What is Valve overlap in engine ?

An engine will have a period of "valve overlap" at the end of the exhaust stroke, when both the intake and exhaust valves are open. The intake valve is opened before the exhaust gases have completely left the cylinder, and their considerable velocity assists in drawing in the fresh charge.

The portion of the operating cycle of a reciprocating engine in which, when the piston is passing TDC (top dead center) on the exhaust stroke, both the intake and exhaust valves are open. Valve overlap increases the efficiency of the engine operation by allowing the low pressure caused by the exhaust gases leaving the cylinder to help the fresh charge of fuel-air mixture in the induction system to start moving into the engine.

 

At the end of the exhaust stroke and the beginning of the intake stroke both the intake and exhaust valves are open at the same time. This period of time (in degrees) is know as the Overlap Period.

There are a couple different ways to calculate the overlap of the cam. The easiest way is to add the intake opening times to the exhaust closing time..

Example:
Comp Cams EX268H
Adv dur: int 268º, exh 280º
110 LSA, 106 ILC (4º advance)
Valve timing @ .006"
intake opens @ 28º BTDC
Intake closes @ 60º ABDC
Exhaust opens @ 76º BBDC
Exhaust closes @ 26º ATDC

In this example the overlap is 54º (28º+26º)