So I'm starting to change my 3.2/901 914 race car into a 3.6/915 and I am wondering if an older magnesium case 915 with a 7/31 ring and pinion will hold up? Any opinions are greatly appreciated! Thanks a ton!!!

No

Are you going to sell old engine and trans ? If so I might be interesred. Thanks Paul

if you be careful with 1st gear i think you will be fine. you will have way more gearing options.

Why is first gear a problem? It's not cantilevered like the 901/914 first gear. In addition...

The mag-case 915's have steel inserts for the bearings to press into, and the addition of a one-piece bearing retainer is the fix for any flex that might happen between the two bearings. The later aluminum cases don't have the steel bosses and are supposedly weaker there. Plus the mag-cases are a little lighter.

The other required piece, and more important in my opinion, is the replacement of the side cover with either a late-model one with stronger webbing, or a billet piece. That's a real weakness in those boxes because they allow the R&P mesh (therefore the backlash/ R&P tooth-contact pattern) to change with the input torque. That's independent of the 7:31 ratio. I think that's good on an autocross course for sure, and tight technical tracks.

I'm still using the 914 transaxle (also 7:31) with my 993 engine, and until I get some more time I'm waiting to put my mag-case, 7:31 into my car as well. I have a later ('84) 915 I could use, but it's heavier and I don't see the need (except for better gas mileage perhaps) to use it yet.

Good luck

you are correct. i kept thinking, "why did i write that" 5th and reverse are under the tail cone correct? i havnt built one yet. are 5th and reverse supported on both ends? regardless, i would thing the amount of torque from 1st gear would be pretty brutal on the ring and pinion. im talking from no practical experience. but am building a new car for a 3.6 so i am all ears.

Yup, there are no cantilevered gears in the 915. There are a several discussions already in the paddock about the relative strengths and weakness of the 914 vs 915 transaxles. Some will argue that to get real strength you'd have to go up to the 930 ($) to really be safe.

In several threads where this is discussed a couple of folks who have run both on the track said that, after several seasons there was no real cost savings to upgrading to the 915. There were none I recall specificaly about the 7:31 vs 8:31 longevity. Except that when I compare the R&P's between the two, the 915's gears are a lot larger in every dimension... just more meat there. Now, the 915 is a lot more expensive to build and install (shift mods) not including any gear modifications.

I have a friend locally that's been running a 7:31 as an AX and DE car for years (911 with a 3.2). It's fast and he doesn't have reliability problems.

I have several 915's (2 mag-case and 1 aluminum-case) 915's but I also have several 914 boxes too, and that makes the 915 just an interesting project I'll certainly do... later. The 914 transaxles are just so much less expensive to build that I can't justify a 915 build except for fun.

Two facts:
1. Over 20 years I've lost exactly one 914 7:31 R&P with broken pinion teeth and the 3.6 since 1999. And that was just a couple of years ago a week before a MUSR in NW Arkansas... dangit. Pretty good track record I think.
2. The 914 and (mag-case) differentials are course-spined so, spaced/shimmed correctly, it can be used in either transaxle. That makes my Guard LSD useable in either one. Of course you can order the diff with either spline-count you want or, while checking the clutches install whichever you want. But if you already have one it could make a difference.

Don't know if you've seen these but here are some pertinent threads.

http://www.914world.com/bbs2/index.php?showtopic=221177
http://www.914world.com/bbs2/index.php?showtopic=161520

Good luck

It can be done ...

I have a vintage 3.5L 911 with an upside down mag 915 with 7:31 (same R&P orientation as a 914 since it's upside down, for what it's worth), and it's held up with no issues at all for many years. No special mods either other than being upside down. With that said, some mods are in order for longevity. I'm planning to put in an 8:31, a stronger side cover, and some cooling.

First gear itself isn't the issue - it's putting the multiplied torque through the trans, given the mechanical advantage that works "against" the tranny with the low first gear ratio. Just don't do silly things like punching the throttle in first, power launches, and don't abuse it when shifting, and it can be made to live.

Scott

The mag-case 915's have steel inserts for the bearings to press into, and the addition of a one-piece bearing retainer is the fix for any flex that might happen between the two bearings. The later aluminum cases don't have the steel bosses and are supposedly weaker there. Plus the mag-cases are a little lighter.
I called WEVO asking the same question and they seemed to really like the Mag case ...
Very nice people with a great product...

A late aluminum case 915 should be fine for a 914. Flipping the ring gear is a simple procedure and normally just requires relieving the case for the bolt heads. I have seen a few 915s that didn't require any shaving. I do recommend the steel bearing retainer and its a fairly inexpensive upgrade but well worth it for longevity. When you flip the ring.. gear walk or thrust is now going the opposite direction so replacing the side cover with a stronger type is not necessary. The standard taper bearing on the cover side will be fine with the ring flipped. For a 911 I do recommend the stronger side covers along with the 930 taper bearing modification as history dictates many failures from thrust. The case wall is plenty strong for mid engine application.

Inverting the 915 upside down (not flipping the ring) will require work and change the height of the drivetrain. Additional lubrication modifications are recommended for longevity since the main shaft and stack are now sitting differently in the oil bath. The 915 was not designed from the start to be run upside down. When set up properly they work well but expect to pay for mods.

For a street car you would need to add a Hall sensor to the trans to run the speedo correctly. Its really easy to add this. Would need to pick up a 911 speedo and change out your old mechanical 914 meter.

There is a lot of gear options for the late 915 that are very suitable for a 914 with the engine you are planning. In fact a long first and second gear can be a lot of fun with that motor.

The 930 option is a different animal. It was designed for torque from birth. Parts and mods are significantly higher but the payoff is strength. You do not see many 930 failures like its smaller cousins in light weight cars. The 930 is close in weight to the 915.

901, 915, 930 are all good units. Just depends on what you want out of them and what engine is in front of it. When torque levels go up service life goes down. Shifting has always been the number one issue on all of these. A properly assembled box will shift fine. These boxes are showing age and they will operate based on the technology at the time they were designed.

Like anything if you maintain it it will last. If you race it expect to spend some duckets to keep it fit.

Last you will need to figure out your shifting solution and axle adaptation. Lot of options out there.