1.6 to 1.8 LSD Swap (3.6 gear ratio) - 1990 Miata
 

Hi All,

I just finished a 1.6 Open Differential to 1.8 Torsen 2 LSD swap on my 1990 Miata. I have a 6 speed transmission from an 1.8 that the original owner had installed, so I was running very short gears with a 4.3 differential gear ratio.
Long story short, I had issues when I initially registered the Miata in CA (from OR) where they warned me that the car won't pass smog in 2 years because the RPM/Speed/Gear don't match up and is considered "Customized". One of the nice benefits of living in the Bay Area, CA... Location (beach, mountain, desert access), Weather & Sunshine (although it's been cold and raining for the past month), Housing (one of the most expensive areas in the nation), and Emissions (the never ending slist of non-practical strict laws). So after doing much research and contemplating if I should revert back to a 5 speed or not, I decided to keep the 6 speed and try to find a differential with a 3.6 gear ratio, where the first 3 gears is almost identical output to a stock 1.6 5 speed with a 4.3 gear ratio.

After months of searching, I found a local 1.8 Torsen 2 LSD kit with the axles and drive shaft. I picked up the 3.6 ring and pinion from Yossi on the forums. I had a local shop install the 3.6 R&P into the 1.8 Torsen LSD and managed to do the rest of the swap.
I put together a DIY writeup from my recent experience to share with all of you. It is attached as a Word file. It's not perfect but I hope this helps anybody thinking of doing a differential swap.

~Aki

Glad it worked out for you the way you wanted.
The manual is awesome.