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This is done to maximize cylinder fill, as the incoming fuel has inertia and the cylinder continues to fill for a little while even after the piston has started coming up.But the amount of time the cylinder fills stays pretty constant regardless of the rpm of the engine.These numbers tell how much the valves are open as the piston passes through top dead center during the overlap period. It's the valve lift as the piston passes through TDC on overlap that creates clearance problems.So if you're looking at cams with high TDC lift figures, plan on claying your motor to check your clearance.After the heads have been properly prepared, however, this changes dramatically.So consult your head porter about how much lift your heads can make use of."Bolt-In" Cams: "Bolt-In" means that the cam manufacturer believes the cams will go into your motor with the rest of the parts all stock, and generally without any clearancing work as well.In other words, they install just like a set of the original cams.
While both valves are open, the exhaust and intake are effectively connected together.Street grinds typically close the intake valve anywhere from 35 to 55 degrees after bottom dead center (ABDC) Race grinds tend to close it anywhere from 55 to 70 degrees ABDC.One other note on intake close timing: the later you close it, the higher compression ratio you need, since no compression takes place until the valve is closed.Proper cam selection is critical to the performance of your XL engine.At HAMMER PERFORMANCE we have many years experience building XL engines and we've used literally every XL cam available at one time or another.