Well for one thing, you cannot pull air thru the supercharger in a roots blower without it turning. Any air that will get thru isn't enough to supply the engine. And after driving millions of miles in turbo-super charged diesels, these get their optimum fuel mileage with about ten pounds of boost. The only time the mileage is better is coasting downhill under no boost. Nothing wrong with partial boost as far as fuel economy is concerned.
Not all roots style blowers have the "corkscrew" rotors as a matter of fact they are not really roots type blowers. In most circles they are called twin screw. These actually do compress the air between the rotors, Wherein a true roots style does not use the case or rotors to compress the air. They use the manifold as a shall we say an accumulator. The roots 2 or 3 lobed rotor actually only moves the air, But it moves more than an engine can ingest, therefore the air "stacks up" in the manifold. This measurement of restriction is called BOOST ( I LOVE IT). As far as a supercharger needing to be turned off there would be only the benefit of less wear on the Drive train of the supercharger. As it takes hardly any power to turn the charger under vacuum. Some do have a internal bypass but mostly this is to take the load off the rotors after a full throttle opening. Where one like mine depends more on belt slippage to do the same. I also read on here that a motor wont run on a roots unless the rotors is spinning, This is not totally true, It wont run at a very high rpm but it will run enough to safely drive the vehicle anywhere you would need to go. But if the belt breaks the motor will never know the difference (rotors turning) till you try to get in the throttle hard then you will notice a very rich mixture and no power. As far as fuel mileage this is all determined by the tune. I can get upward of 18+ MPG with my tune, But you have to tune for cruise and wide open throttle. This involves using the power valve circuit for 20-30 jet sizes, And using the jets and idle circuits for mostly cruise conditions. As always this is just my opinion based on personal experience.
it wold be hella nice to have a swich just like mad max!!! but...back to the point. a AC compressor, some older ac compressors are piston driven, how much pressure could you get? ive seen setups where you can use a ac compressor to feed a air tank, for suspension or air tools. Botom line could i make my 250 spin a little more tire with; a old ac pump, a tap and some fittings, and a boost controlled fule pressure reg? that would be clever! just tap a 1.5" fitting into the intake "log" run some hose? run a smaller pully on the pump, and tune away!!
Ahh arent the movies great...The "Aussiewoods"...(he he play on Hollywood) Falcons blower did not actually work, they emptied out the Weiand roots style blower and stuck it over a carb. Then they used an electric 12V motor to spin the belt and pulley. http://www.madmaxmovies.com/cars/mad-max-interceptor/index.html http://www.madmaxmovies.com/cars/Replica/Black/index.html If you could set it up with an actuated bypass utilizing a seperate fuel and air intake you could theoretically switch on and off the supercharger and show little or no loss. But also in the same sense you could hook up a high torque 12v electric motor to run the blower and possibly not loose as much horsepower even though you are putting an added strain on the alternator.....
