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Old 07-06-2011, 10:05 PM
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A engine with a long duration camshaft with lots of valve open overlap
has a lot of exhaust gas dilution in the air fuel mix when the engine is idleing or running at low rpm. (exhaust gas does not burn, it has the same effect on engine combustion as throwing water on a fire) This exhaust gas dilution slows down the burn speed of the air fuel mix in the chambers. Therefor the spark has to occur earlier in the cycle , when idling, to compensate for the slower fuel burn time.

The big cammed engine needs a lot more initial base timing to idle nice.

So much so that a mechanical timing curve becomes so short that its impractical. The engine want the same {or more} idle spark timing as it wants at WOT high rpm.

If the fuel burns too late in the cycle the combustion is lazy and tends to heat up the cylinder walls and exhaust ports instead of pushing on the piston.

A short duration cammed motor does not have the exhaust gas dilution problem when running at idle and low speed.

At higher running rpms when the motor is "on the cam" exhaust reversion in the intake track is reduced to near 0. Combustion burn speed is more normal. (fast)
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