Check out Hennessey’s 850 hp Mustang GT350R upgrade in action.

When Ford announced the Shelby Mustang GT350 way back in 2015, it was to much critical acclaim. Here was an American muscle car with the power and performance to take on the world, and even had the amenities to compare with European luxury sport coupes.

When Ford also said there would be a GT350R made with carbon fiber wheels, Michelin Pilot Sport Cup 2 tires, and larger aerodynamic elements made out of carbon fiber, people got even more excited. The very first one was even auctioned off for $1 million.

But it’s been a few years and the GT350 has gotten a little stale. The GT500 is on its way to keep things exciting with over 700 hp and a proper dual-clutch transmission, but they’re not set to arrive until 2020. What can keep your blood pumping in the meantime?

Hennessey has got you covered.

The HPE850 upgrade for the Ford Shelby Mustang GT350 and GT350R boosts power from the 5.2-L V8 from 526 horses to an incredible 858 horsepower. Torque goes from 429 lb-ft to 673 lb-ft. Zero to sixty drops from 3.5 to 3.3 seconds while 0.4 seconds are shaved off the car’s quarter-mile time.

Hennessey doesn’t provide a top speed, but we’re sure it’s more than the 202 mph that the GT350R could manage stock.

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Getting to those kinds of performance numbers involves a larger 2.9-L supercharger being installed that operates between 5-6 psi of boost. Also installed are high-flow throttle body and fuel injectors to meet the engine’s new thirst. A new fuel pump and high-flow air induction system complete Hennessey’s tweaks.

It still has the same 6-speed manual transmission, and nothing is added in terms of brakes, so some caution should be advised when driving this beast.

Hennessey only offers a 2-year, 24,000 warranty for this bad boy. Obviously, the expectation is that this kind of performance is taxing on a car, and getting the full 3 years is financially untenable for the company. But then again, if you’re spending money on an 850 hp Mustang, you probably don’t mind the extra repair bills.

Contact Hennessey for pricing, and be sure to bring your own GT350 or GT350R from 2016-2019 model years.

NEXT: SEE HOW MUCH POWER THIS HENNESSEY HPE750 C6-GEN CORVETTE REALLY PUTS OUT