Cooling issues maybe? Your car will possibly run hotter on the RR due to the fact that normal air cooling is substituted by a big fan pointed just at your radiator.
I worked briefly at a RR centre many moons ago, saw a few engines let go on the rollers. But hopefully the technology has moved on from then.
The revs on the machine I saw back then were gained as Markc said, with a sensor wire coming from a plug lead and entry of the number of cylinders.