These next few posts aren’t going to come out in order, but I’ve been bad about updating the journal so I’m just going by what’s off the top of my head. Yes – this is diffs again. 2nd diff (technically 3rd if you include the factory helical) for the front and 4th (? stock, Cusco 1.5 Type RS, OS Giken/modified OS Giken by TRE, ATS Carbon Hybrid 1.5-way) differential for the rear.

Front diff is being replaced because it and/or the transfer case broke at the last event (June 1, 2024 – LRP Proving Grounds). Massive whining began at higher speeds/gears, as well as a decel whine after the event. Likely caused by hitting a curb on one of my runs hard enough that I seem to have jumped it, causing a massive speed differential in the front wheels, while staying flat out. It’s possible this is the transfer case, but regardless, I haven’t loved the ATS front differential. It was a hell of a break-in process, but I was also always a little uncomfortable with it, given that they revised the housing shortly after I purchased mine. Turns out many of these diffs had been imploding in competitions, often taking the transfer case with it. More than that though, I haven’t felt any difference from it. The car feels a little pushy on power, so I’m looking to change the setup. Either way, it’s both time to rebuild the transfer case from a performance aspect as well as from time of use aspect (~58k miles on the car with the stock t-case). The Cusco I’m running in the front is the 1-way 450F. As I don’t think the ATS was ever set up right, I’m hoping the Cusco will perform as intended, and with much, much more durability as well. Plan is for minimum preload springs (8) and 60% of plates engaged.

The rear diff has been a journey. It’s almost a full circle, as I’m going back to a Cusco Type RS, but with one key change – I’m going from a 1.5 way, which I’ve always run in the rear since going aftermarket – to a 1-way. Specifically, going to the 141 FG Cusco. This is the “gymkhana” version, meaning it uses the most aggressive ramp angle (55 degrees, same as I had in my Cusco 141 L15), and also uses much more preload, to offset the fact that the inside rear is in the air for pretty much every corner. The key here is really going to a 1-way diff for the rear. 1.5-way for the rear has always been a standard in the Evo world for the rear diff. The idea was that the 1.5-way would provide additional stability in the rear for things like running on the track. This has shifted over the last few years, with more and more competition evos running a 1-way diff. The “added stability” was never something the Evo really needed on braking, and, based on how clutch type diffs work, didn’t really do anything on braking either. This is due to the fact that the diff engagement on decel solely depends on what the driveshaft sees from engine braking, which is not significant. This means the only time that the diff engages is during coasting – i.e., for my case, the in-between stage of getting off the brakes and getting on the throttle. While brief, this could potentially induce more understeer for me just as I want the car to turn in. I’ve always battled the car’s understeer, and while it has become more neutral over time, I still want it to have a little more kick from the rear on power. I think this will help, retaining the aggressive lockup I knew the Cusco for in the past, with zero engagement off the power, letting the wheels rotate different speeds freely.

The plan is to bring the car to the shop over the holiday, and pick it up after it gets a few more jobs done…