Gear-obsessed editors choose every product we review. We may earn commission if you buy from a link. How we test gear.

A crew chief and engineer with some liberal interpretations of the rules, Smokey Yunick also cooked up the ultimately ill-fated hot-vapor engine.

If you’re at all interested in NASCAR folklore, you’ve no doubt heard of Smokey Yunick. He was a stock car racing crew chief, builder, owner, driver, engineer, engine builder, and car designer, but an engineering genius first and foremost. Yunick turned stock car racing on its head during the 1960s thanks to his bold—often blatantly outside-the-box—interpretations of the rules and regulations governing car setup. However, when he wasn’t giving NASCAR officials headaches at tech inspections, he was equally committed to fiddling with road cars.

One such project of his was the hot-vapor engine. Per its name, it would vaporize the fuel mixture before shooting it into the cylinder head. Yunick had found that vaporizing the fuel mix would lead to higher thermal efficiency, taking the energy that’s normally wasted as heat in the exhaust and cooling system and instead harnessing it to add power. Rather than rebuilding a complete engine on his own, Smokey chose to modify the 2.5-liter four-cylinder powerplant from the Pontiac Fiero. From the factory, it was no firebreather, spitting out a meager 90 horsepower and 125 pound-feet of torque—launching the four-wheeled bottle rocket to 60 mph in roughly 12 seconds.

Meanwhile, the same engine modified with Smokey’s hot-vapor technology could pump out 250 hp and 250 lb-ft of torque. If that wasn’t already impressive, the engine produced those numbers while sipping fuel at a steady rate of just 50 mpg. It also didn’t need a catalytic converter, burning nearly all of the gas that entered the cylinder head.

So what made these numbers possible? Conventional four-stroke engines are only capable of using 25 percent of the fuel’s potential energy to make power. The remaining 75 is lost through the exhaust system or transferred as heat into the radiator and other cooling systems. However, Yunick’s hot-vapor engine harnessed this otherwise-lost heat and used it to vaporize the incoming fuel mixture to over 450 degrees Fahrenheit.

The process of recapturing this thermal energy began with a heat exchanger located under the carburetor. This used hot engine coolant (no, the irony isn’t lost on us) to warm the air-fuel mixture to 200 degrees. After the initial warm up, this mix was then directed to a turbine (Smokey referred to it as a “homogenizer,” but it was really a turbocharger of sorts), which sent the mixture through an intake manifold where it reached its final scorching temperature.

The engine operated at these extremely high temperatures while using a super lean air-fuel ratio—defying the laws of conventional internal combustion. Simply put, all engines need air, fuel, and spark to function properly. A lean air-fuel ratio has more air than fuel, while a rich ratio has more fuel than air. Ideally, engines should be in the goldilocks zone between rich and lean, where they produce optimal efficiency and power.

By now you’re probably wondering why the hot-vapor engine never caught on—especially with those crazy efficiency and power numbers. It’s said that the astronomical temperatures required for the concept to work necessitated advanced metallurgies that would’ve been too expensive for a production car. Hot-vapor engines also worked super close to their breaking point, meaning any minor issue like improper air-fuel ratio or temperature would lead to instant engine failure.

Maybe modern technology and engineering might be able to solve some of those fatal flaws.