Cool thing about steam power is the HP rating is at 0 RPM, the uncool thing about steam power is having to stoke up a fire and get a head of steam before you go out for a ride, oh say about an hour's work.
Mercedes was actually testing a steam power assist system on a car a few years ago but I never heard where it went. They took a diesel, put a heat exchanger on the exhaust to build up steam and it would get the car going from a stop then the diesel would kick in. It had a small 2 cylinder steam engine that was rated at around 100 HP.