Quote:
Originally Posted by Old Mechanic
An engine "idles" because you have starved it for air until it can't run any faster than idle speed, at which point your efficiency is 0 since you are using fuel just to spin the engine over, which is the most extreme example of manifold vacuum. You would not have enough power to move your car unless you reduce the manifold vacuum by increasing the throttle position.
Most power per unit of fuel consumed 1300-2000 RPM with no enrichment and no manifold vacuum. You can achieve 0 manifold vacuum with very little throttle applied at low RPM as long as the load is high enough.
regards
Mech
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In actuality your manifold vacuum is reduce quite a bit due to EGR.
Also any time you increase the air induced to the engine you increase fuel since mixture is maintained by the carb or injectors via the computer (assuming a gas engine of course). If you didn't your engine would run lean.
Regards
Conn