Quote:
Originally Posted by Duffman
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.
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.
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Ok, I agree about the vacuum on the piston but a late closing intake valve does lower the vacuum in the intake manifold and that should be about the same as the vacuum on the piston when the intake valve is open. The late valve closing causes some of the air to return to the manifold at the end of each intake event. That lowers the average air flow. To regain the lost air flow so you still make the same power, you will have to open the throttle more. That lowers the restriction and the vacuum. The reduced vacuum would be applied to the piston during the whole intake stroke.
I also agree about the lazy exhaust flow when the exhaust valve is opened late. That would increase the pressure during the exhaust stroke which is another factor in the pumping loss. There is an optimum opening point depending on engine speed. For low speed, the valve should open later and for high speed the valve should open earlier. When I'm driving I only rarely let the engine get over 3000 RPM so I prefer to have the timing efficient for lower speed operation. It would be best if all the valve events could be adjusted while the engine is running. It's not an easy or cheap thing to do.
I like those valve timing diagrams.