In Reply to: Is a M a dual VANOS engine? posted by John D. on October 24, 2000 at 07:58:56:
The VTEC system has two sets of cam lobes on the camshaft. At higher rpm's, the second set is hydraulically locked in place and take over operation of the valves and allow greater lift and overlap (the time that both intake and exhaust valves are open). At higher rpm's, more overlap allows the pressure of the incoming intake charge to help push the exhaust out of the cylinders more quickly.
At low rpm's, such overlap would result in a portion of the intake charge going out the exhaust pipe with the exhaust -- not good for emmissions or power (you want all that intake air and fuel to hang around for the big bang).
Which is better? Good question. The final answer probably lies in electronically controlled pneumatic operation of the valves, so that timing, lift and overlap can be optimized at every point. Expensive and complicated? Yes. Will we see it in production cars? Probably.
Formula one cars rely on pneumatic operation of the valves, which has the added advantage of allowing super high revs -- 18,000rpm. Conventional valve springs, which close the valves just can't keep up at these engine speeds. Downside? F1 pneumatic valve operation relies on a pressurized reservoir, which is designed to last the length of a race. A pump would be possible to keep up the pneumatic pressure, but would rob horsepower. No free lunch.