stock diffs on our cars are totally rebuildable. Now VLSD's are another story. The viscous unit should it go out, does need to be replaced in its entireity.
Ok that said. the basic VLSD is a very long lasting sturdy unit. its why after several years of installing clutch types Nissan switched to putting it in consumer vehicles.
Clutch types do burn out even with just daily average use. i.e. not getting on it all the time.
Now , some of the drag racers in our crowd have switched out to clutch types because they claim the VLSD gives up under extreme hard acceleration i.e. they burn them out. Unless you are drag racing constantly or drift every day of the week it may not be an issue for you.
I also know of the other drivers, who have had clutch types as well as VLSD and swear up and down a tree that the VLSD hooks up in a much smoother way. Now that is technically a given due to viscous versus clutchs sliding together and grabbing. Of course with a clutch type you can totally adjust the lock up time i.e. how long it takes the clutch pack to engage. this can be very usefull depending on what your trying to do.
what do you need?? Also how much is your pocketbook willing to set you back?
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Only you can make that decision. Cost is a little simpler . Indeed a KAAZ, Cusco or Quaife is going to cost anywhere from $1200-2000 fully setup depending on which unit you go with.
Now if you do weekend drags occasionally or drifting on occasion a much cheaper way to go is with a VLSD because you can get them for $300-$500 depending. Which is a lot cheaper than the clutch types. Ya I know we would all like helical out of an S15 I am not even going to go there . <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt=':p'> <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/hehe.gif" border="0" valign="absmiddle" alt=':hehe:'>
I am going to be running a J30 VLSD for two reasons . One is I am going Turbo so I want a rear end with different gearing so I can keep the engine under boost for longer amounts of time in each gear(J30 3.92 gears 4% difference).
A thing to remember in all this is Nissan stuck 3.70 gears in all the SR powered Turbo cars in Japan and Europe , as well as the 300zx TT. Thats a 10% difference from our NA gears which are 4.08.
The second reason for me running a J30 is cost. I can get these rear ends for $300 thats one third the cost of KAAZ fully setup. hmmmm if it does blow up its cheap to replace. works for me. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt=':p'> <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/hehe.gif" border="0" valign="absmiddle" alt=':hehe:'>
In the end its all up to you. though I will reiterate this the standard VLSD is a long life unit that under normal conditions will not blow up unless it loses its fluid and goes dry. It will last an extremely long time.
The R200 VLSD is the size of a Ford 9 inch . I.E. Our cars have differentials that can handle V8 power without sweating. They are by far one of the strongest links in our drivetrains. You will blow up transmissions long before differentials.