Quote:
Originally Posted by plym49
To roflwaffle:
I'm not sure that I follow you. You would have to run with a larger throttle opening to produce the same power. Your statement about filling the cylinders with air seems to neglect the effect of the throttle plate. Or am I not understanding you in some other way?
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What I'm saying is that it doesn't matter if the throttle or the valve limits air flow into the cylinder, either way the engine will see pumping losses of the big kind. By that I mean the ones associated with having the cylinder not fill with gases all the way. EGR in modern gasoline engines allows for pretty decent efficiency above 30-50% throttle, but below that, since the cylinder can't fill up with air, BSFC gets progressively worse.
Here's a fairly new gasoline engine, and
here's a heavy duty diesel. Notice how the diesel is fairly efficient over the entire range of power that it makes, which the gasser isn't until the torque it's making/air it's pulling in is sufficiently large. Those low load losses with gassers are what I refer to when I'm talking about big pumping losses. Little pumping losses are volumetric efficiency at different engine speeds. So, while changing the lift may help flow at certain engine speeds, for a few percent increase in efficiency, changing load via gearing, or other methods, can change efficiency much more than a few percent.
That being said, changing the lift after changing the gearing may result in way more benefit since changes in VE are magnified at high load, at least that's what I've read.