Quote:
Originally Posted by Andyman
If the camshaft is set more retarded then the engine can tolerate a little more advanced ignition timing without knocking. This combination is likely to give the best fuel economy. If you retard the camshaft and it retards the ignition timing at the same time, I would expect lower fuel economy.
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The reason the engine is tollerating more ignition advance is that the dynamic compression is down. If the engine likes more advance but you cant accomplish this without lower compression then you need more octane.
Quote:
Originally Posted by Andyman
I think that there are two reasons that retarded valve timing should help efficiency, at least at low RPM. One is that a later closing intake valve reduces vacuum during the intake stroke, reducing pumping power loss. The other is that a later opening exhaust valve increases the expansion ratio during the power stroke, recovering more energy from the burned gases before they are released to the exhaust system.
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The vacuum that is at the throttle means diddly, its the vacuum at the top of the piston that is creating drag. Ideally the piston top sees atmospheric pressure for the whole intake stroke instead of near the end of the stroke. You need max valve lift early to minimize pumping loss throughout the whole stroke instead of just the end.
A late exhaust event makes for a lazy flow. This could be advantageous in that you may get more EGR which reduces your pumping losses.