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The Lamborghini Murciélago uses the M4 configuration.

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 roadwheels.

It is a type of car powertrain layout. Although the term "mid-engine" can mean the engine is placed anywhere between the front and rear wheels, it is usually used for sports cars and racing cars where the engine is behind the passenger compartment. The layout or placement of engine and transmission are reversed in comparison to a front engined car to allow the gears to be changed directly from the passenger compartment using a gear stick. 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 is not the same as the differential on a two-wheel drive[citation needed] car where torque is distributed between the two roadwheels, depending on the speed and traction of each wheel. Rather it is used in conjunction with the two axle differentials to control the torque going to each whole axle, depending on the aforementioned circumstances. It will also “lock”[citation needed] if there is too much torque going to a set of wheels causing them to spin. This will send the excess torque to the other set of wheels, reducing wheel spin. The centre differential in some four-wheel drive sports cars contain a viscous coupling unit that will only provide power to the front wheels if the back wheels are spinning, such as in a Lamborghini Murciélago.[citation needed] 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.

Benefits

The engine is usually where the weight of a car is most concentrated so placing it between the front and rear axles gives a car a much better handling balance. Assuming the engine is behind the passenger compartment, the engine will also be pushing down on the rear wheels. Because the weight of a car is shifted toward the rear under acceleration in all cars as a rule, this further improves the amount of grip on the rear wheels, decreasing the amount of the engine's power wasted on wheel spin.[citation needed]

Because the engine is not in the front, the car can be designed with a minimum amount of frontal area perpendicular to the wind, greatly increasing aerodynamic efficiency.

A good four-wheel drive system allows a car to both accelerate and corner more quickly, since it can vary the amount of torque going to the front and rear wheels, and therefore vary how much the car behaves like a front- or rear-wheel drive car. This means that through a fast corner the car is able to display more “neutral” handling - with less oversteer or understeer. This is a much more efficient means of turning and allows for faster cornering speeds as opposed to a two-wheel drive system.

Drawbacks

Such a system is very difficult[citation needed] and expensive to design and engineer, which is why it is only usually found on race cars and very expensive sports cars.

Most mid-engine cars, because of the size and position of the engine and transmission, compromise heavily on both passenger and boot/trunk space.

Four-wheel drive systems tend to be quite heavy and some of the engine's power can be lost through the various differentials in the car, in addition to the frictional losses of the powertrain.

The variable handling characteristics of a four-wheel drive car mean that when travelling round a corner at high speeds the car may enter the corner and understeer and then half-way through the corner suddenly start to oversteer.[1]

References

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