As we can see above these twin engine where side-by-side...front-to-back...many combinations in a quest for more 'scoot'. One question I have is.... how were they connected(side-by-side) a chain OR gears OR ?? The front-to-back pretty much same question... A coupling...a clutch set-up....WELDED crank !?? The side-by-sides....all the power was focused through one clutch. Was it twin disc....multi disc?! When they 'fired 'em up'...I wonder what happened IF one engine didn't 'catch' and didn't fire ? Did they start with just ONE starter...or were they all still push-started ! The gear ratio in the diff.....Did they run a taller gear to take advantage of DOUBLE the horsepower OR did they still use the same gear ratio as single engine rails and depend on the tires to "try" and bite enough to propel them down the track? How critical was it for the tuner to get BOTH engines to make the same power ?! (Prudhomme was just a kid here.....wonder how old he was when he got into pro drag racing)
On the front-to-backs I remember seeing sprockets with a double roller chain. But there were probably other ways too. On the side-by-sides, I think they meshed the flywheels which means one engine had to rotate opposite direction, so a special cam (or marine cam). Can’t answer about clutch or gear ratios.
In 1966 some friends and I were out cruising around on a Wednesday night and as we drove past the Phillips Motel in Oak Lawn, IL we saw Don and his wife Lynn in the parking lot. The Phillips was a popular spot for touring drag racers to stay and Don had been at US30 that evening. He was a very approachable guy and opened the trailer and showed us his B&M Tork Master rail. I had to check when he was born and it was April 1941. I think the Tork Master was his first car to tour by himself so I’ll say he was 25 when he turned pro.
Several WW2 era engines had this format (GM Model 6046 is a good example) so maybe some racers with a military background took there ideas from that.
I’ve seen pics of a WWII tank engine that was produced by Chrysler….5 inline flathead sixes connected at flywheels, stacked up in a weird circle. Imagine doing a tuneup on that monster!
Necessity is the mother of invention they say... https://www.theshermantank.com/about/t******rman-tank-engine-page/the-chrysler-a57-page/ Also check out this twin v12 ww2 monster that went into production.. https://oldmachinepress.com/2021/07/20/daimler-benz-db-606-db-610-and-db-613-doppelmotoren/
Not sure about the one pictured, but Eddie Hill's twin Pontiac dragster actually "used two complete and separate drivetrains all the way to the rear axle. Each engine had its own flywheel, clutch, driveshaft, and ring-and-pinion. The ring gears were bolted solidly to a huge rear axle that was machined out of oil-field, drill-stem tubing". more:https://www.motortrend.com/news/vintage-twin-engine-dragsters-from-the-1960s/
Someone, on here @Toymaker Built a twin engine dragster & went on to race it Thread here https://www.jalopyjournal.com/forum/threads/toymakers-twin-engine-dragster-build.533613/ And a follow up here https://www.jalopyjournal.com/forum/threads/toymakers-evil-twin-engine-dragster.671912/ There was a fellow in the UK who was building a twin engine T powered special, he made alot of progress, but the thread has been dead for years, I can find the thread at the moment but a very interesting one. I can't help you other than that @6sally6
And back in the 60s, there was that twin engine show car on the cover of Hot Rod magazine. Twin Pontiac engines, and 2 Jaguar rear ends. Short shaft between the two differentials.