So here in the next month or so I’m supercharging my 90 NA 1.6 Miata, I was told to get a Torsen LSD rear but not sure what ratio to go with, what do y’all recommend?

 

You’ll need to provide some more info before you can expect a decent answer.

what is the intended usage of the car? Street driven down to the shops and back, street driven only on the interstate, track driven at COTA, autocross?
What’s the hp goal?

Each of those options would potentially give a different ideal ratio..

 

Sorry about that, we’re looking hp wise at 250 eventually. It will just be street driven and interstate for now. Just something to cruise around in and have a little fun, nothing to crazy.

 

My vote out of the factory options would be for a 3.6. Keeps the revs down in top gear when cruising and you don’t spend your life changing gears.

Fwiw, my track car is a supercharged 240whp nb which runs a 4.1, which is great for most tracks (though too short for the longer tracks) but would be painful on the interstate. (4000rpm at 70)

 

I had similar initial plans and currently have 262whp running E85 through the stock 1.6. I currently have a 4.1 Torsen in the car and it's perfect for my car and where I drive. Granted, I don't do much highway driving and my commute to work is a fun mountain road and on base (read not much traffic). The more highway you plan on doing the lower I'd go. I wouldn't mind trying out 3.90 but I don't think I'd go all the way down to 3.6, personally.

 

Also, 5-speed or 6-speed transmission?

 

6 speed with 3.9 was perfect for NCM Grand Full last month. I had a 4.3 in it in May, and had to go into 6th 3 times per lap, which was painful. The 3.9 made it far less busy. I have a C30-94 making 260ish WHP.

 

for a track car you ‘want’ to use 6th, as the rev drop from 5th to 6th is less than 4th to 5th,, meaning the car will pull harder in the higher gear as it’s further up the rpm band, especially a rotrex’d car that makes more power the more revs you have.

So ideally you’d run the shortest diff ratio (highest numerically ) you can that doesn’t exceed the limiter in 6th on the longest straight. That way you can use eg 4-5-6 instead of 3-4-5 and the car stays on the boil better after gear changes.

though this is completely different to OPs needs where his is street/highway based.

 

I don’t disagree in principle, but I was never accelerating for long in 6th (and at a slower rate since I would be making less power than the same speed in 5th with the 3.9), and it made for trickier shifting at other points on the track, as well as having a double downshift for the next turns. If I was able to get the speed up higher it would be a different story.

 

Right on! I don’t travel the interstate often, maybe once or twice a month if that. my job route consists of back roads and mountain roads as well. I was looking at getting the 4.1 but was curious of other peoples thoughts.

 

It’s currently a 5-Speed.

 

With the 5 speed, 4.3 is not bad at all.
Other choice would be 4.1.