Fun with legal LSD

            First things first, what is an LSD?  It stands for Limited Slip Differential,  why is this important? Because it allows your car to get an incredible holeshot due to increased traction.   The differential is the thing in the rear of the car that splits the power coming from the driveshaft up between the 2 rear tires.  

             Now the factory 240 (with a few exceptions) use what's called an 'open rear end'.  In this setup both tires are basically free to spin at whatever speed they want to, this has one good side and a bad side.  The good side is that when going around a turn the tires don't 'scrub' or 'hop' around since they will be traveling at different speeds (outside tire has farther to go so it spins faster).  If you had a solid axle (or welded the 2 halves of the differential together) you would have killer grip in a straight line but handling would be extremely bad.   The bad side of the open rear end is that since both tires are free to spin at whatever speed they want, whichever tire has less grip (the one spinning) will get more power transferred to it which will cause it to spin more, which will cause it to loose more grip and spin more which............. anyhow it's not a good cycle. 

             The LSD comes into play right here, LIMITED SLIP comes to the rescue by stopping the loss in grip by recognizing that one tire is spinning and then transferring the power to the other wheel that has grip.   The type of rear end I'm using is called a Viscous LSD which means it uses silicone and 2 plates to operate.   The simplified explanation of how this works is that there is a plate stuck on the end of each shaft.  These plates spin at the same speed as the wheel they are attached to.   When one wheel looses grip and starts to spin faster it creates friction between the two plates due to the difference in speed.  This friction heats up the silicone floating around causing it to expand.  When the silicone expands it pushes on the other plate 'locking' it to the one that's spinning.   This locking is what causes the other wheel to push.   

             The fact that both wheels are now pushing is VERY apparent while driving.  My friend's turbo 240 dropped a full second off his quarter mile time after I installed an LSD from an Infiniti J30 into it.  My car can be launched at 5,500 RPM's by sidestepping the clutch and flooring it with almost no wheelspin.   But this isn't just useful to dragracers, the difference while autocrossing is even more pronounced.  The car can start accelerating earlier and harder out of corners than you could ever hope to do with an open differential.   Lastly the car is much smoother to drift (but it dose take a lot more power to get the rear end to start coming around).

             Now that you know what an LSD is I'm sure you want to know where to get one.  The easiest way is to find a 240sx with one on it already and take it.   There is a lot of talk about what year and options a 240 has to have to be equipped with an LSD.  From what I've heard SE's with ABS, HICAS, or S14 5 lug SE's w/ ABS all should have the LSD.  Canadian 240's also have VLSD (all of them).  The easiest way to check is that  the LSD will have an orange sticker above the fill hole that identifies it as an LSD.  Also if you turn one side the other should turn the same direction.   You'll also notice that the LSD is harder to spin by hand than the open differential.    If your like me you can't find a 240 in a scrap yard anyplace with an LSD in it, so you have to improvise.......... here comes the fun part.   The R200V (VLSD) differential is used in the Infiniti J30, and Z32 300zx.   The Infiniti one has an ABS sensor at the front which adds about an inch to the length of it.  The flanges for the halfshafts (the shafts that go to the wheels) are the same pattern for the 95 and up J30's (pre '95 used a different pattern at least on the ones I saw in the yard)  Also if you own an S13 (89-94) you will need to get a new differential cover gasket and swap the rear cover off your R200 (stock diff) onto the new one.   The S13's used a different bolt pattern (4 bolts) than the S14 (2 bolts).    The 90+ 300zx one also fits but takes more work.  First thing DO NOT get the twin turbo diff, it's got different gear ratio's in it and it will really slow your car down (3.86)  The NON turbo has the same ratio as the 240 (4.06).   The 300sx also uses the 2 bolt rear cover so if you own an S13 you'll need to switch it.   The biggest problem with the 300zx  is that the output flanges are different.  The look like a '5 star' flange where the 240sx uses 3 sets of 2.  The parts to convert a 300 to a 240 can be had for around 200-300 new.   I got mine at a scrap yard for 150. 

             The M30 and Q45 diff's also work with the same basic modifications I mentioned earlier.   The big thing to look for is the flanges, they need to be the 3 sets of 2 or you will have to replace them.

             I have heard rumors that the J30 differential requires you to shorten the driveshaft by an inch due to the position of the ABS sensor but my buddy hasn't had any problems with his when he installed it in an S14.   It's only been on for 2 weeks and the fit is tight but it works.

             The install is pretty straightforward, just unbolt the stock diff and then stick the new one in.  The hard part is getting the new diff up and in place so you can bolt it on (these things are HEAVY).  I needed another guy to help me and 2 jacks to get the thing where I wanted it.    Care must be taken when tightening it down that it's level when you start tightening so that you don't' crack the cover.   I hung it in the back by sticking the studs on the cover through their holes and then lining up the bolts in the front and tightening them down then I put the nuts on the rear.  After that put on the 6 bolts on each side that holds the flanges to the driveshaft.  Finally bolt the driveshaft to the differential.  

             While you have it out I highly recommend changing the fluid in diff while you have it out since it's a major pain to do it while it's still in the car.  You MUST use either a fluid designed for LSD (royal purple or Redline) or add the LSD additive (this additive contains the silicone I mentioned earlier that heats up to make the differential lock up).   Another thing you might want to do at this point is take the bottom drain plug off the stock diff and put it on the new one at the filler point.  The bottom plug has a magnet on it that picks up metal shavings floating around in the case, it's not really necessary but since your throwing the other diff out anyhow you might as well keep the magnet for the new one to help keep stuff from floating around the case.

*might need to have driveshaft shortened one inch due to ABS sensor but this isn't' confirmed and my friend has no problem with this setup

Written by Paul [email protected] 

people want to know about this swap!