We’re used to seeing some exotic swaps into old muscle cars, and with Mopars, modern Hemi transplants are becoming an increasingly fashionable and pragmatic option. Jon Shaw, the owner of this particular ’70 Road Runner, apparently didn’t get that message, as he’s opted for something a bit different: A Detroit Diesel 3-53.

Jon explains that he picked up the 383 V8-powered Plymouth back in 1986 for a paltry sum of $300 and then went about installing the  a 159 ci (2.6 L) Detroit Diesel 3-53 inline-three engine, which is connected to a Chrysler four-speed gearbox. Running on a 50/50 mix of diesel and cooling oil, the engine isn’t exactly a screamer with 101 horsepower and 205 pound-feet, but it does return admirable fuel economy at 25 mpg.

“I’ve put almost 12,000 miles on it with no heat problems,” says Jon. “I don’t even have a fan on it.”

Jon says he’s a fan of the three cylinder’s sound and plans to add a turbo and swap in bigger fuel injectors, an upgrade he estimates will double the engine’s power.

As the clip demonstrates, the Road Runner isn’t exactly quick, but it certainly does have a unique engine note and character due to the unusual power train. “And it’s fun to drive,” notes Jon as he takes us for a spin around his rural neighborhood in upstate New York. It might not win any stoplight drags, but this oddball swap is certain to strike up a conversation or two at any car show.