Alright, here's what I've found.


The PRO4 diff is 'sorta' a drop-in replacement.

I took the PRO4 Ball Differential Set 39Tooth (P/N 75141) and cross-bred it
with the stock T-Spec diff. The only part I retained from the T-Spec was the
ring gear. Everything else (clamp bolt, thrust bearing, spring, diff plates,
diff balls, and outdrives) came from the PRO4 kit. One nice feature is that
the two bronze bushings that the outdrives run on inside the ring gear are
replaced by ball bearings with the HPI kit. The diff set cost me $40+tax.

The unit thus built dropped right into the T-Spec diff case, though I did
replace the outdrive bearings with new parts, since mine were a little
gritty.

There are only 2 things I'm not 100% happy about, and I think they're
related. First, the diff balls running in the stock ring gear run right on
the very edge of the PRO4 diff plates. The second was that the diff
seemed to loosen up after every race. I'm hoping the problem is that
the diff screw keeps loosening. Worst-case, the diff balls may be wearing
agains the ring gear, which will ultimately cause the gear/diff to fail.

As it is, I ran a hard practice session, 3 heats, and a main with it last
night, and the only problem I had was a fragged outdrive on the new
diff (of course!!!) from a mis-adjusted dogbone. I may look into running
PRO4 CVDs too, just to get rid of that problem.

So there's the bad.

The good news is that this thing is so silky smooth it's scary! The stock
diffs in the T-Spec were gritty from the day I bought the car, so it's
been really hard to tell whether they need to be rebuilt or not. The HPI
diff is so much better, I can't wait to put one in the front, too. The car
was very much smoother throughout the track, and it just seemed more
planted.

In addition to the improved performance, the diff plates on the HPI
diff are NOT chromed, so you don't have to worry about it peeling off.
Also, they are about 2x the thickness of the T-SPec, so I expect them
to last a LOT longer, even without being chromed.

Our first points season is over, so I won't have a chance to run it
against other cars until after the new year, but by then I'll have swapped
the front as well and repaired the rear, so I'll be able to give a much
better report on performance. I'm going to try some Loctite to keep the
diff from loosening, so I'll let you know how it worked. Frankly, even if
I had to replace the ring gear every few weeks, the improvement in the
diff action would be worth the expense.

Now, a word on legality - at our track, we are allowed to do pretty much
anything. As long as we run the stock body, motor, and battery, anything
else goes. My understanding is that this modification would be very illegal
for a true T-Spec class, and certainly for the national race they've got
planned.

Also, at $40/diff, $80 seems a little steep to spend on a car that only cost
$150 new. That said, I'm hoping to start running my T-Spec in the stock
class (with the HPI diffs, of course), so I expect this will help quite a bit.
Compared with the $325 for a new PRO4, it's not THAT bad a deal just to
get my feet wet.

Depending on your local rules and your pocketbook, this may or may not
make sense for you. If you can't find any T-Spec parts, though, and your
track is willing to let you, this seems like the best option going right now.
Otherwise, I'd suggest upgrading the diff balls to a quality aftermarket set,
keep new diff plates in it as often as you can afford, and try to find some
thicker washers for the thrust bearing.