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Actually when vacuum is lowest the engine is running most efficiently, when throttle position is less than 75%.
In those carbureted engines of yesteryear, there was a point in acceleration where the volume of fuel and the rate of delivery was matched to a certain rate of acceleration.
When Fuel Injection came along efficiency would be improved at different rates of acceleration.
While the highest vacuum reading at steady speeds would indicate best mileage, it does not indicate best efficiency. This is clearly demonstrated by Pulse and Glide strategies, which, when combined with engine off coasting, eliminate all engine operation with any significant manifold vacuum..
Try to understand that vacuum is detrimental to efficiency because it indicates a reduction in the compression of the mixture at the point of combustion. If your vacuum is 50% of atmospheric pressure, say 15 inches, it is the same as WOT at 18,000 feet altitude where atmospheric pressure is only 15 inches. Engine power is significantly lower, and in many cases in aircraft, supercharging or turbocharging is necessary to restore engine power at high altitudes.
Another way to understand it is to consider a dyno test on a 4 cylinder GM push rod, old tech engine.
With a 20 HP load at 1500 RPM the engine burned 1 unit of fuel. Increasing the load to 50 HP only resulted in a 50% increase in fuel consumption.
1 unit = 20 HP
1.5 unit = 50HP
You can see the difference easily. The greater load, which would reduce manifold vacuum and increase the in cylinder compression resulted in;
16.66 HP on .5 units of fuel
Versus
20 HP on 1 unit of fuel
When you consider the accomplishments of dedicated hyper milers like Pale Melanasian and the super high over EPA ratings mileages they achieve, they do so by eliminating any part load high vacuum engine operation. It's either accelerating at 0 vacuum, or engine off at 0 vacuum.
Most important point, although a high vacuum reading indicates better mileage, it does not indicate better engine efficiency. Those who achieve the highest mileage understand this perfectly, or they should.
regards
Mech
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