What would a front limited slip differential do for a WRX?
 

I am assuming that it would reduce understeer when you get on the power while the wheels are still turned in a corner. Is this right?

Fearing that Gary will chime in and crush anything I say, I will none the less comment
I guess teh answer starts with it all depends...

A perfect LSD would be electronically controlled tied to actual ground speed at each corner and steering angle... Anything else is compromise.

Since a mechanical LSD can't know what the steering angle is, it can't know how much to let the wheels slip when you actually want to turn as compared to when a wheel is actually slipping.

In my experience with Audis, front LSDs can actually increase understeer. Matching all 3 differentials with the suspension is a huge challenge.

As you can tell from Gary's list of differentials for his USTCC car, the diff configuration may vary by course, and conditions...

This complexity is probably why none of the manufacturers I am aware of ship an AWD car with three LSDs, and IIRC I am only aware of one Audi that I beieve had a front LSD, and an open rear. I realize this didn't help much, but it is a really complex question.

Except the STi and EVO... and probably a butt load of 4WD trucks.

See, it didn't even take Gary...

I'll go out on a limb again and claim that most if not all the 4WDs are either open, or "locker" fronts, not LSD.

Now the STI, and EVO RS have Helical Limited Slip Differential fronts, and Mechanical rears. The normal EVO has an open front by the way.

I need to do more research to understand the Helical LSD. I am more familiar with the straight mechanicals, and torsionals on the Audis... My guess is that on the Helicals, there needs to be significant wheel spin prior to power transfer.

I stick with my earlier assertion... Front LSDs are a huge challenge due to there impact on steering. There are very few situations where you want to compromise front steering in favor of front power...

Going back to your original "it depends" answer, an aggressively locking front diff would severly increase understeer. However, I htink a more moderate locking diff, just enough to prevent inside wheelspin, could really help. After driving an STi somewhat spiritedly for the first time, I could definitely feel the difference under power with high steering angles. The STi is much less inclined to wash out. It really tries to hold the line. I think that if the diff isn't locking, just "encouraging" the outside tire to try to keep up, it would work great.

The other issue is that you definitely have to make sure all 3 diffs work together well, or you'll have problems. Nate was going to just run the Cusco RS tarmac center diff in his coupe, but learned that Cusco strongly discourages that because it doesn't work well, especially with viscous LSDs like stock. Because of that, I think that even just adding a front SureTrac out of an STi to the WRX wouldn't make it as good as the full STi setup. I suppose if you got a DCCD 5MT and used the R180 mechanical LSD from an STi/22b (are they helical or clutch type? I'm not sure actually) it would work well. But, that would probably cost more than just getting a DCCD 6MT and rear diff.

Actually, for the most part only the hardcore guys run lockers up front. Most people run LSDs because it's much more streetable and more affordable/reliable than an air locker or cable locker.

Newer "4WD" sport utes mostly run LSDs. Some have automatic lockers in the rear. Couple more years and most SUVs will be AWD instead of 4WD.

Assuming the car has 4 wheels ... what is the difference?

AWD: Lack of low range, viscous center LSD (or something more complicated like DCCD or VCD like 4EATs have), no locking diffs.

4WD: 2WD and hi/low range, open or locking diffs.