What is “overlap?
With stock or extremely mild cams, the exhaust valve is closed (or nearly so) when the intake valve begins to draw in the new air/fuel charge. The norm for performance cams is that while the exhaust is closing, the intake is opening. The period of time when both intake and exhaust are open is called “overlap.” Depending on the RPM range you are running with an unrestricted exhaust systems, the intake charge can be pulled across the top of the piston and out the other side by the velocity of the exiting exhaust gasses. This greatly increases emissions, hurts mileage, and (if there are any air leaks in the exhaust) can create a secondary burn in the exhaust system. Also, in a non-restrictive exhaust system, you have and event that can occur called reversion. At lower RPMs, the velocity of the exhaust can create a negative pressure in the cylinder and a reverse pulse wave returns the exhaust gasses back into the cylinder, polluting the new intake charge. In extreme cases, I have seen wher
