Mechanical compression and vacuum release

An internal combustion engine (10) having a vacuum release mechanism (70, 70') that includes a centrifugally actuated member (72, 72') movably attached to the camshaft (30, 30') and having a vacuum release cam (84, 84') extending therefrom. The vacuum release cam is in lifting engagement with the exhaust valve assembly at crankshaft cranking speeds during a portion of the power stroke to relieve vacuum forces opposing motion of the piston (14). The vacuum release cam centrifugally pivots out of engagement with the exhaust valve assembly in response to the crankshaft attaining running speeds.