Question for the Motor Heads

[Blackdog_kt]

The efficiency of direct-drive supercharges is tied to the RPM. This is true.

The above effect is not always linear and probably each supercharger has its own "powerband" so to speak.

There's also another thing to consider. RPMs=amount of combustion cycles per minute. The engine is essentially a vacuum pump, it sucks air, mixes it with fuel and burns it to produce power.

If you lower the RPM you essentially lower the amount of combustion cycles during a given time frame. This means that for the same throttle position, less air is being "used up" by the engine.

Where does this air go then? I guess it stays in the manifold for a while longer because at one end (the intake) air is still being forced in, while at the other end (the actual engine/carbs/pistons) less air is being drawn out of the system. This would easily result in an increase of pressure in the intake manifold system and since this is what the boost/ata/manifold pressure gauges measure (just with different units), it shows up in the instruments.

As another interesting bit of information, the less amount of stress on the manifold is not with the throttle closed. At low throttle settings the intake "tube" is trying to implode, because the inside pressure is less than the outside pressure. In fact, the lower amount of stress on the intakes occurs when running throttle that gives a manifold pressure equal to the outside (ambient) air pressure: at sea level this would mean running full throttle on a non-supercharged engine.

Just goes to show how things are not that much set in stone but there's a lot of inter-dependency between different conditions.

If any of you want some pretty long winded explanations that cover everything, check out the following links. Someone else posted them here and i didn't miss a chance to bookmark them after reading, very useful stuff.


Manifold pressure: http://www.avweb.com/news/pelican/182081-1.html

Propellers: http://www.avweb.com/news/pelican/182082-1.html

Mixture: http://www.avweb.com/news/pelican/182084-1.html