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915 trans 7.31 Pinion with 3.3 Turbo Issues

I appreciate everyone's help with this....

I recently purchased an '84 M491 modded with a 3.3 turbo motor knowing the
915 trans had an issue. After having my mechanic take it apart, two splines were found broke off the 7.31.




I've been reading what others have done but Im not absolutely sure whats the best going forward. My mechanic has an 8.31 pinion in house and it looks thicker and more robust but is it much more different than the 7.31? Sounds like another 7.31 wont last with 3.3 turbo. I also ordered an OS Giken LSD to go with my car. (does not currently have LSD).
I also read that 8.35 pinion is another option.
Should I change the other gears while its all apart?
I would like to drive spiritedly but not have to drive a 911 with glass slippers...I mean whats the point, right?
Thanks for any help....Eddie
Old 10-17-2019, 05:39 PM
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Ive got a similar setup to you, I have now bought a 930 gearbox to do a conversion. The 915 is not really up to the task of handling a big turbo motor. However some people have had some luck, using an 8:31 ring and pinion, I also have heard the 8:35 is even stronger. If you go with one of those, since your gearbox is taken apart, change the bearing plate to a single billet style for strength and buy a billet diff cover also. That way it might hold up for some time longer, depending on how much power you are running
Old 10-17-2019, 07:27 PM
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Crotchety Old Bastard
 
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The 7:31 is too weak and gearing is too short for the 930. As suggested, swap out for the tallest gears you can get and add the WEVO robust parts. If you can add a limited slip that will also help. Let the turbo do the work, keep the torque below 350 ft/lb, don't use sticky tires and the tranny will live.
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Old 10-17-2019, 08:01 PM
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I forget the exact number, but I believe the strength delta between the 7:31 and 8:31 is about 14%. As mentioned, you don’t want the short one in there anyway. Install the 8:31 and cross your fingers. Hopefully you ordered the right LSD.

Do your stub axles have coarse or fine splines. 7:31 were from coarse gearboxes, but using a 5mm spacer ring one can put one on a fine spline lsd.
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Old 10-17-2019, 08:49 PM
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Quote:
Originally Posted by Matt Monson View Post
I forget the exact number, but I believe the strength delta between the 7:31 and 8:31 is about 14%. As mentioned, you donít want the short one in there anyway. Install the 8:31 and cross your fingers. Hopefully you ordered the right LSD.

Do your stub axles have coarse or fine splines. 7:31 were from coarse gearboxes, but using a 5mm spacer ring one can put one on a fine spline lsd.
Follow these instructions.
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Old 10-18-2019, 11:36 AM
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Thanks for the responses guys...I'll have to check which LSD I ordered and report back. I didnt realize there may be a few varying choices on that.
So, maybe I'll take the 8.31, add the billet diff cover and Wevo single bearing plate.
I'll have to ask about the stub axles and report back...
Any others items to fortify the unit?
Old 10-18-2019, 03:20 PM
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Just wanted to update this post. I went with OS Giken LSD part number PR013-HA.
I added the Wevo single bearing plate and 8.31 pinion.
My mechanic says as long you dont drop the clutch and drive like an idiot, this set up should give me no troubles.
Old 11-25-2019, 05:24 PM
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Maybe I’m the local idiot, and correct me if I’m wrong, but Your ‘84 would have had an 8.31 ring and pinion originally unless someone did a swap at some point. I believe the 7.31 was last used in the ‘74 transmission and technically the ‘76 912.
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Old 11-25-2019, 05:58 PM
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Quote:
Originally Posted by Rich76_911s View Post
Maybe Iím the local idiot, and correct me if Iím wrong, but Your Ď84 would have had an 8.31 ring and pinion originally unless someone did a swap at some point. I believe the 7.31 was last used in the Ď74 transmission and technically the Ď76 912.
I'm pretty sure previous owner did swap it. It likely had an 8.31 originally...Quite a few things were changed on it from original.
Old 11-25-2019, 08:54 PM
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I have a 930 regeared 4spd with guard lsd for sale if you need an option. Pm or email me. Bluedogmechanical@hotmail.com
Cheers, Mick
Old 12-02-2019, 03:10 PM
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my 78 SC/930 conversion had a 73-74 915 with a 7:31

wonder why both POs went with that. I am about to have my tub painted and begin putting the car back together. got a short BF 930 gearbox. sold the 915 to a friend, when rebuilding was surprised it had not broken. lot of burned gears/parts.
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Old 12-03-2019, 05:56 AM
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