difference between that an a 40mm g-lader I assume?
It's a totally different type of supercharger. They use two twisted rotors (not quite screw shaped but similar). It's a development of the Roots supercharger which has been in use since the early 20th century, initially on planes, then on powerful racing cars of the day and then on factory cars. When you see those big American V8s with huge superchargers sat on top of the engine poking out of the bonnet, these are Roots blowers.
It's the Eaton M45 from a BMW-Mini Cooper S that people use, lots of different sizes of Eatons are available on other cars such as Merc Kompressors, Jaguars to name just a couple. They are a strong, chunky design which can be revved pretty highly without damaging it. It's actually not as efficient as a g-lader supecharger in that it requires more engine effort to provide a certain air flow than a g-lader does and it produces more heat.
People have used them to get powers approaching 200bhp. Eaton M45s are usually found on ebay or for sale sections of here or club polo. They come from John Cooper Works conversions on new Mini Cooper S. What happens is JCW takes in brand new minis and simply swaps the original charger for a worked on replacement. So the standard hcarger is surplus to requirements and fairly worthless. Although more and more people have now cottoned onto the fact that they are useful for boosting smallish engines so they are more sought after these days. Plus the current mini cooper s uses a turbocharger so the supply of brand new Eaton M45s is drying/dried up. My point is, they will get rarer and more expensive as time goes on.
Some further reading:
http://en.wikipedia.org/wiki/G-Laderhttp://en.wikipedia.org/wiki/Roots_type_superchargerhttp://www.eaton.com/EatonCom/ProductsServices/PerformanceProducts/Products/Superchargers/M45/index.htm
