The rear differential affects how your 240SX puts power from the rear wheels to the ground. Several types of diff are available for different types of driving.
Weld locking the stock differential is a popular but controversial modification. Welding a differential is typically done to a drift-use car, and is unfavorable to daily-driving conditions. With the proper knowledge daily driving a welded differential can be a livable experience.
The "welded diff" is done by welding the spider gears together in the factory differential. Special care should be taken to make the most durable welds possible, some have welded thick bolts onto the spider gears to make for a stronger weld. Failure could result in an immobilized diff which would cause an unsafe condition for you, and those around you.
- Breaks traction consistently, makes drifting much easier than open diff.
- It's cheap, a welder will charge something near 50$ USD to weld. Replacement differentials can be found used for around 40$ USD.
- It's extremely durable when done properly.
- Little to no maintenance.
- In certain situations, and if driven correctly, you will have more grip in cornering.
- Requires a different way of taking corners. Must accelerate through a turn, to the limit of adhesion, taking care not to break the tires loose and cause oversteer.
- Tires make noise at very low speeds (typically parking, or U-turns) as the wheels "fight" each other. The wheels are traveling at the same speed, but along different radii, which causes the outside wheel to shake, hop, and break loose at low speeds. This will increase tire wear in normal use, but if you really care about tire wear over performance, you wouldn't be welding a differential. Let it also be known that most 2-way LSD's exhibit this same characteristic, especially the more aggressive ones.
- Car tends to "push" at low speed.
- Not recommended for inexperienced grip driving. Can be very effective and far more consistent than an open differential, but requires advanced driving techniques.
- If the welds break, they can cause differential lock, and lead to a potential accident.
How to weld your stock diff
A viscous differential works by heating a thick fluid, making it expand. The diff has 2 plates close together (one for each axle), similar to a clutch type diff, but they never touch each other. When the wheels spin in opposite directions the fluid between the plates is heated up and expands, locking the plates together. As you can imagine its not a very efficient lock, but the smooth power transfer makes it the most streetable limited slip differenital.
VLSD may not be a direct drop into the S chassis cars. The LSD that works best on a 240SX is the R200V. Depending on what vehicle the R200 comes from, the differential cover, output shafts, and drive shaft may need to be swapped as well. The S14 only has a 2 bolt rear cover while the S13 has a 4 bolt. Also the output shafts may be 5 bolt outputs, so the axles may also need to be changed from the stock 6 bolt axles. They were an option for all 240sx, but are more rare in the US. The drive shaft on a factory LSD equipped 240sx is about 3/4" shorter than non-LSD models, and likewise the VLSD unit is about 3/4" longer than the open differential. Some have reported using a non-LSD drive shaft with a VLSD and not having issues. The R200V differential came on many other cars, such as the Infiniti J30, Q45, Skyline R32, S13 240sx W/ Hicas, and 300zx (n/a only; the turbo models have an R230 that won't fit without more complex modifications). Because of the availability of these differentials, the VLSD is an obtainable and practical LSD upgrade for the stock 240sx. Be careful about the gear ratio, though. There can be a difference in the gear ratio depending on the vehicle you source the R200V from, and this can affect the accuracy of the speedometer and also make the engine run at a different RPM at a given speed when compared to the stock unit.
The S15 differential has gears that are cut in a fashion that when the car is turning the gears mesh together and transfer power to the tire with more traction. It does not lock like a clutch diff, but is very smooth. The best thing about a helical diff is that it practically never wears out. You can treat it the same as an open diff: change the oil regularly and it will likely outlast the car.
The S15 helical differential is probably the best diff for road racing applications. It is not recomended for drifting because it never actually locks. The differential can only multiply the torque it can apply at the side with low traction, so if one tire has very little traction (such as on ice or in the air) then the differential will act the same as an open differential.
The Clutch type differential works much like you would expect. There are a number of wet clutch plates close together and when the power needs to be transfered the clutch plates lock together helping the power get to the wheel with more traction. There are three common types of clutch differentials used: 1-way, 1.5way and 2 way, each with its own advantages.