I understand the difference in gear ratios, but fundamentally how much difference is there in strength? Syncro's, gear strength, bearings? Anything to help me in my decision. I know the gearing might be a little short, but will it fail alot quicker under load? Tranny in question, if that matters, is an 87 NA.

 

i can give you no hard facts as to the strength of the internals but i can tell you this much for referance.... A have a friend (who will remain nameless at the moment) who owned an NA for a little over 2 years, he drove the car HARD and I know that he went through atleast 4, i think it was 5 transaxles. He has owned a turbo for the last couple of years, he drives the car just as hard as the NA all the time, plus it has twice the power. He has only managed to destroy one 951 transaxle and that one had over 200k on it. Aside from that I have never heard of another 951 transaxle breaking.

 

Also when subjected to increased power, like in super/turbocharging an NA, you want to get a Turbo tranny. Here's what happens to an NA tranny if you don't:

 

Danno - why are you showing us the bottom of your cocktail blender??

 

that's one heavy duty cocktail blender... industrial size! hehe... makes 50 gallons of ur favorite drink in 3 seconds!

 

I will second Zu. I went through 2 and it needed a 3rd trans. when I sold my last NA. Looked just about like Dannos picture.
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Danno's picture shows the weak spot; the ring and pinion gear. The 944 and 944S trannies have 9:35 ring and pinion gears, for a drive ratio of 3.889:1. The 944S2 has 8:31 R&P for a nearly identical ratio of 3.875:1 (0.4% taller.) Since the gears are the same size, each tooth is thicker and stronger. The turbo gets the same 8 tooth pinion but a 27 tooth ring gear.

 

DANNO,

I noticed that's an open diff and it got me thinking.....

if that was one of your failures, did it happen during a shift at low speed, like a 1-2? was it running mobile 1?

I've seen so many failures w/ the mobile 1/ open combo....

I know open diffs are much more common and that would sway the statistics one way, but I''m pretty sure there's something to do w/ the slip rating of mobile 1 and the diff (not appropriate for our gearbox; i know, but lotsa people run it)

 

Luke, diff type doesn't change the ring gear, it's separate. It changes the pinion even less, and the pinion is usually what grenades.

Sam

 

Sam Lin,

I'm well aware of that. But diff type will determine the severity of tq transfer thru all gearbox components

 

Open diff is most likely to let a wheel break free and ease the forces, so that's not a likely hypothesis. How many factory LSD NAs have you heard of?

Sam

 

But, an open diff, will let go, and have only the little bit of resistance created by that 'one' tire spinning quickly... and then if it grabs all of a sudden..... 100% tq immediately

vs. with a plate type, only a percentage tq might only be shifted around

of course this is under rare circumstances, like loading and unloading laterally (you know there's alot of weight transfer with soft 944 butts), and shifting, modulating throttle at the right time.

~or~ hitting a slick spot on the inside of the apex, be it ice or gravel... and snapping into traction.

~or~ breaking free in a apex on an elavation change.

I guess If I were going to run a 944 gearbox with an open diff, with ANY ring and pinion; I would reccomend stiffening up the *** to cut down on the weight transfer, and making sure you were running the correct fluid with the appropriate slip rating

I'm talking about 44' gearboxes in general, not just na's and i'm not singling out the ring and pinion... Ive seen failures else where that "I" think sound very related

how many pre 's' turbo gearboxes with LSD's do you know of

If you think about it Sam Lin, all the failures i've seen / heard about under these kinda situations are either ring/pinion or the integral 1/2's

and what did porsche do, the started shot peening all the 1 and 2's and putting LSD in all gearboxes (turbo s and 89-91)

i think the 944 turbo (non s) with 217hp is pushing the limits of an open diff gearbox

 

But, and open diff, will let go, and have only the little bit of resistance created by that 'one' tire spinning quickly... and then if it grabs all of a sudden..... 100% tq immediately

You are pretty good. You have just described both failures. When the other tire grabbed it was bango and gone. BTW - they were both open diff, one with Redline and one with Mobil 1.
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thank you very much


i'm an student engineer that is in love w/ failure analysis almost as much as motorsports (I think they go hand in hand, especially in rally)

 

Thanks for the responses. Not exactly what I was hoping for, but it was what I suspected. I guess, even without details of what might make it weaker (other than R&P), testimonials are convincing enough to let it pass. I "plan" to have or make reasonable power and was just looking at picking up a back up trans for a good price. Never mind I guess. Thanks for the feedback.

Open rearends were notorious for breaking axles in the 60's with thin tires, weak rears and burn outs around corners. Tire grabs and snap crackle pop. No I wasn't envolved in 60's, myself.