Best replacement Tranny for 74 911S

[David Borden]

I purchased a 74 911s from another pelican member recently. The 915 tranny isnt quite right.

Car has 134K miles and the tranny whiring sound in 3rd only and the shifter moves forward and back quite a bit durring accel and decel(all bushings are new) It grinds at times even when making slow deliberate shifts.

Ive been looking for a used box here on pelican and thinking of going through my existing box.

What is the newest year trans I can put in the car without major re-work?

Whats the average number of miles these trans last before needing work?

Any other advice would be greatly appreciated!

David

[RWebb]

newest or best?

what does best mean to you?

budget??????

wanna change gearing?

i can do a core dump right here if you are ready to mull thru the hex....

[turborat]

The 915 was used through the 1986 MY. Any of these should bolt into your '74 with little to no issues. There are quite a few versions of the transmission however. Gearing along with ring and pinion make some of the differences. Also, the early 915 cases were magnesium. Later cases are aluminum and are arguably more desirable. Your '74 is probably a 915/16 with a 7:31 R&P and magnesium case. Installing a later 915, say from an SC with an 8:31 and taller gears would change the way your car drives. This may not be desirable. Decide what you want to do with the car. If it will be put it on the track, one may need shorter gears with closer spacing than most of the factory gear sets combinations offer (as installed). To accomplish this, one will usually have to custom build a box. If you will do strictly street driving and want fuel mileage high in the priority list, a completely different set of gears is desired. Here are some links to get you started.

http://www.pelicanparts.com/techarticles/mult_tranny_ratios/mult_tranny_ratios.htm

http://www.roadglue.com/wiki/index.php/915_transmission

http://members.rennlist.com/chuxter/901&915ratios.htm

[Bill Verburg]

Originally Posted by David Borden

I purchased a 74 911s from another pelican member recently. The 915 tranny isnt quite right.

Car has 134K miles and the tranny whiring sound in 3rd only and the shifter moves forward and back quite a bit durring accel and decel(all bushings are new) It grinds at times even when making slow deliberate shifts.

Ive been looking for a used box here on pelican and thinking of going through my existing box.

What is the newest year trans I can put in the car without major re-work?

Whats the average number of miles these trans last before needing work?

Any other advice would be greatly appreciated!

David

Are your engine and trans mounts ok?

It could be an internal trans issue but why not check the easy stuff first.

If you are happy w/ the stock gearing(you probably should be, becasue back in those days the engineers were still in charge of that area) then the biggest issue for you ill be the speedomter drive.

From '76 thu '86 the speedos are electriclly driven, from '72 thru '75 a mechanical flex shaft drives the speedo.

'75 was the year that the cwp changed from 7:31 to 8:31

So most any '72 thu '74 will be the best choice for you, all of these are mg case.

[stlrj]

I went for the stronger aluminum '78 915 on my '74. I still have plenty of performance with the 8:31 along with better gas mileage too.

[RWebb]

a reinforced Mg case will be stronger than the Al case - search and/or look at wevo catalog - term = bearing retainer + reinforced side plate
- the last set of Al trannys did have big cv's on them tho - only needed if you hot rod your motor a lot

also examine tables posted on guide tube problems - there is a fix for that too

definitely do every thing Bill said first
also shifter rebuild & blueprint ala Sherwood (search terms)
and replace the 3 bushings involved; be sure it has fresh Swepco in it

search up that table of trans. changes by year and see what is what - expect you will stay with your own box

[David Borden]

Guys,

Thanks very much for the advice and feedback... and agree I could have been more specific in my request for assistance. I will try to outline that here.

I would prefer to get a good working transmission and spend somewhere south of $1500. Im not afraid of a good used trans and Ive also entertained pulling my box out and opening it up to see what Im working with and possibly freshen it up myself. Ive built a number of cars from the frame up, so it might be fun to explore a 915 trans. I will definitely check the motor mounts and clutch adjustment. I have a slight concern with whats inside due to what I found after a post purchase gear oil change. What appears to be 3 small gear tips. Im hoping its dog teeth or some wear items.

The trans shifts OK, definitely need a pretty slow engagement. Its pretty quiet, except for 3rd gear, which howls a bit, like the sound many cars make in reverse. My main concern is hurting the existing box due to the small bits of metal and the 3rd gear noise.

Car is used a a cruiser and for some back road fun from time to time.

I do like the existing gearing very much, and Im not clear on how much of a change the later gearing would make on the car for cruising around and some spirited driving. I wasn't sure if the Magnesium boxes are something to worry about. I appreciate the tips and links to beefing them up.

I went for the stronger aluminum '78 915 on my '74. I still have plenty of performance with the 8:31 along with better gas mileage too.

Can you tell me how you addressed the speedo issue, and any other changes you needed to make it work?

The car is being broken down for paint in a few weeks, so doing my research and trying to get my ducks in a row.

Thanks again!

David

[RoninLB]

915 doesn't last long before it needs $750-$1,500 in maintence.

call up rebuilder Gary Fairbanks and see if he'll sell you a rebuilt.

[stlrj]

Originally Posted by David Borden

Can you tell me how you addressed the speedo issue, and any other changes you needed to make it work?

To make the later 915 speedo work you'll need to replace your speedo with a '78 or later and run a pair of wires from it through the tunnel.