[zjrog]

Superchargers are powered off the crankshaft, and make power pretty low, and lose efficiency as you go faster. They require a good amount of engine power just to turn (naturally, the more boost you want from one, the more power it takes to turn one...). As I recall, the production GM 3.8 liter V6 cars used some sort of bypass to avoid intake restrictions at higher speeds. But I could be wrong. Centrifugal superchargers are more efficient than the roots style, but again are limited by how fast they can be turned by the crankshaft... A turbo is powered off the exhaust, free power. Has very little parasitic drag on the engine.

There are turbo technologies to allow one turbo to make power throughout the powerband. These "Variable Nozzle Turbos" can easily be referred to as variable displacement too. Basically, the vanes in the compressor side can be adjusted to allow very little or a great deal of boost. If sized correctly, a VNT can act like a smaller turbo at lower engine RPMs and adjust the vanes to act like a larger turbo for more power or efficiency at higher RPMs.

Chrysler toyed with these in the late 80s early 90s, but they are found in very few cars. Chrysler turbo cars are lumped into 4 basic categories, Turbo I which is non-intercooled (early models were draw through, later were blow through, and even later still had increased displacement), Turbo II were always 2.2 liter and intercooled blow through. Not exactly rare, but hard to find (forged crankshaft and crossdrilled heads for better head cooling). Turbo III which was only offered in Dodge Spirit RT and Dodge Daytona RT was a 224 HP DOHC package that consisted of a 2.2 block with special heads. Last was the Turbo IV which was the rarest of all, 2.2 SOHC was essentially the same as any other 2.2 turubo motor, the magic was the VNT turbo and electronics.

Well, Probably rarest of all Chrysler turbos were the 6 3.0 liter V6 engineering mules that escaped from Chrysler. Originally intended for a special model of Daytona for 92 or so, the performance was such that Chrysler felt the Daytona would have drawn too many sales from the Dodge Stealth (same as a Mitsubishi 3000GT) and was nixed. How the motors escaped is anyone's guess. I do know where three of them are, two are in running cars (one is running around in Columbus, Ohio!)... I really only mention them in passing because they were the motors that got me thinking about turbocharging my own 3.0 liter V6 in my old minivan...

And I guess if I mention the Mitsubishi cars sold by Chrysler, I also need to mention the Starion with a tubocharged 2.6 and the Mitsu Eclipse/Plymouth Laser/Eagle talon triplets too. The triplets could be had with turbos and AWD... But I digress from the VNT discussion.

It is my understanding some diesel turbos are VNT. But not having paid a great deal of attention to diesels, I don't know which use them, or if any foreign manufacturer uses them. I can't imagine why not though.