A new engine, the Cobra Jet, made its way under the hood of the Shelby Mustang. The big-block powerplant had 428 cubic inches (seven liters) of displacement and whipped 335 hp (389 PS) and 440 lb ...

An abandoned 1968 Shelby GT500KR 428 Cobra Jet. Believe it or not, this God-tier ultra-rare muscle car was rotting in a Windham, New Hampshire, property.

But what about the actual KR-8 Mustang? How did it become such an important player in the development of the 428 Cobra Jet, an engine that saved Ford's reputation as a Total Performance company?

From the November 1988 issue of HOT ROD: Conservatively rated at 335 hp, the 428ci Cobra Jet engine was, at the time, the biggest displacement production powerplant ever fitted to a Mustang.

The 1968 GT500-KR routed air through its functional hood scoop to a 428 cubic-inch Cobra Jet V-8 with 360 horsepower, mated to either a three-speed automatic transmission or a four-speed manual ...

Craig Jackson has restored what is likely the rarest Shelby Mustang of all, this 1968 Shelby GT500 Lime Gold prototype.

The Ford Mustang "Green Hornet" prototype is no less than one of the rarest and most valuable Shelby Mustangs on the planet.

KR stands for “King of the Road,” and the initials were first used in 1968 on an evolution of the Mustang GT500 powered by a 428-cubic-inch V8 engine called Cobra Jet.

Instead of the 355-horsepower 428 FE engine of the 1967 model, the plan for a more powerful 1968 model was to go with the same-sized Cobra Jet V8 for this higher-spec car.

Those who could usually opted for the Shelby Mustang. Gilbert came up with a formula that combined high-flow 427-sourced cylinder heads with a police-fleet tough 428-cid rotating assembly.

In 1967, the New York auto show played host to the launch of the original GT500KR, a version of the Shelby Mustang powered by a 428-cubic-inch Cobra Jet V-8 rated at 335 horsepower.

The KR—for “King of the Road”—was a development of the ’68 GT500, with the so-called Cobra Jet 428-cubic-inch engine in place of the Police Interceptor unit.