"If a pilot has an actual engine failure and there is absolutely no hope of a restart, then allowing the prop to stop usually is a good thing. It improves the glide ratio of a typical lightplane by about 20 percent."
"As a seaplane instructor, Morrel would simulate engine failures the way most other instructors do, by simply retarding the throttle. During advanced training, however, he occasionally would fail the engine by retarding the mixture control—not a problem when practicing in a floatplane over 40-mile-long Moosehead Lake.
Students and experienced pilots alike were shocked when he did this, but not because Morrel had really killed the engine. They were stunned by how differently the airplane performed. With the engine actually shut down, Morrel says, the airplane sinks noticeably steeper; glide performance is less than when the engine is idling and contributing a modicum of thrust.
In other words, Morrel says, pilots who have genuine engine failures experience less glide performance than they have been trained to expect because their engine-out training includes the benefit of idle thrust."
"....is there a way to keep a propeller windmilling during a glide with the engine shut down? Well, yes, but this is going to sound a little screwy. To increase propeller windmilling speed with a dead engine, all you have to do is open the throttle and increase manifold pressure. I know. This does sound nuts. This is because we usually consider manifold pressure as a measure of power, but this is valid only when the engine is operating.
With the throttle closed, the flow of induction air into a dead engine is restricted (or choked), and the windmilling propeller must work hard to pump air out of such a “closed” engine (create a vacuum); manifold pressure and rpm are reduced. With the throttle open, induction air flows freely into the engine, and this makes it easier for a windmilling propeller to turn the engine; manifold pressure and rpm are increased.
This means that throttle position can be used to vary windmilling rpm following an actual engine failure...........Glide performance can suffer following an actual engine failure, depending on throttle position, windmilling rpm......"