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Old 11-20-2012, 10:59 AM   #1
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Smile Rear End

1990 Miata FM2 conversion Aug 2012. PRior to that the car ws Vodoo II for 2 years. Last nite serious vibration thru the 5 speed shifter on deceleration. Seconds later boom. The driveshaft decided it no longer wanted to be attached to the rear diff.

Going to upgrade the 1990 rear with a later model rear. Whats the best way to go here. I hear stories of later model torsen rear ends dying at 240 hp, and other stories saying they can take a lot more. From a flat out longevity perspective, will an open rear or torsen rear last longer? I know the more a tire spins, the less strain there is on the rest of the drivetrain. When my 1971 455 convertible GTO with posi rear hooks up, it stressses the entire car. When the wheels spin...well, all that power goes up in smoke.

thanks
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Old 11-20-2012, 01:09 PM   #2
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You definitely want something '94+ since that's when Mazda upgraded to a 7" rearend. You'll need to swap the driveshaft, differential, and axles from the newer car.

I can't stand to drive an open diff car, but if you don't have any personal preference, you should be able to pick up an open diff considerably cheaper. Any 7" miata rear should be considerably stronger than your 5 speed or motor (assuming those are still stock). There are plenty of guys pushing them beyond 300whp without issues. Not sure I've seen anyone compare torsen vs open as far as longevity, but neither really seems to be a major weakpoint.
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Old 11-21-2012, 01:55 PM   #3
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The only way you're going to break a Torsen at <500whp is by drag racing. If you don't drag race, reliability of the 7" rear end is a non-issue.
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Old 11-21-2012, 02:08 PM   #4
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The only way you're going to break a Torsen at <500whp is by drag racing. If you don't drag race, reliability of the 7" rear end is a non-issue.
I broke a torsen autocrossing. I guess I unweighted a tire enough to get one of them spinning. When it got traction again it liberated some teeth from its innards. I switched to a 1.5-way clutch pack, but I miss the torsen.
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Old 11-21-2012, 02:37 PM   #5
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Would it be ideal to get an rx-7 turbo 2 diff or a 1.8 torsen?
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Old 11-21-2012, 02:57 PM   #6
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Can we differentiate here between the two different failure modes of the rear-end?

1: Fracture or shearing of the ring & pinion gear. Fairly common, typically happens to the 1.6-style rearends when turbocharged. Almost unheard of in the 1.8-style rearends except in extreme circumstances.

2: Destruction of the differential itself, which is the thing that the ring gear attaches to. This is the failure mode reported by DammitBeavis, wherein the gears in the limited-slip assembly were destroyed due to extreme tire slippage. Can theoretically happen to any differential which contains internal gearing, be it open or Torsen. This failure mode is uncommon as compared to the first.

That said, forget about trying to piece together a bunch of RX7 parts. If your stock 1.6 rearend stayed alive for this long, the chances of you destroying any component inside a 1.8 unit are extremely small.
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Old 11-21-2012, 03:00 PM   #7
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Torsen all the way. Turbo 2 diff has no mounting point for the PPF, so you will have to do much fab work to get it to fit. Since the torsen is stronger than the 6-speed, you would be stupid to do all that work unless you were replacing the whole drive train.

Dem768 has broken a bunch of axles, but not his torsen.
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Old 11-21-2012, 03:06 PM   #8
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Quote:
Originally Posted by Savington View Post
The only way you're going to break a Torsen at <500whp is by drag racing. If you don't drag race, reliability of the 7" rear end is a non-issue.
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Old 11-21-2012, 04:04 PM   #9
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Well since i've been boosted in march i've broken 2 opens and 2 vlsd diffs all 1.6s and i just broke one my last one Saturday. Idk if they are just that weak or its just my driving style, im addicted to roasting second gear and kicking her out sideways! Im trying to figure out whats my next step here because though its probably worth it i cant see myself spending 1000 bucks on a torsen right now!i know you can get an rx7 diff for 3-400 bucks id have to get custom brackkets to mount it and a custom drive haft. But comfort knowing im not gonna brake it! Right now im at 200 hp when tuned im trying fo 300 hp. What would you guys do? Oh op sorry for stealing your thread!
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Old 11-21-2012, 04:19 PM   #10
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Did you bother to read the thread that you hijacked, or any of the other 948201290 threads that mention how the 1.6 diffs are weak and have been known to break even under 95 whp abuse?

Open or Torsen 1.8 diff. I've seen the opens for sale for as little as $100. Why? Because they don't break, so there's tons of used ones coming off of wrecked cars, and people are removing them for the (high-demand, low supply and therefore $$$) Torsen units.

If you can't afford a Torsen, I wonder about the lapse in logic that would prompt you to shoot for 300 whp.
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Old 11-21-2012, 04:34 PM   #11
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Quote:
Originally Posted by Eunos93 View Post
Well since i've been boosted in march i've broken 2 opens and 2 vlsd diffs all 1.6s and i just broke one my last one Saturday. Idk if they are just that weak or its just my driving style, im addicted to roasting second gear and kicking her out sideways! Im trying to figure out whats my next step here because though its probably worth it i cant see myself spending 1000 bucks on a torsen right now!i know you can get an rx7 diff for 3-400 bucks id have to get custom brackkets to mount it and a custom drive haft. But comfort knowing im not gonna brake it! Right now im at 200 hp when tuned im trying fo 300 hp. What would you guys do? Oh op sorry for stealing your thread!
Did you seriously replace 4 broken diffs with 1.6 parts?

Don't be a glutten for punishment. Do the 1.8 swap. Too cheap for a torsen? Weld the damn thing until you can afford parts. Don't worry, after you drive on a welded rear, you will find a way get the torsen soon enough.
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Old 11-21-2012, 04:38 PM   #12
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I have read it and i understand 1.6s are weak. Im just trying to see where my next step should be because im tired of braking diffs and i dont want an open i need lsd so im just weighing out my options. And what makes you think I can't make 300 hp if I already have 200 at 8psi. Who 10-12 more psi and a good tune i think 300 is reacheable if not close!
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Old 11-21-2012, 04:40 PM   #13
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Quote:
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Im just trying to see where my next step should be
Your next step should be a complete Torsen assembly out of a '94-'05 Miata.
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Old 11-21-2012, 04:59 PM   #14
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If ballin': Torsen.

If no money: 1.8 open which can be had very cheaply and will get your car running as well as before, but won't break. Realize that adding more power to a drivetrain that you already can't afford to properly support may not be advisable. Engines and transmissions are pricier to change than diffs.
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Old 11-21-2012, 05:19 PM   #15
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I converted my 1.6 VLSD to a clutch pack LSD for $370

$170 of that was converting the 1.8 open to the RX-7 clutch pack diff.

You just have to be frugal. Get the RX-7 diff for cheap either at a junk yard, here, or on an RX-7 forum. Rebuild it with a shim. I used the stock ECU case. Recycle, Reduce, Reuse!
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Old 11-21-2012, 05:30 PM   #16
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I like that idea curly that's another option that's all!
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Old 10-23-2013, 11:48 AM   #17
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Quote:
Originally Posted by curly View Post
I converted my 1.6 VLSD to a clutch pack LSD for $370

$170 of that was converting the 1.8 open to the RX-7 clutch pack diff.

You just have to be frugal. Get the RX-7 diff for cheap either at a junk yard, here, or on an RX-7 forum. Rebuild it with a shim. I used the stock ECU case. Recycle, Reduce, Reuse!
Curly, is there a write-up on how-to do the rebuild? I get just a little nervous when it comes to things unknown.

Greatly appreciate it!
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Old 10-23-2013, 11:54 AM   #18
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you do the rebuild by buying a torsen and not dealing with clutches.


I did my torsen upgrade for $400 total. that includes the axles, driveshaft, and torsen rear end.
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Old 10-23-2013, 12:00 PM   #19
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My Torsen is going.
I somehow have gotten the notion that clutch would perform better in my style of driving (every time Torsen feels like an open diff, and, no, this isn't my first Torsen (lost one due to housing failure.)

And, yeah, using Joe Perez's advice, I got mine for $250.

FC diffs can be had near here for $100. I have the 1.8 in place, so no other hardware is required (save for circlips).
That much I've figured out. As far as preload, shimming, rebuilding, I'm still completely in the dark.

Scott, if you could shed a little light, I'd greatly appreciate it.
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Old 10-23-2013, 12:09 PM   #20
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I find it hard to believe you've broken 2 torsens at stock power. WTF are you doing to your car?
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