 A secondary effect of increased duration can be increased "overlap", which is the number of crankshaft degrees during which both intake and exhaust valves are off their seats.It is overlap which most affects idle quality, in as much as the "blow-through" of the intake charge immediately back out through the exhaust valve which occurs during overlap reduces engine efficiency, and is greatest during low RPM operation.In general, increasing a camshaft's duration typically increases the overlap, unless the intake and exhaust lobe centers are moved apart to compensate.