In automotive design, an M4, or Mid-engine, Four-wheel-drive layout places the internal combustion engine in the middle of the vehicle, between both axles and drives all four road wheels.
It is a type of car powertrain layout. Although the term "mid-engine" can mean the engine is placed anywhere in the car such that the centre of gravity of the engine lies between the front and rear axles[citation needed], it is usually used for sports cars and racing cars where the engine is behind the passenger compartment. The motive output is then sent down a shaft to a differential in the centre of the car, which in the case of an M4 layout, distributes power to both front and rear axles.
The centre differential contained within many 4 wheel drive cars is similar to the conventional differential in a 2-wheel drive car. It allows torque to be distributed to both drive axles whilst allowing them to spin at different speeds, which vastly improves the cornering of a 4-wheel drive car on surfaces with high grip such as tarmac. However, unlike the differentials on the drive axles which are configured to provide torque equally to both wheels, the centre differential is usually set to have a bias to one set of drive wheels or the other, depending on application of the car.
Some 4 wheel drive cars use a centre viscous coupling unit that will provide most power to the rear wheels unless the amount of torque being supplied to the rear wheels is in excess of the traction limits of the rear tyres, such as in a Lamborghini Murciélago.[1] Others contain a computer that will decide how much power to distribute to any wheel at any time depending on the circumstances of each wheel. In general the M4 system is not widely used[citation needed] as it is suited toward sports cars and some off-road racing vehicles.