Jack Dempsey, in his PRIME, would not be an easy fight for Muhammad Ali, but I think Ali would win it.
Ali did not have easy fights against Cooper either time (cuts helped him) , a swarmer like Frazier ever, Chuvalo the first time, Bonavena, Blin for a few rounds ... my point is very physically strong swarmers with terrific chins, stamina and decent speed were fights for him .. Dempsey had terrific speed, extreme power, an iron chin, proven stamina an underrated 77" reach and an insane fighting heart ... he'd be a very game match up but similar to Superfight 2, a ref could make it a stinker allowing Ali to hold all night ...
Another question is, if Ali egregiously head yanks in 1919, how many ph-ques will the referee give where Dempsey's blows land?
I think it would be competitive even though Dempsey is a 190 pounder. He punched well above his weight class and proved he could beat bigger men. Dempsey would be better than Cooper, Jones and Chuvalo. Would favor Dempsey over those 3 anyways. But Ali wins either a decision or perhaps late stoppage.
Cliched answer perhaps but Dempsey presents reasonable danger in the first, say, 4 rounds - of course his left hook being the real danger punch. Not that Dempsey couldn’t remain viable for rounds thereafter but given the opponent at hand, that’s Jack’s best window, his chances of winning dropping significantly each and every round thereafter. Just imo.
It would be a very interesting 3 or 4 rounds. No one is going to shut down Dempsey (unless he is taking a dive) but Ali would likely take control over matters as the rounds went on and ultimately wear down Jack to a stoppage. Or... Dempsey hooked as hard as Henry Cooper, who for all intents KO'd Clay.
Ali had issues with Cooper x2, Jones, Mildenberger… those were 190lbs men that were not as good as JD he’d absolutely be competing WINNING? idk in his era yes, Ali’s? he gets a 30% chance from me.
If Ali also gets floored like with Cooper… he may be stopped, being basically sucker punched would be a new experience, it’s possible he gets floored with a low blow and king hit on the rise refs were more “foul for a foul”
I don't see Ali be being beat by a 4 round fighter without a great Ring IQ. Still, Jack was enough of a force of nature to have a chance early here. And I seriously doubt that Jack would take kindly and meekly to Ali's clutching and pulling.
With the smaller gloves and the rules of the 1900's-20's I give Dempsey a fair shot vs Ali under that rule set, any later and I dont know.
Early on give JD 6 rounds, we may get an unexpected outcome. It’s unlikely… in JD’s era it gets scary, Ali fought with relative pillows by comparison and couldn’t just hide in his shell or clamp down the same way, JD would fight him there and the ref would let him get away with bloody murder agreed?