Grinnall used the same RV8s as anyone else so I doubt that it would be worth anymore than a non-Grinnal V8 unless you had a Grinnall that needed a new motor. It may have been tuned depending on the customer's request but you would have to strip it to find out. I know they occaisionally asked to put a wild motor in their cars but I thought that Grinnalls were more about the body styling than the motors.
