A week with the Y58 differential
Disclaimer: I’m not a mechanic, just a guy who Googles things and then visits my local Subaru shop.
I’ll start with the conclusion. I love it and it’s great for my specific scenario. I wish I had done it earlier. Ymmv based on what you’re looking for and what you prefer. More details below and happy to take questions. I know there’s plenty of other folks who have done it already but figured I’d share anyways.
WHAT: it’s a LSD from the facelift brz that has a 4.3 final drive ratio. I bought mine from a salvage yard for about $560 before tax and shipping. I also got the KDT925 differential bushing inserts (might as well).
HOW: paid a shop, it’s a direct swap and they did new seals, fluid, and inserts at the same time.
WHY: I’m a Miata guy at heart and I wanted more responsiveness. I also still have CPO warranty so I didn’t want to do headers or a tune yet. My daily commute is a lot of stop and go with inclines and I live in the city so more torque is more good.
So what’s it like?
The awesome part is that it makes the car feel more eager in every single gear. Sure I’m shifting more, but that’s the fun part for me. My gas mileage results are interesting - I’m up from 22.4 mpg average to 22.7 mpg average. This is likely very specific to my commute, as I’m now able to spend more time consistently in 2nd gear and less time bouncing between the higher end of 1st and the lower end of 2nd. Cruising is where I take the hit the most because I’m at a higher RPM but I don’t mind.
The bushing inserts increased NVH slightly. I can feel a gentle vibration in the shifter if I think about it and a bit in the steering wheel at a stop. I now hear a small amount of (what I assume is) gear whine at low speeds. It firms up the shifting feel which is great and there’s less bucking during shifting, especially if I downshift awkwardly or am doing some low speed maneuvers in parking garages.