Originally Posted by Staples

I've posted an explanation on another thread, can't find it... here's a brief copied and pasted explanation.

The VVEL system in our cars and extremely aggressive at about a 300 cam duration;

Valve Overlap

"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."

When you're pushing so much air into a car (forced induction) with an aggressive cam - there's air that isn't going to be used and is just going to pushed out of the motor and wasted because both the intake / exhaust valves are open at the same-time. Drop the cam duration, you'll have more time between the valve overlap process so when the air enters the head it will give the bottom end enough time to use that air for the combustion process, building more psi, creating more power.

Let me know if there's anything I'm incorrect about, but that's what I've gathered.