half the speed of the crankshaft. Both gears turn the same way when using a chain.

Cam
When meshed timing gears are used, the cam rotates in the opposite direction of the crankshaft.