ThirdPedalNirvana

I thought about posting this in the 1st gen forum I started to discuss LSD options, but I realize this is a larger topic that not only applies to all RX-7's but all cars in general.

https://www.ebay.com/itm/LSD-Limited-Slip-Differential-Conversion-Kit

I have been skeptical of LSD conversion kits for open differentials (as linked above) for as long as I've seen them on eBay, which has been a long time. I've mostly just sort of ignored them as some kind of snake oil or other bogus device that can't possibly function as described.

Finally, my mechanical curiosity got the better of me and I started researching videos of people installing these kits to see what they are and how they work. From what I can tell, the idea is to insert this spring-block device into your differential, insert the cross shaft through it, then remove the retaining pins which hold the two halves together. The springs then push the two blocks so the device becomes wedged between the bevel gears. The device rotates with the differential carrier, so whenever the wheels spin at a different speed and the bevel gears rotate with respect to the carrier, this simple spring & block device pushes up against the bevel gears to provide resistance to this turning. Aha, I thought, so that's how such a simple device works.

But if it's that simple, why do no OEM's use such a device?

The realities of metallurgy make this quite troubling. The friction surface is not clutches nor shims designed to provide friction without disintegrating, its the face of the actual bevel gears, gears which I presume are produced with powder metallurgy and sintering. I can easily see how such a device would, in an alarmingly few amount of miles, wreck the differential by turning the bevel gears into shavings and clogging up the bearings with those shavings. Or, I could see how this effect could take hundreds of thousands of miles to do any real damage. The real test is real world empirical data.

Have any of you had any experience with this type of differential conversion kit over a significant mileage (30k+ miles)?

Considering the cost for an open diff is roughly half that of a torsen diff, it might be a worthwhile experiment to try it and see how it goes, unless I can draw on the experiences of those who have tried it and have ruined their differentials. Almost all the reviews for this type of product come from people who have just installed them, and they are all on the websites of the manufacturers who make them.

For reference, I have an '85 GSL with an OEM clutch type diff that doesn't lockup anymore, unable to find rebuild parts. I'm considering OBX torsens, OEM torsens, and now OEM miata open diffs so I can get the 4.10 gears from it.

Jake the Snake

If I'm understanding how that thing works, I don't think the breakaway torque would be very high. It looks like the friction is in a pretty small area compared to a clutch lsd.

As for longevity I couldn't say for sure, but I'm not optimistic at all.

There's a thread somewhere about shimming an old clutch lsd with round sections cut out of beer cans. That would be cheaper and work better. I think the guy got it to be nearly locked