Chrysler's Slant-6 Was An Oddly Tilted Engine That Refused To Die

In the late 1950s, as American drivers were discovering the benefits of small and efficient automobiles, Chrysler knew it had to get in on the action. As a result, it set up separate projects to develop both a new Valiant car line and a new engine to power those machines. Somewhere along the line, however, something got lost in the translation, and the new car's engine bay wasn't big enough to handle the new motor.

The six-cylinder engine — with an inline configuration and 170 cubic inches of displacement — was too tall to allow the hood to close, and it was also too long with its front-mounted water pump. A larger 225 cubic inch mill was already in development, as well. Now, Chrysler could have redesigned either the motors or the cars, but instead, the engineering team had a stroke of genius.

Rather than installing the engine with the cylinders pointing straight up, they tilted it over by 30 degrees as if it were one half of a V-shaped engine. This wasn't the first time an automaker pulled off the trick, as Mercedes-Benz used a tilted straight six engine in the legendary 300SL in 1954. But it did allow Chrysler's engine to become a legend in its own right, eating up millions of miles in millions of vehicles — including what may be the most reliable American car ever built.

The original 170 cubic inch Slant-Six engine debuted in the compact 1960 Plymouth Valiant — although the mill didn't always use its now-famous moniker. Before that caught on, Plymouth tried calling them 30-D engines (for their 30-degree tilt). Regardless, the smaller motor proved a bit too small in real-world applications. It only kicked out 117 horsepower and 155 pound-feet of torque, which was too much of a trade-off for its improved efficiency.

Also launching in 1960, the 225 cubic inch Slant-Six started life making 140 horses complemented by a nearly 30% increase in torque over its smaller sibling, raising the bar to 215 pound-feet. Key upgrades for the engine included a whole new block with a taller deck height, longer stroke, and a revised crankshaft. Note that the same block, with a shorter throw, was used with a 198 cubic inch Slant-Six. It replaced the 170 cubic inch motor in 1970 and stayed in production until 1974.

The 225 Slant-Six wasn't perfect, of course, but a cascade of updates kept it alive in Chrysler vehicles well into the 1980s. You could still find it in some passenger cars as late as 1984 and in certain trucks until 1987. Moreover, the motor was manufactured for a variety of marine and industrial applications up to 1991,when it was killed off — primarily for emissions reasons. (Maybe we can bring it back now that emissions standards are a thing of the past.)

Chrysler's Slant-Six engine also inspired one of the most stunning concept cars of jet-age design — the 1960 Plymouth XNR. In fact, the XNR was built on the bones of a Valiant and used a 170-cubic-inch version of the Slant-Six for motivation. Granted, it was tuned to contemporary NASCAR specifications, which are very different from what's expected for the next generation of racing. But it did power the XNR to 146 mph, and it reached 152 mph with the addition of an aerodynamic nose cone.

The concept was created by Virgil Exner, one of the masters of automotive design, who wanted to emulate the basic layout of the era's Indy cars. After seeing the Slant-Six, though, he moved the driver's position from the center of the car (as in a racer) and put everything far to the left of the vehicle, including the hood scoop, small windshield, and vertical rear stabilizer.

The original Plymouth XNR led quite the journey in the later years of its life. It was owned by the Shah of Iran, survived the Lebanese civil war in the 1980s, and eventually was restored and sold at auction in 2012 for the tidy sum of $935,000. Now owned by the Paul and Linda Gould Collection, the XNR also made a brief appearance in the recent Amazon series based on the "Fallout" video game.