Ever wondered why all recent World Rally Championships (since 1982) have been won by full time 4 wheel drive turbo charged cars? There must be something special about these vehicles that makes them unbeatable in world rallying. Well there is.

There have been many attempts to build this kind of car. The first I know of, the Jensen FF back in 1966 (only 7 cars where made), not only had a full time 4WD drivetrain but also antilock brakes! This car was a total commercial failure. The advantages of full time 4WD are quite clear. Since a car has 4 wheels why should power be applied to only 2 of them? Applying power to all 4 wheels not only distributes engine torque (thus avoiding wheel spin) but also provides cars that handle more precisely. Why aren't all cars made that way you might ask. Well, like always, it's a question of price. 4WD drivetrains cost much more to implement than do 2WD ones. For instance you must have at least 3 differentials in a full time 4WD car. One between each opposing wheel and one between the front and rear axles. For a 2WD car one differential between the driven wheels is enough. With the price argument cleared, car manufacturers reserve this kind of vehicle to niche and specialized markets. In rallying power is nothing without traction. Naturally all major rally cars are 4WD nowadays. Not a long time ago The Fédération Internationale de l'Automobile (FIA) forced all manufacturers to produce 2500 cars in order to get the necessary homologation so that they could race in World rally. This enabled people like myself to get our hands one some of these very special cars. Cars that were, in fact, made solely for racing but had the looks of everyday sedans (well almost...).

A full time 4WD performance car, as mentioned earlier, needs 3 differentials in order to operate properly. All attempts to build such cars (with the exception of the Citroën BX 4x4 group B car which never made it to racing and had no central differential) involved 3 differentials. Now this is where things get complicated. The differentials in these cars and their locking make all the difference. Their type and settings can make a car handle exceptionally well or incredibly bad. Comparably to a 2WD car, where if a wheel spins the engine tends to send all its torque to that wheel, thus immobilizing the car, in a 4WD car the same one wheel spinning would also draw all engine torque and immobilize the car. To avoid this phenomenon most 4WD vehicles use differential locking techniques.

Most implementations use the classic Ferguson layout  which consists of 3 classic differentials 3 of which (the central and rear) are coupled to the wheels they drive through viscous couplers. A viscous coupler can be seen as a tube containing a viscous fluid in which discs are rotating. Half of the discs are attached to the incoming axle and the other half to the outgoing one. The discs are pierced and the viscous fluid completely surrounds them. Minor speed differences are allowed between the two axles but increased slip leads to a rapid increase in the viscosity of the fluid which then locks up the coupling.

Viscous couplers are convenient devices mainly because they are not expensive. Their major drawbacks are:

• An exponential increase of their locking to speed difference curve (their are not very progressive)

• A delay in their locking ability induced by the time the viscous fluid need to increase its viscosity

• Difficult to handle under braking (they lock in braking situations)

Some of the more known examples of this 4WD layout are:

• The Ford Escort (and Sierra) RS Cosworth

• The Subaru Impreza (and Legacy)

Of course the efficiency of the Fergusson layout greatly depends on the characteristics of the viscous couplers (type of viscous fluid, design and spacing of the discs, etc).

A more efficient (and expensive) 4WD layout is the one involving a Torsen ( which stands for TORque SENsing) differential. This extraordinary device, invented by the American company Gleason corporation, is based on the non-reversibility of worm gears and worm wheels (i.e. when you turn the worm wheel the worm gear turns but not vice versa). The torsen differential has the advantage of being a fully mechanical device which guarantees its instantaneous response and progressiveness. Its main advantages therefore resume to:

• Instantaneous response

• Linear increase of their locking to speed difference curve

• No locking or speed difference inhibition under braking

The Torsen differential A: Differential housing B: Out axle C: Worm wheel D: Worm gears E: Synchromeshes F: Hypoid wheel (from engine) G: Out axle

Torsen differentials are expensive devices. They split torque in a 50:50 proportion in no-slip conditions and can manage slips up to 20:80 ratios between the wheels they drive. The main examples of torsen differential applications are:

• The Lancia Integrale

• The Toyota Celica GT4

• The Audi Quattro Turbo (the earliest series used a manually locking differential)

The most important difference between torsen differentials and viscous couplers is that the torsen has a torque sensing characteristic while the VC has a rotation sensing characteristic. That's why torsen differentials only lock when power is applied to them whereas viscous couplers lock both when power is applied and while braking.

There are so many details and technical stuff I could mention here that the information would probably be rendered unusable. I will therefore go into no more detail (unless you suggest otherwise).

When you add a turbo charged, fire-spitting engine to a 4WD car the mixture becomes explosive. To sum up the situation in those high performance full time 4WD turbo charged devils you have automatic differential locking and slip control, high output turbo engines exceptional road holding abilities and performance.