915 questions

[Larry_Ratcliff]

Ok I will preface this question with a little back ground. I purchased an 83 SC as a donor drive train for my 69 911. The 915 in the SC has a bad first gear. I was thinking to just rebuild the SC transmission but also at the same time drop in a 7:31 ring and pinion.

Here are my questions:
1) are there any issues with the electric speedo when converting to the 7:31
2) are there any gotchas to be aware of in doing this.

I did a search through other threads. I am aware that the 7:31 is not as strong.. I read the arguments for switching all of the gears (seems like a much more expensive option). I am also aware that when I switch to the early exhaust that the heat exchanger will have to be "massaged" to clear the helper spring on the later box.

I just remember back when I had my stock 74 911 with the 7:31 ring and pinion and stock 2.7 and it was a lot of fun to drive.

My reasoning to rebuild this transmission is as follows:
1) I can used the SC pedal assembly and axles
2) rebuilding this trans I wind up with a known quantity
3) I dont end up doing the job twice by purchasing a bad mag case 7:31 box (I guess just #2 worded differently)

Thanks in advance to any information
Larry

[Evan Fullerton]

I do believe that the electric speedo and 7/31 ring and pinion don't play well to together which is why the '76 912E 7/31 is considered to be valuable as it has the magnets for the electric speedo. Don't know this for sure as I haven't tried it but my '75 is a cable speedo and the '76 912E my Dad has is electric.

I think it comes down to what wheel and tire set up you want to run in your '69 as to if putting in a 7/31 will be valuable. I have a 8/31 in my 3.0L powered '75 and with 23" (225/45/15) tires the gearing is fantastic and would be too short for street/ax/race use with a 7/31 but with the 25" tires on it is a little taller geared and noticeably slower under performance driving but the freeway RPM is better as is ground clearance and ride quality. Partially because the 15" tires are Rcomps (stiff side wall) and partially due to the extra .5" of side wall height.

If you have stock fenders on your '69, I say stay with the 8/31 and embrace the short tire on 15" wheels look but if you have large fenders and need similarly large and tall tires to fill them, the 7/31 will give better gearing but be even more taxed with a large and heavy wheel and tire combo.