915 tranny Options

[john914somers]

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!!!

[pcar916]

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

[stownsen914]

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