I have some encouraging information to share about the planned Torsen limited slip for the 2010 model IS-F. Please keep in mind this info is unofficial until a swap is actually performed and tested!
The 2010 IS F offers a Torsen limited slip differential installed in a type
FD21A rear differential.
This is the same type of differential case that was standard in the 2009 IS
F and means that the Torsen LSD could be a complete bolt-in for earlier IS Fs.
Based on this first look at the technical literature, it looks encouraging.
You wouldn't know for sure until someone lines up two cars on the lifts and
starts dropping parts on the floor. There's always the possibility that
TMC might have changed something like the number of splines on the side
gears, or the threads for the mounting bushings, etc.
The new IS F Torsen unit also seems likely to fit in earlier IS 350s, since
they come with a type FS21F (though not IS 250s, which have a narrower type
F19SX diff with different mountings for the subframe).
Preliminary info we have so far does not indicate any change to the gear
ratio of the IS F final drive. TMC rarely changes the final drive ratio
unless there are other drivetrain changes (engine output increase,
transmission change, tire size change, etc.).
Based on that, we assume that the 2010 IS F final drive gear ratio is
unchanged from the current 2.937:1.
Now, as for the auto-limited slip (A-LSD) function of the brake control
system on the 2008-09 IS F:
It's not clear that the LSD-equipped IS F would have to lose the A-LSD
function. We've had models in the past that integrated a LSD with VSC (a
Torsen was available on the original IS 300), and the two did not conflict
with each other.
Theoretically, the Torsen acts instantaneously, while all electro-hydraulic
systems have some time-lag. So if the Torsen works first and can limit
side-to-side wheelslip, the A-LSD may not be called upon, and there may be
In any case, I cannot think of any way to permamently disable A-LSD: there
are no specific parts or sensors dedicated just to A-LSD that could be
removed from the circuit. It's just completely integrated into the brake
control network & hardware for VDIM. A-LSD is going to come on when the
VDIM system feels it's needed.