915 trans questions

[rattlsnak]

When did the 915 box change from mech to elec speedo? I read somehwere that all the 7:31 boxes (pre 75?) were mech and all the 8:31 boxes were elec, but I found someone selling a 75 box, with a 8:31, with a mech speedo, all factory he says. How can i check this?


Also, Im switching my 72 box for one with a 8:31 which is the reason for these questions. What year did they change the CVs? My 72 has the 6 bolts. I thought the newer ones did too?

Or can i just change my 7:31 to a 8:31 on my box along with the race mods, (retainers, etc) and it be just as strong?

Anyone???

[Jeff Alton]

The later 912 had a 915 with 7:31 and electronic speedo. I know that does not help, probably makes it more confusing!! I was under the impression, not sure excactly why, that all the 915 8:31s were electronic. Not 100% though.

Jeff

[rattlsnak]

Actually the one Im looking at is a Mag cased unit also, but supposedly has the 8:31 gears and a mech speedo. -middle year '75 unit- For some reason it appears that this combo was never made.

Is there any way to tell from the case number whats in it??

Noah, yes its for sale.

[Geary]

I don't know the case numbers, but I can confirm that the 8:31 with mechanical speedo drive was manufactured in limited numbers. This transmission also had an oddball-dimensioned differential.

[wevoid]

The 1975 915's were definitely available with 8:31 and gear driven speedo. By 1976, the speedo were electric.

In 1975 there were two sizes of drive flange, earlier the large pattern with 4 x M10 and two dowels and then later, the smaller pattern with 6 x M8.
The change was mid 1975 year at chassis # 911 520 1530 coupe, 521 0860 targa, 540 0174 USA carrera, 541 0089 USA carrera traga.

In both cases, your 1972 trans and the 1975 trans, the input shaft is still the dodgy version where the 2nd gear abuttment on the input shaft has the spline cut through it and is a known and trusted failure point.

The 1972 and 1975 cases are equally strong, a 1975 final drive casing is one of my favorites for an out and out racing transmission core.

Regards

Hayden

[rattlsnak]

Thanks everyone, that helps alot!

So if I were to go to a newer version, like from an SC, whats involved with making the speedo work? Just getting a newer speedo head?

[Britain Smith]

Is the electronic sending unit one wire or two? I need to convert my car to electronic now since I have a '76 915.

-Britain

[john walker's workshop]

two wires up the tunnel to the speedo and a 12v and ground. you could even lay the wires next to the tunnel under the carpet.

[wevoid]

Noah,

It's based on many things, mostly preference, but consider these facts;

1975 911 had about 160 hp, low power and stress on the trans, many of the CA cars suffered early nuclear melt down from their reactors, leaving 1975 transmissions sitting around with low mileage.

The absence of the '76 onwards thinwall (only 1mm thick) speedo pocket eliminates a potential leak point after a piece of errant debris is dragged past the inside of it and makes it a window instead of a pocket.

The '75's like all mag trans has the iron insert in the pinion bulkhead, making it about a 5:1 chance (these days) that the pinion roller bearing is still tight in the case. Unlike the aluminum cases where it seems to be about 1:10 that you find a good one.

We do a lot of work to the cases for full race prep, so it's not really an issue what we start with, but given my choice, for the lightest possible 915 race transmission I will put a '75 Final drive with any year mag maincase and a '76 or '77 (mag) front cover - meaning no casing provision for a speedo of either design.

Regards

Hayden