What happens when you add 25 percent more power to a McLaren 720S?

Texas tuning company Hennessey Performance is well-known for extracting big power from American muscle cars, but they're more than a one-trick pony. In fact, they originally got their start tuning the Mitsubishi 3000GT in the 1990s before turning their attention to the Dodge Vipers that made them famous.

Not content to merely turn pony cars into supercars, Hennessey also uses their tuning know-how to elevate supercars well beyond their stock capability. In this video from their official YouTube channel, Hennessey performance employee Alex slides behind the wheel of their tuned McLaren 720S for a few hot laps.

With 809 rear-wheel bhp, it's a substantial boost over the stock car, which boasts 710 bhp at the crankshaft. At the wheels, that number is more likely to be a little over 600, if our math is right (we're assuming 15 percent drivetrain loss, which is pretty standard).

Adding 25% more wheel horsepower to an already-impressive supercar is no small task. In addition to custom tuning, Hennessey has employed several parts to get the McLaren 720S to that point.

Hennessey Performance has fitted a custom intake and exhaust, and added blow-off valves for the twin turbochargers. These modifications not only allow the 4.0 litre forced induction V8 to breathe easier – they noticeably improve the car's sound as well.

The aggressive exhaust note is accentuated by the whistle of the turbochargers and that trademark blow-off valve sound. We were already hot on the McLaren 720S here at Motor1, and this Hennessey version makes us want one even more.

Alex notes in the video that, as you'd expect from any supercar, the McLaren 720S has a fantastic chassis that's more than capable of handling the extra power. Under normal driving conditions, it drives more or less just like the stock car. Open it up a bit, though, and this McLaren 720S is a whole new animal.