Abstract: An internal combustion engine is provided with variable duration valve lifters on the exhaust valve train having response characteristics that are different from the valve lifters on the intake valve train. Either the intake valve train can be fitted with variable duration valve lifters having diminished or reduced bleed channel characteristics or stock valve (no bleed channel) lifters may be employed. In both events, the exhaust valves fitted with variable duration valve lifters with relatively greater bleed channel flow characteristics (as compared to intake valves) will open to a progressively greater degree in the mid range to high RPM engine speeds. The particular optimal engine functioning is achieved by precisely matching in a complementary fashion the appropriate degree of variable duration valve bleed flow channel size to the particular cam profile of the engine in use.