Ahead of the 2015 Detroit motor show, BMW has given its range-topping M6 sportscar so subtle a makeover it could be difficult to spot.

However, look for long enough and you'll see that car -- whether in coupé, Gran Coupé or convertible form -- has a redesigned kidney grille, front and rear aprons and bigger, more menacing air intakes.

BMW has also developed new forged 19-inch alloy wheels (20-inch on the Gran Coupé) and, on the hard-top variants, redesigned the carbon fiber roof. It is now more contoured, lighter and sits slightly lower, improving the car's already excellent center of gravity.

Inside the differences are quicker to spot. The levels of trim and styling have been taken up a notch as have the range of optional extras potential owners can choose from.

Under the hood it's business as usual. The new M6 has the same (admittedly wonderful) 4.4-liter V8 twin-turbocharged engine as before and its figures are also the same - 0-100km/h in 4.2 seconds and an electronically limited top speed of 250kph (155mph).

However a competition package is available that will add an extra 15bhp to the engine's already impressive 552bhp and will knock a whole 0.1 seconds off its 0-100km/h dash. What will be a new addition to the 2015 model is a reworked two-stage exhaust system that BMW says produces a more satisfying engine note.

The updates aren't limited to the flagship 6-Series variants; all models in the range are getting similar though less aggressive external and internal tweaks. However, in the case of the standard range, the engines are also getting improvements. BMW says that six-cylinder and eight-cylinder gas engines now all meet the strictest European EU 6 regulations, their efficiency has been improved and so have acceleration and responsiveness.

The company is also for the first time offering its XDrive all-wheel drive system as an option on the standard range, as well as passive rear-wheel steering for keeping the back end in check when making progress.

The face-lifted 6-Series range will make its global debut at the Detroit motor show in January and will go on sale in March.