I finally got a chance to put the new tires and differential to proper use last weekend at the KC Region SCCA’s first autocross event, and things went well. Very well.

Pardon the shaky video, as I’m currently stuck with an early mount from the manufacturer’s Kickstarter and have a replacement on the way ... also the swearing. If not for the cone, this would have been my fastest run by 0.9s and good for 12th in overall PAX out of 143 drivers. Instead, I had to “settle” for 20th and a STR class win (out of 5 with a 0.9s lead), which I’m still extremely happy about, considering that I spent the entirety of last year in the neighborhood of 45th-65th place (top 35-45%) finishes and usually without any STR competition.

Here’s one from last year with a similar-ish layout for comparison:

The tires: I can now fully appreciate the hype behind the RE-71Rs. Compared to the ZIIs (non-Star-Spec) I was riding on last season, they just do everything better. Simply put, there is a mountain of grip available and they do a good job of being communicative too. I don’t think they respond to sharp inputs quite as well as the ZIIs did, but that’s more of a driving style adjustment that I should be able to adjust to in short order. The day was a little too cold to get a complete idea of their temperature profile, but I can at least say that they still work very well even if they don’t have a lot of heat in them. The sweet spot for tire pressure seems to be around 29psi for me, though I still need to experiment some more to say for sure.

The diff: If you aren’t familiar, OSG’s SuperLock LSD is a clutch-type unit that is comes set up as a 1.5 way with 100% locking capability from the factory. There is some adjustability available, but the general consensus is that it’s good to go right out of the box for most people. Given my current level of experience (about 12 autocross events), I’d definitely consider myself in the “most people” category at this time, so I left it alone. For what it’s worth, the shop that installed it was also impressed with its build quality. It didn’t require any substantial break-in, and backlash was still well within spec without having to change shims.

On the street, the diff is very driveable. It stays entirely quiet, save for an occasional soft click when engaging, and the ramp-up to lock when it does happen is nice and gradual. There is a more pronounced shift in the car’s handling attitude between acceleration and deceleration, but it’s all very benign.

During autocross, it really comes into its element. I’m still trying to figure out how to best launch with it, but even with how cold and grimy the course was from rain the night before, it put power down everywhere with ease. I was genuinely surprised at how early and how hard I could put the throttle down throughout the course. Even better, the car is much more stable and predictable than when it had the open OEM diff, which works wonders for confidence and consistency. It felt easy to use and I didn’t encounter any surprise handling upsets in throttle/brake transitions, bumps, or wet/muddy spots on the course. The only downside I’ve noticed so far is that the partial lock under deceleration makes the car a little pushier in those moments, but that too is something that I should be able to adjust to. Well, that and the prospect of replacing its oil at $90/L.

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TL;DR: both are awesome upgrades and were worth the investment IMO.

Oh, and here’s some bonus Factory Five 818C goodness for your time:

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