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Basically you are correct.
WOT is wide open throttle-floored-butterfly at its maximum open position.
Fuel injection systems are very precise mixture delivery systems.
Carburetors are always a compromise.
You can have 0 manifold vacuum and 0 valve restriction (or at least very close to 0) if you are in your highest gear accelerating from 1000 RPM. Throttle position could be as low as 15% of maximum and would not affect your manifold vacuum (easily confirmed with vacuum gauge readings).
The complete induction system is designed for significantly more air delivery than any engine requirement at 1000, 2000, or even 3000 RPM. WOT airflow will not produce significant manifold vacuum until you get close to max RPM.
Air pressure at sea level is a nominal 29.92 inches of mercury.
In a strong hurricane that pressure can drop below 26 inches of mercury.
At 18,000 feet altitude atmospheric pressure is half of the 29.92 sea level amount.
Another factor is density. Air density drops about 25% from 32 to 200 degrees fahrenheit.
This is not pressure but density, even if the pressure reads the same amount the density can be lower as the ambient temperature rises.
Carburetors can respond to density and pressure, but the ventures portion of the carburetor will always create some pressure drop. This is not true with fuel injection because no venturi is required to create suction to draw the fuel from the float chamber as it is in a carburetor. The old SU carburetors used a variable venturi to maintain a somewhat constant velocity of the air passing over the needle and seat, even when the volume of air was increasing or decreasing, the velocity was relatively constant.
Most BSFC maps for engines are fairly similar as long as you separate the diesels from the gasoline engines that are throttled. Unthrottled diesels have no manifold vacuum unless they are near their maximum RPM. Even then it would be a very small amount.
The reason most engines produce best BSFC at 80% of max load and in RPM ranges of 1500-3000 is because higher loads would require WOT and enrichment.
Below that sweet spot you would see manifold vacuum and reduced air incoming to the cylinder on the intake stroke, which reduces the actual compression ratio of the engine when combustion occurs.
Basically you control the speed of the engine by choking off its air supply, which is effectively the same thing as moving it to a much higher altitude than sea level. Its the same reason why engines loose much of their power at high altitudes and require supercharging or turbocharging in high flying aircraft to increase the effective pressure to the cylinders for more power.
In easy to understand terms, you can find your BSFC with no instrumentation by accelerating using only enough throttle to get your best rate of acceleration in the highest gear you can use. Increasing the throttle position will not increase your rate of acceleration by any significant degree.
Never use WOT unless you absolutely have to have that power to merge into traffic or to avoid an emergency situation.
While blocking the throttle to eliminate WOT altogether would accomplish the objective, it would also eliminate the availability of max power for collision avoidance or any other potentially dangerous situation.
I am not saying BSFC maps are identical in all engines, and I am not trying to say there is absolutely no residual manifold vacuum at lower throttle settings.
As a general rule the restrictions at low RPM are very small, to the point of insignificance.
BSFC sweet spots are generally similar in passenger car engines that are designed for EFFICIENCY.
regards
Mech
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