The 1967 Shelby GT500’s factory-rated 355 horsepower was almost certainly a lie — a deliberately conservative number stamped on an engine most historians believe was closer to 400 real horsepower. Carroll Shelby swapped in Ford’s 428 Police Interceptor V8 after the standard 390 GT proved too tame for the badge. Here’s the story behind the engine, and why Ford didn’t want anyone to know its real output.
Ford’s press materials for the 1967 Shelby GT500 listed 355 horsepower, a number that sounded impressive enough on paper but that almost nobody in the industry actually believed. Under that long hood sat something Ford hadn’t originally planned to use — a 428-cubic-inch ‘Police Interceptor’ V8, swapped in after Carroll Shelby decided the standard 390 GT engine simply wasn’t dramatic enough for a car wearing his name. Two Holley four-barrel carburetors sat mounted backward on a mid-rise aluminum intake, actuated by a progressive linkage system complicated enough to intimidate most home mechanics. The official horsepower figure was suspiciously conservative, and everyone from period road testers to modern restorers has spent decades arguing about what the engine could actually do. So what was Ford really hiding behind that number?
The Engine Shelby Swapped In After the 390 Fell Short
When Shelby American launched the GT500 for 1967, it needed an engine that matched the car’s bigger, more aggressive new bodywork, and the 390 GT simply didn’t deliver the drama the nameplate demanded. The solution was Ford’s FE-series 428 Police Interceptor V8, topped with an aluminum mid-rise intake manifold and a pair of 600-CFM Holley carburetors. It was the same basic engine architecture used in Ford’s full-size police cruisers, repurposed here to turn a pony car into something considerably more serious. Only 2,048 GT500s were built for 1967, and it was the final year Shelby American assembled the cars entirely in-house at its Los Angeles facility before production shifted closer to Ford itself.
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Horsepower Numbers Ford Didn’t Want You to Believe
Officially, the 428 Police Interceptor was rated at 355 horsepower at 5,400 rpm and 420 lb-ft of torque at 3,200 rpm — figures that most enthusiasts and historians now believe were deliberately understated. Real-world output is widely estimated closer to 400 horsepower, a gap Ford is generally believed to have created on purpose to keep insurance premiums manageable for buyers. That kind of quiet sandbagging was common across Detroit during the era, but few engines wear the reputation as well as the GT500’s 428 — a motor that undersold itself on paper and still became one of the most respected big-blocks Shelby ever offered.
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