With its 4.4-liter twin-turbo V8 gasoline engine producing 617 horsepower and 553 lb-ft of torque, the BMW M8 Competition is already one of the most powerful grand tourers on the market. But that hasn't stopped tuners from extracting more power, of course. If 617 horsepower isn't enough for you to handle, G-Power is offering a new tuning package for the sporty BMW M8 Competition Convertible that offers supercar levels of performance.

Dubbed the G8M Bi-TURBO, the most powerful version increases the output to 820 horsepower and 737 lb-f of torque. To achieve this impressive power boost, G-Power uses a combination of software and hardware upgrades such as upgraded turbochargers with a larger compressor wheel.

Other modifications include an improved turbine wheel and CNC-milled housings, downpipes with or without sports catalytic converters that reduce exhaust gas back pressure, and a titanium exhaust system with carbon tips that produces a racier sound reminiscent of motorsport vehicles.

G-Power hasn't said how the upgrades affect the performance, but it's safe to assume the customer BMW M8 Competition Convertible is quicker off the line than the standard version, which can accelerate from 0-62 mph in 3.1 seconds. If 820 horsepower is too much power for you, G-Power is also offering two additional tuning packages.

The entry-level upgrade increases the output to 720 hp and 626 lb-ft of torque, which is achieved by G-Power's GP-720 performance software. Slotting between the two packages is a mid-range version with 770 hp and 685 lb-ft of torque achieved with the tuner's GP-770 performance software and the downpipes with sports catalytic converters. Completing the package are 21-inch twin-spoke forged wheels with a black diamond cut finish and Michelin tires sized 285/30 ZR21 and 295/30 ZR21.

Sadly, the BMW 8 Series isn't the sales success BMW had been hoping for as US dealers have been struggling to sell the revived grand tourer.