Swapped Miata diff, now driveline vibration.

thestig99 HalfDork
10/29/14 9:04 p.m.

Just barely got home from swapping from open to type 1 torsen LSD in my '96 Miata with my dad.

Now I have a vibration above 65mph. This was not there earlier tonight when I drove up there on the open diff. This vibration was not present in the car that this torsen came out of (I owned it).

This leads me to believe that something with the installation is causing this vibration. What is the most probable cause of this?

Parts that are different:

T1 Torsen LSD, complete unopened. With poly mounts (will be replacing these with OEM, holy noise)
PPF

Everything else is as it was. We DID notice that the rear driveshaft u-joint has a kind of "notch" in it. I'm hesitant to blame this since it was never an issue before, but could it be amplified by the stupid poly mounts so that it's now noticeable?

Tired and not quite thinking straight, and won't be able to get back under the car for a few days so. Any thoughts from the brain trust in the meantime?

codrus GRM+ Memberand Dork
10/29/14 11:55 p.m.

Check if the companion flange has come apart. There's a big rubber cross-shaped damper on the input diff flange, it's the thing that's annoying and in the way when you're trying to torque the bolts holding the driveshaft on. They tend to break around the circumference of the flange and can then spin freely, which obviously isn't doing much damping. :)

That said, when I took this off the car, I hadn't actually noticed a vibration as a result of it. I'd taken it off to swap 4.3 -> 3.9, then when I decided to 3.636 I took the 4.3 diff to the rear end shop and they called up and said I needed a new companion flange as well. I put in mazdaspeed diff bushings (stiffer rubber, not poly) when I went from the broken one to a non-broken one, so it's possible that's why I didn't notice a reduction in vibration.

The poly diff bushings are likely the cause though.

Leafy Reader
10/30/14 7:35 a.m.

Its probably the PPF alignment. Check to see if its within spec.

That damper ring thing is really only for noise damping.

thestig99 HalfDork
10/30/14 11:37 a.m.

Googles "Miata PPF alignment"

Innnteresting. I had no idea this was a thing, I'll check that.