A 24v motor driven at 12v and connected directly to the spit would be fun to watch!.... see a chicken spinning at approx 1500RPM would be entertaining even if not very effective at cooking it. you'd need to have a gearbox with a reduction ratio of about 500:1... or to put it another way, assuming a on stage gearbox with a 10 tooth gear on the motor you'd need a rather large 5000 tooth gear on the spit to bring it down to a usable speed (it would have huge amounts of torque though, you'd never be able to stop the chicken spinning!)