rear open diff vs rear viscous diff
 

I have a 98 RS and am in the process of swapping motors (WRX). My car, for this year atleast, was given the open rear differential. I've been debating whether or not to swap in the WRX rear end along with the motor. I understand that the WRX rear offers the viscous... but I'm wonder which is more practical. I've searched and read for hours on end, but I've gotten mixed reviews (and no real solid details). So, I turn to my fellow I-clubbers for their knowledge and opinions!! Under certain driving situtation, which would be better to have? How exactly does an open diff work by the way? I plan on using this car for everyday driving, many sprited drives thought canyons and backroads..maybe some fun in the rain? I need some factual information to compare the two, ASAP!! Thanks in advance.

You would have to use the WRX diff if you swapped the transmission also. If you have the RS gearbox, using the WRX LSD means disassembling everything, swapping the guts, and putting it back together. It would be a far more economical solution to just go find a used 99-01 RS rear diff.

I understand that, which is why I'm debating over this. I can go either way (financially). If I choose to use the WRX viscous, I'm going with the wrx tranny. If not, I'll keep my RS stuff. I just want to know what the performance aspects of each are and the pros/cons of driving with each. I'm not concerned with the financial aspect/practicality of things right now, as I have those dialed. I just need to know all else to make my final decision.

Oh, okay. Basically, the debate comes down solely to which effective gear ratios you want. The RS gears will make the car accelerate faster, especially at lower speeds, while the WRX ratios will give you better fuel economy. Both have equal strength in terms of how much torque they can handle. I think I would personally go with the WRX drivetrain, because most of the time, you're gonna want the higher gears. Plus, you get the benefit of the easier LSD swap. The RS gears will be fun for drag racing and low speed autocross, but on higher speed courses and racetracks, I often wish I had taller gears for higher speeds. It sounds counterproductive, but when your car tops out 4th gear on a straightaway, slightly lengthening it is preferable to having to shift to useless 5th gear. ;) Besides, with all the extra power, you'll still get to the top of each gear in less actual time under full throttle. Also, you can easily shorten your effective ratios by using shorter diameter tires- 215/45R16, or 215/40R17 for example. My apologies if you're wanting us to just say "use the RS transmission", but hopefully I've given you enough info to decide for yourself. :)

Edit: whoa, post #10,000. I need another hobby...