Additional Photography by WPC
You finally purchased your first car and savor every second driving home. The asphalt is noticeably wet so you decide to have a little play. You bring the car to a complete stop and rev up the rpm before quickly dumping the clutch. To your dismay, that burnout you left behind was unwelcomed by a loud clunking noise followed by random pieces of metal ejecting out the back of your car.
In a mechanical sense, the importance of building the rear end in any race vehicle should be equivalent to assembling the engine or transmission. Without a proper limited-slip differential (LSD) and final gear, you might as well be throwing your money away thanks to a severe loss in traction. You've spent all that time and effort squeezing power out of the engine, so why not apply it to the tarmac? Just as how people would invest in getting stronger head studs and heat treating rotational components, the rear differential is also a vital investment that should never be taken for granted.
In the case of an open differential (non-LSD), significant power is lost through the driveline spinning freely, especially during cornering maneuvers. In its design theory, a mechanical clutch type limited-slip differential is made to lock when it senses uneven wheel rotation on either side of the axle. The uneven torque feedback compresses the clutch plates together for a lockup reaction, effectively putting the power down to the surface of the road.
Limited Slip Differential Power Loss - Fact Or Fiction
These days, various limited-slip differential characteristics can be adjusted to suit specific driving styles and the genre of motorsports it will be used for. Today we will explore in entirety the rear end rebuild process including adjustment of the LSD lockup rate, WPC surface treatment, and assembling the ring-and-pinion gear on a '86 Toyota AE86 (Corolla GT-S).
A Cusco Type-MZ LSD is a cone spring type of limited-slip differential, which is a very common design nowadays. In addition, Cusco offers their LSD that comes enabled for two different profiles by incorporating two different cam angles on the pressure ring. For example, this particular unit has 1.5- and 2-way capabilities, which is suitable for drifting, since vehicle control greatly depends on throttle operation.
The initial torque (aka breakaway torque) refers to how much torque it will require for the LSD to reach full lock. The Type-MZ LSD has the highest rated initial torque rating out of any of Cusco's different type of limited-slip differentials. In layman's terms, this unit won't lockup unless enough torque is applied to fully crush the outer cone springs and compress all the clutch plates together.
Disassembly & Wash
The first and foremost task when building anything on a car begins with washing away the components free of metal shavings and foreign elements. A single metal shaving naked to the eye has the ability to create catastrophic failures, especially on rotational components that have strict clearance guidelines. A simple blast of brake parts cleaner will never suffice, so most legitimate shops come equipped with industrial hot tank washers to effectively clean with the use of temperature, solvent, and pressure.
WPC Surface Treatment
Extreme heat is what kills even the strongest metal known to man, and it primarily relies on some form of lubrication to prevent these meltdowns. Friction equals heat. To minimize friction, the proper grade gear oil must be used to effectively lubricate and keep down the heat as much as possible within extremities of the given operating temperature range. But what if more protection and strengthening are necessary? Luckily, in these times there is a scientific method that has been tried and true by many professional motorsport teams, and it is called WPC treatment.
This process is not a coating but a surface treatment, where the procedure is similar to shot-peening, but WPC is done at a micron level. Very small media is fired at high speeds against the metal surface to change its metallurgy for a stronger and more durable surface. This results in a thermal discharge that creates micro-dimples in the surface, which serves as oil reservoirs, retaining superior lubrication.
Sandblasting vs. Shot-Peening vs. WPC Treatment
Moreover, the WPC treated components also become stronger and more durable. Imagine tiny cracks on the surface of the metal that can only be seen through a microscope. A sufficient amount of these cracks will undoubtedly be vulnerable to major a fracture at some point. WPC basically creates compressive stress to areas that are treated, and the micro-thermal reaction within seals these tiny cracks. The dimensional changes on the surface of the component are negligible since all is performed at a micronic level (+/- 0.00004-0.00008 inch).