We set up a track day with Erik Shauver, designer of the HPI Blitz at
our local indoor track OC/RC Raceway in Huntington Beach, Calif. The
track was recently changed and that provided lower grip levels than
normal, which in this case would help us see the difference more. We
equipped the Blitz with a Tekin Brushless setup with a 10.5 mod motor
and Pro-Line Bow-Tie tires. This will give us some increased grip and
the power to spotlight handling characteristics that could be overlooked
in slower classes.
We had five diff options and each was installed and run to get an
overall feel and handling characteristic. We didn't do lap times because
the track grip levels aren't super consistent and with a new track the
more we drove the better we got, but we did keep an eye on speed on the
track. Fastest lap doesn't always mean better, but we're pretty
confident with our SC racing that the changes are easily going to equate
to faster more consistent laps.
1: HPI Z164 Heavy Duty "Green" Grease (about the viscosity
of full-size automotive bearing grease, gears lightly greased)
Acceleration - 1 Stars
This was clearly the worst of the bunch on many levels. The truck was
very darty under acceleration, and almost felt like it never wanted to
go straight. Hitting timing jumps was also difficult, and required more
rounded corners to maintain speed.
Corner Speed - 4 stars
As long as I was off throttle, I had lots of corner speed and could hold
tight lines. But since I was always on the threshold of losing traction
it was very inconsistent.
Stability - 0 Stars
This yielded the least stability in the test: going out of corners,
braking, and even down the straight at full throttle felt like the
chassis never settled; it was dancing all over.
Overall feel: 1 - Stars
Terrible, unless you're required by some spec rule, or have super high
traction (maybe the track is made of glue). We can't recommend a time
where you'd want just grease in the diff. It creates an inherent
instability that might be fun for a few minutes in the street, but not
so fun on the track.
2: 4,000 CST silicone oil (same as a stock Blitz RTR diff)
Acceleration - 1 Stars
Our test vehicle was an HPI Blitz and it was surprising that the light
stock diff oil wasn't much better than the grease. The rear end was
still dancing out of corners and the ability to clear tight timing jumps
was still very low.
Corner Speed - 4 Stars
Like with the grease, if you coasted through the corner you could
maintain a tight line, but at any second or sudden input the rear end
instability was immediate.
Stability - 1.5 Stars
Slightly improved over grease, but still felt very skittish on the
straight and could spin out if you just barely hit the gas.
Overall feel - 1 Stars
Still way too light and unstable. Corner speed is still pretty good off
throttle, but with the instability everywhere else, was a waste.
3: 10,000 CST silicone oil
Acceleration - 2 Stars
This was getting closer to where the truck would start to accelerate out
of corners harder and hit the jumps coming off a slightly tighter line.
It was still not completely stable or comfortable but better than the
previous two tests.
Corner Speed - 4 Stars
You had to start to use the throttle in the corners to maintain speed,
which is a good thing since now it was starting to feel like it wanted
to have power.
Stability - 2 Stars
This was the first diff that we felt added some stability. Down the
straight the rear end wasn't dancing back and forth and you could use
the brakes and not spin out on a dime in some corners. There was also a
noticeable increase in out-of-the-corner punch that allowed the truck to
clear jumps using tighter lines.
Overall feel - 3 Stars
Still not rock solid in stability here, but there was a noticeable
difference in stability and acceleration that made you feel more
confident. This did require that you start using a steady smooth
throttle in the sweeper to go faster, but that's much easier to do than
holding on to car on the verge of spinning out all the time.
4: 30,000 CST silicone oil
Acceleration - 5 stars
It was pretty clear from the first lap that we'd reached a spot where
the Blitz felt right. You could now punch the throttle just about
anywhere, but still overpower the rear end. Hitting jumps was now easier
as the truck launched forward.
Corner Speed - 4 Stars
This is an odd rating, because off power, the truck would be slower than
an open diff and in some tight high-speed corners. But with the
stability and natural tendency to slow down quicker, drivers just had to
use throttle in the turn to get the speed back. The natural coasting
speed comes back with some throttle, and you can maintain a higher
average corner speed because you can use the brakes to slow the truck
down or rotate it.
Stability - 5 Stars
This was the most stable feel of the test. Out of corners the truck
squared up. If you left off the throttle or if you got the rear end out
of shape, the truck squared up and you could just go full tilt down the
straight and ease on the brakes with little drama.
Overall feel - 5 Stars
We've done lots of SC racing and our previous testing showed us that
these trucks are way easier to drive with a heavier diff. The gains in
traction from this weight were immediate and we feel that for 99.9
percent of drivers the confidence will make for faster lap times.
5: Prototype ball diff
Acceleration - 3.5 Stars
This had some better acceleration out of corners than the lighter oils,
but after driving the heavy oil timing jumps showed decreased forward
traction. It did track straight, but the lose in forward movement was
Corner Speed - 5 Stars
This was a pretty good mix of both the lightweight oil and the heavy oil.
You could use some throttle but it coasted a higher, tighter line than
Stability - 4 Stars
Out of corners and down the straight the truck was pretty rock solid,
only really bothered by abrupt use of the brakes. You could get the rear
end to get out of shape pretty quick, but overall it was still pretty
Overall feel - 3.5 Stars
Back in the day ball diffs were king. We were low in power and weight
meant everything. The prototype ball diff we tested is considerably
lighter than the seal diff options but at higher-power motor levels that
means nothing. This was shown by the heavier, thicker oil diff
accelerating better and harder than the lighter ball diff. If you're
racing stock motor you probably can't use this diff anyway, plus the
maintenance is going to be tenfold higher, which to us is a negative.
Contrary to popular thought, the ball diff is not the best option. While
it did show us that it is a good compromise, the fact that we preferred
the heavier (mass) and thicker oil gear diffs is proof that times have
changed. Most of us have so much power and low enough grip that having
more mass might actually help in feel. We were a little surprised that
our steering didn't fade out as much as we thought, but since the truck
started with a severe over-steer tendency that reduction brought it into
the sweet spot. You
will lose some turn-in and response as you go heavier, so you will have
to figure out the balance you need to go fast. If you're having trouble
with the rear end on your SC truck, going to thicker oil will solve most
of that issue. It won't teach you to drive or throttle control but it
will help remove some of the natural handling characteristics of a high
power 2WD car with low traction, A.K.A. spinning out on power.
Where to Start
We'd love to generalize and say run 30,000 wt. in every SC truck, but
that would be dumb. Differences in tolerances and diff volume will make
the "sweet spot" different for every truck (even though our
Slash setup suggests 30 - 50K oil) and can vary with track conditions.
Higher traction will allow you to run some lighter oils to get some
steering back, and if you have some terrible setup to start, that can
make things worse in every change you make. Our suggestion is to start
to go thicker with oil and get a feel for the handling. Our testing has
shown us time and time again having a more stable truck makes us faster.