Posted this in the garage for a day but no response...
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I am building two transaxles for my 3.6L 914;

1. a 914 transaxle and
2. a 915 transaxle

Stuff I have around:
1. several 914 transaxles
2. two mag-case 7:31 915's (circa 1973)
3. one alum-case 8:31 915 (1984)
4. one early style clutch-type LSD (GT, nearly new)
5. one late style TBD with one AX season on it.

The skinny:
Since I got the TBD (Quaife) I've discovered it's a fine-splined QDF1Q for the later 915/930 application. Now I have one later (aluminum) transaxle and could use it in there directly, but it's a heavier aluminum case version with an 8:31 R&P.

I also have two early mag-case 915's that use the same diff (Quaife QDF5Q or Guard equivalent) as the early 911/914/901 do. Advantages you ask?

1. Mag case is MUCH lighter
2. The bearing bosses are steel inserts rather than using the aluminum in the later boxes, which lessens the need for a one-piece bearing retainer.
3. I can use the stronger late ring-cover plate on the mag case to reduce thrust flex... easier on the R&P. I haven't measured to see if the two plates are the exact same critical dimensions.

I'm not worried about flange splines since I have 100mm Carrera flanges and the reluctor from the late case as well, so no spacer is needed to set up the diff. I'm using GPS for a speedo anyway.

The reason I'm asking:
The spigot on the ring-side of the diff is slightly shorter (.05in or a shade less than 3/16in) and that's ok, however the ring flange is the equally thicker than the earlier diff. So when I shim for that, the ring/bolt-heads might wind up closer to the case on that side. Other than that I think I can space the cover-side spigot to the right dimensions if there's enough wiggle room to get the proper R&P tooth engagement/preload.

Sooooo. I know I can use either earlier style differential in 914 and mag-case 915's. Frankly, I'd rather use the mag case 915 (7:31 R&P) and the LSD but that would require using this late TBD diff in the 914 trans. It's critical dimensions show me it might be adapted but I'm not sure it's worth the trouble (see "reason" below.

Finally, the question:
Have any of you used the later diff in an early case?

My only other option is to swap this late 911/930 TBD for another one that fits early cases directly. That's the easiest solution but not one I have in hand.

Thoughts? beerchug.gif

Plenty, but nothing helpful for this thread.

I've never even dived into the matter that deep. I have a spare rebuild 915 that i was going to drop in a TB style LSD at some point and make it a cable shifter.

That's how far i have gotten ...

So, here's your free

Thanks for the bump!

I'm gonna cable-shift this 915 too. Frankly, searching for an easy way to get this working I'd run it inverted (whichever config I decide on) just to make it easier to get the R&P out when I want to, but the axle angle is just too severe unless I run the entire powerplant lower than a street/track car should... don't want to give up the horsepower and create that kind of heat in the cv's.

This diff question is a conundrum though. Apparently no one here has done this or I'd have heard so far... or they think I'm crazy. To be fair I just wanted to save myself the RTFM time but I'll calculate through the dimensions and discover for myself. If I knew of some reliable person who wanted this TBD in trade for a low-mileage early TBD I'd consider a trade.

No matter how it turns out I'm really lucky to have all of this stuff around to experiment with. Time and active hunting has a few perks.

Now... which cable shifter and clutch/flywheel combo to use as a base for the fabrication. Hmmmm.

McMark and i have wrapped our brains around a few beers and the same question.

Since he has been playing with ideas and parts to make a cable-shifter kit for the 901 he has already found a few parts that may work well in the 914.

I actually think the 915 will be easier than a 901. I really need to get going on this.

Now, if i could only make up my mind about which LSD to use ...

I really prefer the clutch LSD on both the street and track, but it sucks on the AX circuit. TBD rules there as far as I'm concerned, which is why I want to put one in the 914 trans.

I've just started thinking about the cable setup. On the front end it looks to me like the side-shifter would be easier to modify using the existing console as a base structure, but I haven't done any quality noodling-time on it yet... can't imagine buying anything when this is such an interesting project.

Now that you mention it, a beer-laden shop time with some mechanically inclined cohorts standing around a mocked-up isn't a bad idea. Ok... beer after machine tools.

pcar916,

It might be possible to use the early mag case housing (where the gear stack resides) and bolt it to the late aluminum differential housing section, thereby saving some weight. I don't know if it'll work, but the tranny gaskets are the same for both
early and late 915s (up to '86), so there's a chance they might interchange, I dunno.

I mocked-up a 915 and an empty t4 case to see how well the fit was with the Rod
Simpson V8 mount. It wasn't perfectly in line and contorted the stock -4 donuts, but
if solid mounts are used then it might not make a difference.

The Renegade V8/915 mount also had issues, and ended up cutting off all the
pieces and will re-weld them.

I have the Renegade 915 cable shifter set-up.

I seem to vaguely recall a thread or article on a 915 r/p flip that the bolt heads needed to be shaved down for ring gear clearance, or something like that...

Good luck

Good thought about mixing the cases so I'll weight all of the components to see which way to go. One of my mag cases has a bad R&P but all else is in great shape, so I'll use that one for spare "stuff". I don't remember which years they did it without some reading but Porsche mixed those 915 cases as well.

The biggest advantage to the mag diff-housing, besides the weight, is that it has steel bearing bosses. The later side cover however, is much stronger and will be used regardless.

I haven't taken the time to think through the rear mount system. I'm thinking a cradle mounted to the stock perches will be the answer. Inverted would be SO much simpler in that respect... and no more solid mounts for me. The 911 "sport" mounts used on the competition cars and 911 cabriolets are very good, and that little bit of compliance is helpful when the mount parts are just a tiny bit off.

I've plumbed all cases for an external cooler system and had to relieve them for the 3.6L speed sensor, but that's another topic altogether. This is a really fun mod so I suppose I'll have to get on with it.

Of course a real side-shift conversion is the ultimate answer but I'm unwilling to let go of that much cash.

Exactly my thinking. The stock 915 mounts are just asking for a cradle setup. This should be easy to fab.
And since i don't run a banana muffler, there's plenty of space back there to work with.

This may or may not help you, but Guard Transmission used to have a nice PDF file with dimensions of the various differentials used up through the 930. I have a copy, but based upon Google searches the URL for that file is no longer working. In short, that file is no longer on the GT site. I don't feel comfortable posting that PDF directly here as it belongs to GT.

Richard

Thanks Richard, I found that drawing at the house! Based on those dimensions it looks (at first glance) like it'll just be easier to swap this TBD for an earlier version if there be anyone out there with one. I'll keep all abreast of this.

The biggest hurdle I see is that the ring flange is thicker on the later model, which will force the ring bolts even closer to the side of the case. Your suggestion about measuring the clearance with clay is a good one. I'll do that and post the results. That won't be immediate since I have to crank out a trans for my 914 NOW with existing parts.