Lift at Overlap
What is Lift at overlap?
Valve overlap is the time when both the exhaust and the intake valve are open simultaneously. When the piston passes through top dead center (TDC) between the exhaust and the intake stroke there is a period where both valves are open. At roughly 10 degrees before top dead center (BTDC) for the exhaust and 10 degrees after top dead center (ATDC) for the intake is when the valves are closest to the piston. This time period is what is looked at when calculating valve pocket depths. While total lift is needed information it does not even scratch the surface when you need to calculate valve pocket depths. Cam timing is all very critical as well. Duration, lobe separation and intake centerline or installed angle are all necessary to help us determine how far the valves will be open at TDC and from there we can figure the lift just before and after TDC for both lobes. You may be surprised to know that even stroke and rod length will have an impact on pocket depths. We go as far as taking the stroke and rod length to determine how far down the hole the piston will travel during that 10 degrees of rotation before and after TDC. Of course it is best to physically measure your lobe lift but a lot of times customers do not have cams yet and that is not possible. But with the right amount of parts research you can get us the data we need to accurately determine your pocket depths and ensure fitment in your engine.