TurboTim offered help, so I sent him a link for a reman unit.
He bought it and shipped it to me in record time.
Tim rocks!

 

Serious question:

Even though the diff casing looks centered in the rear subframe, the pinion yoke is offset towards the passenger side by 25mm or so because of the ring-pinion relationship.
Since the transmission tail is in the center of the car, that means the driveshaft is angled laterally towards the passenger side.

Would it have catastrophic consequences if I move the diff another 25 mm towards the passenger side for a healthier clearance for the 3" exhaust?
Halfshafts will not be a problem since they will be custom units. PPF will also be a tubular frame custom deal.

The driveshaft will be custom made, as well, and a lot of cars and trucks have considerable angles in their driveshafts.
(Fore and aft yokes will be perfectly parallel to each other)

 

Yes I do. I am glad I was able to help.

So you want 50mm offset? That's fine. Like you said though, technically any amount is fine as long as either both shafts are parallel or the angle between the input & output shafts to the driveshaft are equal. This assuming you are using standard universal joints that are properly aligned.

 

Its a bit tight for a 3" exhaust around the bmw diff but its not impossible. I have mine on the center so i don't get bigger driveline angles and because the e36 cv axles that i use are equal length. Using an E38 type 188 diff with Z3 torsen case in mine.

 

How / where can I find Miata VSS specs, namely, pulses per revolution of the said sensor please?

I searched and searched, all I can find is pulses per mile.

As in, how many of those ugly, sawtooth waves are produced when you spin the sensor one revolution, and how many pulses per wheel OR shaft revolution...
The bimmer diff puts out 9 pulses per wheel revolution. I just need the Miata data to make the speedo work.

This is what I am arriving at with the Miata - please correct me if I'm wrong.
Pulses per mile: 4014
Wheel revs per mile: 886
Diff ratio: 4.10

4014/886= 4.53 pulses per wheel revolution
4.53*4.10= 18.57 pulses per shaft revolution.

BMW VSS: 886*9= 7974 pulses per mile



If this is indeed the correct way to look at the speedo thing, all I need to do is halve the pulses the BMW diff puts out to get the speedo in my car to ballpark accuracy.
The rest will take some fiddling with a pot or something to get things bang on.

 

Any and all garage work is suspended due to a massive snow storm we are weathering now. Businesses are closed, civilian traffic is restricted. Defcon snow!

Um.. there is a couple inches of accumulation so far... It has been almost 3 days.

 

Good luck with everything.

 

It has been a while...

There is significant progress despite a hectic schedule, eventful days, and general mayhem in my life.

Let me summarize the current situation with a video:




I will document every step thoroughly once I see the light at the end of the tunnel.

 

Update:

The project is back on after lengthy delays...
I finally received the body from BB Auto - they did a bang up job on the detail repairs, rust elimination and painted it a beautiful shade of red.

I promptly installed both subframes, engine, gearbox, diff and suspension to verify all the work I did while the body was away.

I am running into a gearbox flange - pinion angle problem...

The diff points UP by 2 degrees. And the gearbox needs to point DOWN by the same amount.
Best I can get it to point down is 3 degrees. There is a full degree difference.

I could slot the engine mount bolt holes on the block side and lower the engine by 15 mm. That will give me 1 degree at the gearbox tail.
(780 mm from engine mount to gearbox flange - 15 mm down. That triangle gives me 1.1 degrees.)

While reading on this issue, I found a ton of conflicting information.

While the theory dictates gearbox and diff flanges must absolutely be parallel to each other, manufacturers like Spicer insists there should be a 1/2 to 1 degree difference.
I talked to driveline specialists who told me 1 degree is perfectly negligible, and that is as close as one can get to perfection.
Also, live axle cars and trucks experience considerable diff rotation which means their angles are never parallel.

So what should I do?

Shall I pursue perfection and get it bang on, or just call 1 degree difference good enough?

 

You should be fine.
If you are still using the guibo (round flex coupling shown on your transmission in previous pictures) you should be golden...
These allow slightly greater angles than a standard u-joint.
A real constant velocity joint will allow even further angularity difference.
This is why the really serious off road stuff uses CVs.
But even normal u-joints can live with a single degree just fine...

Oh, put a brand new guibo in if you still have it. Buy from BMW or verify that the part you buy comes from Germany.
Some aftermarket guibos suck big time.
Guibo should be changed every 50K on your set up.

 

Agree, it will be fine.
What Spicer are on about, is that with conventional uni joints, if everything is dead in line and totally rigid, the needle rollers never move and will wear away in the exact same spot.
A very slight rocking motion of the needles encourages the grease to circulate, and spreads the wear around the needles, and everything is then happy.

 

It is now 4:35 am.

Took some fiddling, but I took the difference down to 0.1 degree.
I'll call that good enough.

It involved removing the aluminum transmission tunnel liner to gain room at the top of the tunnel to raise the end of the gearbox, and dropping the engine by 14 mm by carefully drilling new mounting holes in the engine mount faces on the block side. Gave me a ton of room to get the engine to a perfect angle.

Pics and full thread soon.

 

Gents,
I started a new thread for the actual work I did on the car to install the BMW gearbox and the diff.

Please click here if you are interested.