Installed a 3.91 getrag from a solstice into my 02 Miata using the V8R kit. Still running the stock engine/trans for now. I wasn't able to find any kind of instructions of guidance on this process. The transmission crossmember basically located itself fore/aft and left/right in the tunnel when bolted to the transmission. The differential was very solid when hanging from the rear 2 mounting studs before I welded in the front mounting tabs. Everything just seemed to fall into place. I measured driveline angles as I went but as stiff as the rear diff mounts are I was not able to get more or less angle from them. I can take the poly 'mounts' out from under the transmission and lay the trans right on the crossmember to get a little more angle there but the engine is then clearly at an awkward angle. After it was all said and done the driveshaft itself is at 3.5 degrees. I had a hard time hanging the angle gauge off the pinion flange and trans output shaft in a way that yielded consistent readings. Near as I can determine there's about 2 degrees difference in the angles. 3.5 degrees up on the diff and about 1.5 down on the transmission. I went and drove it up to about 70mph and didn't get any noticeable vibrations.

The question is am I over thinking this? I couldn't adjust the angles if I wanted without completely modifying the trans crossmember and/or diff mounting setup from V8R. Since I am not getting a vibration everything must be within tolerance right?

 

If it ain't shaking, don't worry about it. If there was an issue, you would know it.

 

So the Solstice/Sky diffs are interchangeable with the CTS ones using the standard V8R parts then?

 

the solstice/sky diff is not machined out for the front bushing. I had the case swapped from a CTS that already had the bushing. Originally the plan was to buy the bushing sold by Creative Steel but the bushing was too big for the hole that was there. It looked like all the cases are cast the same and only some of the applications got the bushings bore machined out to the correct diameter. End result being I have the 3.91 LSD from a Sky/Solstice in a CTS case. Followed this guide from Kmiata for models and years:

https://cdn.shopify.com/s/files/1/11...25599662246927

the axle place that did the case swapping and gear setup said the cases are even the same from LSD to non LSD applications.

 

Excellent information, thanks!

 

Measure the angle at the front surface of the crankshaft damper pulley on the front of the motor, it's the same as the trans output, then measure the rear end flange. Should be the same or close. Absolute angle is not important if its not extreme, the motor can be nearly level, it doesn't really have to have any particular angle, just enough to keep the ujoints from always being in the same spot. The 3-3 1/2 degree number is a throwback to solid axles.

 

I also purchased a 2009 Pontiac Solstice 3:91 differential which was not bored out. The hole in the differential was as-cast and also this hole has a draft release angle which was used in its original Pontiac application. All housings used the same casting with some having a bored hole as part of the machining process. I used the Creative Steel bushing too in my installation. To make the bushing fit, I carefully opened the cast hole with some emery cloth, some wood dowels, and patience: opening the hole up carefully so the bushing would end up with a light press fit. I also drilled and tapped a hole into bushing hole from the side of the case allowing me to use a bolt and lock nut to pin the bushing in the application. I had been running a 3:42 and after a cam swap the 3:91 was a needed change for my car. I have had no issue with the bushing in the case after installing the differential.