Another long overdue update. Figure I'd share my painful experience of upgrading to a 1.8L Miata 7" diff.
Baseline build was centered around the 1.6L open diff. Picked because it came with my donor, it is the lightest option, and research had me convinced that there is no need for limited slip diff. Four years of fighting the dreaded one-wheel-peel coming out of slow corners, banging off the rev-limiter in 2nd gear, or up/down shifting between 2nd/3rd gears during Auto-X, and a buzzy 4K RPM highway drive made me rethink that diff choice.
Enter the solution. I bought a 3.63 Fuji diff with axles that only the Euro/Aussie MX-5's got.
First issue was getting the bigger diff to fit into my chassis. These larger diffs are a simple drop in swap in every Miata assuming you have the correct axles and driveshaft. Not the case when you make a skinny driveshaft tunnel that functions as the Miata PPF. These better diffs extend the driveshaft mating flange (nose) a few inches forward which makes the driveshaft a little shorter. You'll also need the specific flange yoke to mate with the new diff. I also needed to notch some tubing in the driveshaft tunnel, grind on some of the diff casting, and saw off ~0.5" of the diff casting right next to the PPF mounting bolts. Lastly I needed to make a drop out section of my chassis so that I could install/remove the larger diff. Pics to come.
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First few drives were positive. LSD action made acceleration better out of slow corners. Gearing change from 4.3 to 3.63 was a game changer for Auto-X and highway drives. There is a reason why all higher HP Miata folks love the 3.63 and 3.9 diffs. Next few drives the diff started to make angry noises.
After a few weeks of bearing changes, pinion crush sleeve fighting, back lash setting, and multiple bottles of Redline fluid all I successfully did was make different types of diff noises. I pleaded with my driveshaft gut to recheck the balance of my driveshaft, which of course was perfect. We did notice some bluing/wear marks on the driveshaft yoke that goes into the transmission. So one new yoke and one new transmission tailshaft bushing later and guess what the results where? A different noise.
Getting late. Let's finish tomorrow.