OK, it’s now possible to have a supercharger that can “be switched on and off” (some Toyotas). How does it work? I mean, what happens to the intake air and fuel when the blower is not engaged? Do the rotors just freewheel to let the intake charge through? I’ve got the operating principles down for the typical hot-rod type Roots blower but this thing has me stumped.
Thanks, Lee
Far out! So Mad Max was/is an accurate picture of the future afterall 
I assume it just freewheels, like a turbocharger when you don’t have enough exhaust pressure to produce any boost.
Future? Studebaker had even Mad Max beat by a few years. Their R2 and R3 supercharged Avantis used a McCulloch centrifugal supercharger blowing through a Carter AFB carburetor. In 1962.
The McCulloch had a form of “clutch” which engaged and disengaged the pulley from the internal planetary that drove the impeller.
Even without the drive, air could still get past the impeller and housing, with surprisingly little drag. And since it was ‘dry’ air- the fuel had not been mixed yet- carburetion and distribution was not a problem.
The Roots-type positive displacement supercharges can’t effectively “freewheel”, as there’s very little clearance to allow airflow past, and they’re notorious for taking significant horsepower just to drive.
I’m not familiar with the Toyota version, but I’ll presume it’s a centrifugal (centripetal, you get the idea) in that it has a “turbo” type impeller. And thus when it’s not powered, air can still flow pretty freely past.
And since this all takes place presumably long before the injectors, again there’s no problem with fuel distribution.
The easiest way to think of how it works even if you don’t have a Supercharger is to look at the electric clutch on your Air Conditioner compressor. They’re virtually identical.
Ford has used Eaton superchargers with a bypass valve. This almost eliminates drag from the supercharger when it is not used.
So the Last Of The V8 Interceptors, with a clutched Roots-type blower, was BS. Can’t get air through a Roots unless it’s turning.
I always learn something here!
Unfortunately the whole engine was BS. It was just a stock 351 cleveland with a 2 barrel carb. They put an empty blower case over the carb. Did absolutely nothing. Oh well.
/Markus
This may seem like a dumb question, but why would I want to turn off the supercharger?
If you want to sneak out of the driveway (or back in again) without the old lady hearing the whinning of the blower.
Conserve fuel.
Less wear and tear on the blower when not racing.
Longer life of engine in general.
Blowers put more pressure of every part of the engine, so turning it off would let the engine rest a little.
The guy next to you at the light won`t hear the thing whinning until you get off the line and engage the clutch, then proceed to “blow” him away.
I suppode when you race the car at the track (1/4 mile), you may not want the blower for the launch (too much power) because the tires will spin easier. Engage it when you get into second gear. Then hang on.
Right BLOWN & INJECTED???
Still curious…
How much fuel would you actually save with the blower turned off? Not looking for an exact figure, but an approximate percentage would be great…
Right. But wouldn’t an engine be designed such that it could take, at all times, the extra stresses that the supercharger exerts without compromising on engine life and reliability? I don’t think I’d want to buy a car where the designers didn’t factor in blower loads into the engine design. (Not talking about after market fits)
Now that makes sense to me 
I used to own a 1988 Toyota MR2 with a factory supercharger. The blower would come on if the accelerator was pushed down more than halfway.
With normal aspiration, I got about 27 mpg. With the blower on, I would be lucky to get 20. Bear in mind, I only usually used the supercharger when I was over about 4000 rpm, which would also hurt gas mileage.
Every piston stroke puts some stress on the engine. With the supercharger on, that stress will be higher. The 4AGZE engine in my car was very well built, but a car that was constantly on boost would have engine problems sooner than one that was driven less aggressively.
>>I suppode when you race the car at the track (1/4 mile), you may not want the blower for the launch (too much power) because the tires will spin easier. Engage it when you get into second gear. Then hang on.
Right BLOWN & INJECTED???
That could be a way to ease the shock. Have not ever heard of anybody doin’ it, but things almost always change.
I know lots of people that do not turn on the nitrous until second gear because it will just blow the tires away if hit at the line