I guess you could make up some rotors that actually had a skinnier center that allowed a largerdiameter lobe on the same center line gears, etc, but i think that the displaced volume is what passes thru the middle ---in other words---the volume will be effected by the area between the centerlines of the rotor shafts and any extra put on the rotor lobe will be offset by the amount necessary to remove from the rotor valley where the lobe must fit.

I was the first person to use 8:71 rotors and got them from Ed Isky----Donovan had a
6:71 case mold made longer to put the rotors in---they were the same cross section for the end plates and the 6:71 mounting was how all the manifolds were made--Van Luven assembled it and we were able to run it slower, make more power and go quicker!!!

I will play around next week and see if I can come up with how much you could increase the volume without moving the gears