So about a week ago I posted some preliminary information about attempting to install an IS300 LSD into my IS250. Well it is now installed in the car, so I thought I'd just post a quick information thread along with final install pics and initial thoughts.
The IS300 uses the same differential case/setup as the IS250 RWD and 09+ IS250 AWD. The 06-08 IS250 AWD uses a smaller diff and the IS350 uses a larger diff (same size as the IS F):
01-05 IS300 models have a 205mm rear differential
06-08 IS250 AWD models have a 190mm rear differential (A Torsen from an IS200/RS200 Altezza might work well in this application)
06-11 IS250 RWD & 09-11 IS250 AWD models have a 205mm rear differential (The IS300 Torsen is a bolt-in upgrade for this application)
06-11 IS350 and IS F models have a 208mm rear differential
If you want to do this, what you'll need is an 01-05 IS300 differential with LSD. They come in either 3.73:1 or 3.91:1 ratios, same as the IS250 does.
There are various ways to tell what diff the IS300 has, depending on what information you have. There is the axle code on the car's information label, the sticker on the differential, looking through the axle hole, or pulling the diff cover off. As it is a Torsen, you cannot simply just turn one side and see if the other spins the same way or something like that, as without load the diff will act like a regular open differential.
On the IS300 information label, the axle code will be listed:
B01A = 3.73:1 (MT) Open
B01B or B01C = 3.73:1 (MT) LSD
B03A = 3.91:1 (AT) Open
B03B or B03C = 3.91:1 (AT) LSD
On the IS300 differential sticker, the code will be listed: (My sticker was so old, it could no longer be read, so I had to check the actual diff inside to confirm)
M82 = 3.73:1 (MT) Open
M86 = 3.73:1 (MT) LSD
M96 = 3.91:1 (AT) Open
M98 = 3.91:1 (AT) LSD
: The above list may not be comprehensive, there may be other labels.
As my car is a M/T (thus it has 3.73:1 gearing), I decided to try to find a complete differential out of an automatic IS300 for the 3.91:1 gearing (slightly improved acceleration at the expense of top speed). After finding a used one, I decided to pull the case off and inspect the gears for wear and to double check if it was an LSD. The sticker was unreadable on the differential I got.
After pulling out the IS300 axle stubs (you won't be using these), I cleaned and painted the housing and put it all back together with new seals, drain/fill plugs and a new breather. Not wanting to touch the rear suspension on the car, the exhaust was removed and the diff lowered down enough to get the axles out (they don't bolt-off like on the IS300).
Initial impressions are very good. The turn-in response on harder cornering is better, and 1st gear take-off is (I think) nicer with the 3.91:1 gearing. I've only driven it a few kms, nothing really spirited yet, but so far it seems like it's working very nicely. There wasn't any noticeable wear to speak of on the ring and pinion and it seems very quiet.
So chalk this one up as a great success!