Don’t let Overlap get Over You!


There are hundreds of articles written on Camshaft selections.  Very little is discussed

on the importance of  Valve Overlap. Overlap is the angle in crankshaft degrees that both the intake and exhaust valves are open.

What does it do?

At high engine speeds, overlap allows the rush of exhaust gasses out the exhaust valve to help pull the fresh air/fuel mixture into the cylinder through the intake valve. Increased engine speed enhances the effect. Increasing overlap increases top-end power and reduces low-speed power and idle quality and visa versa.

Some facts:

Engine Displacement:
An increase in Displacement without change to Intake port size with require more overlap to aid in airflow the Engine

Higher cylinder Pressures are a Byproduct of Higher Compression. This increases the speed of Escaping gasses creating a more dynamic scavenging ability, reducing the needed Overlap Duration.

Crankshaft Stroke:

Rod length and stroke ratios affect the amount of time the piston is at TDC; as well as how far the piston travels before and after Top dead center.  Resulting in the amount of time overlap has to perform its task.

Rocker Arm Ratio

Increasing the Rocker arm ratio will reduce needed overlap.