overlap???

[morpower]

does anyone know how over lap effects compression tests???? ive seen some info here but what does it mean (bleeds off compression)at slow engine speeds?? does this mean you would get a low comp. reading if you did a compression test??does the comp. go up when the engines running??? or when your at higher rpm's?? anyone got any ideas or thearys??? thanks in advance

[wedgehead]

Overlap takes place at the end of the exhaust/beginning of the intake stroke. It is defined as the time that both the intake and the exhaust valves are open simultaneously. Add the point where int opens and exhaust closes from your cam card and this is the overlap of the cam. Ex: int opens @ 25 deg and exhaust closes @ 60 deg, then your cam is said to have 85 deg overlap. Overlap is directly related to compression ratio. Too little compression and too much overlap will surely kill your bottom end torque, and vice versa. Too much overlap will lower your cranking pressure. Hope this helps. Now, this is only my perception of how overlap affects an engine. Wedgehead

[panic]

Overlap affects idle quality by allowing residual exhaust gas to remain in the cylinder, contaminating the fresh intake charge. IMHO, the reason longer duration cams need more static compression is not overlap, but the later intake valve closing that usually goes with it. Late closing causes some of the charge to escape back into the port at low to moderate speeds, reducing cylinder pressure. You regain some pressure by raising the ratio at which the remainder is compressed; it's not the same thing. Even though a higher ratio piston will give you back a better gauge reading, the power is still lower: it's a smaller volume of mixture being compressed to a higher compression ratio, so the total of gauge pressure times actual volume captured is still lower than with an early-closing cam and the original compression ratio. Whheeewwww... Sorry to be so long, it's a bit complicated!