@k2k6
Simple answer - because they're better :-)

Seriously though, the main reasons why automatic gearboxes aren't fitted to smaller-engined, cheaper cars are cost and the fact that an automatic gearbox always incurs some sort of degree of transmission losses.

However, once you're into the realms of several hundred horsepower, the transmission losses are virtually un-noticeable, and a decent auto gearbox (such as the ZF 6HP28 that I work with daily) can change gear much, much better than even the best driver with a manual 'box.

Another issue that you tend to run into with manual 'boxes and high-powered engines is the fact that in order to deal with the sheer torque of these engines, the clutch plate needs to be roughly the size of Ipswich. A corresponding degree of force is then needed on the clutch pedal, and it can make for a very uncomfortable drive. Drive a manual US muscle car to see what I mean.

There's some cheaper, simpler alternatives to a proper epicyclic torque converter auto transmission, such as the Borg-Warner twin-clutch DSG box fitted to some VW group models. These aren't bad at all, and a good compromise between driveability and transmission losses. However, they can quite easily be caught out if you do a gearchange that the electronic brain wasn't expecting, and you'll have to wait a fair few seconds for it to catch up with you.

A dead easy way to see why most large/powerful cars are fitted with auto boxes is to drive something that's available with both (the LR Discovery 3 springs to mind). The manual is utterly dreadful, and it's a real slog to keep it in the power band. The auto is effortless, and much, much faster in everyday driving, even though it's the slower of the two on paper.

<grabs anorak on way out>
( rubberduck should only be used after consulting with your GP, Mon 22 Sep 2008, 8:55, Reply)