Porsche transmission type: 915
Original equipment for: Porsche 911 72-86
Description: 4 or 5 speed manual transaxle w/non synchronized reverse and early Porsche synchromesh
Limited Slip Differential: 40%-80% ZF LSD optional equipment
Weight (approx): 123lbs
Oil Capacity (approx): 3L SAE 90 transmission oil
Speedometer: Mechanical speedo 72-75.5; Electric speedo 75.5-86
Trans controls: Rod linkage
Clutch: Mechanical/cable, pull clutch, 225mm single plate

72-73 5 speed 72-73 4 speed

915/02, 915/08

915/12

1st gear 11:35 1st gear 11:35
2nd gear 18:33 2nd gear 18:32
3rd gear 23:29 3rd gear 24:27
4th gear 26:25 4th gear 28:23
5th gear 29:22

Final drive 7:31

74 5 speed 74 4 speed

915/06

915/16

1st gear 11:35 1st gear 11:35
2nd gear 18:33 2nd gear 20:32
3rd gear 23:29 3rd gear 25:26
4th gear 27:25 4th gear 29:21
5th gear 29:21

Final drive 7:31

75-77 4 speed 76 912E 5 speed

915/16, 915/45, 915/48, 915/49, 915/65, 915/66

923/05

1st gear 11:35 1st gear 11:35
2nd gear 20:32 2nd gear 18:33
3rd gear 25:27 3rd gear 23:29
4th gear 28:23 4th gear 26:25
5th gear 29:21
Final drive 8:31 Final Drive 7:31

75-79 5 speed 80-86 5 speed

915/40, 915/43, 915/44, 915/60, 915/61,

915/62, 915/63, 915/67, 916/68, 915/69, 915/70, 915/72, 915/73

1st gear 11:35 1st gear 11:35
2nd gear 18:33 2nd gear 18:32
3rd gear 23:29 3rd gear 23:29
4th gear 26:26 4th gear 26:26
5th gear 28:23 5th gear 38:30
Final Drive 8:31

Installing a 7:31 R&P in place of 8:31

An oft-discussed modification for 1975-1986 911’s is the ring and pinion gear swap. The ring and pinion are the gears that determine the final drive ratio of a transmission, and by swapping them for gears of a different ratio, the final drive ratio can be raised or lowered. In the case of the 915 transmission, the commonly-available 7:31 ring and pinion from the early 915 transmissions can be installed into a 1975-1986 transmission (which used an 8:31 ring and pinion), thereby accomplishing a 12.5 percent lowering of each gear ratio. In this tech article, I’ll describe some of the things that are required of this swap, so that you can make an informed decision and be prepared for all of the expenses and parts-finding legwork associated with this modification, which is especially easy to have performed in the course of a transmission rebuild.

I’d like to start by discussing some of the pros and cons of the 7:31 swap, because it is not a good modification for every application. On the pro side, the 7:31 swap represents a more affordable means of shortening gears than replacing individual gear sets. The latter method, which is preferred for track cars, is significantly more expensive – the 7:31 ring and pinion can often be purchased for the price of a single short-ratio gear set. The 7:31 is easily strong enough for street applications, especially when a couple of other transmission parts are upgraded (these upgrades are detailed below). On the con side, the 7:31 installation requires great precision and should be performed by an experienced Porsche transmission builder. To make the 7:31 more durable, there are a few extra parts to buy, which adds to the cost of the job. Finally, because of the different stress the 7:31 puts on a transmission, it probably should not be used in a car that will experience regular track use while mated to a high-horsepower engine.

The 7:31 ring and pinion is often accused of being weak and prone to rapid wear, but after much investigation on the subject before proceeding on my own project, I concluded that those assertions are mostly false. I contacted a few 911 transmission experts about the reliability of the 7:31 ring and pinion, and learned its longevity problems are due less to the 7-tooth pinion gear, and more to the weakness of some of its surrounding components. Hayden Burvill of Wevo is a well-known Porsche 915 transmission expert, and says that under load, “the 7:31 makes a special effort to exit through the side differential cover because the 7:31 has a force vector skewed more towards the side cover, whereas the 8:31 delivers this force more along the axis of the pinion shaft. The flexing of the magnesium side cover leads to excess backlash and excess stress in the pinion gear teeth. The 7:31 components are not inherently weak, but in the O.E. installation, rigidity was challenged by higher output motors.”

There are a couple of relatively inexpensive upgrades that can be performed when installing the 7:31 to increase its durability: the addition of the later-style reinforced differential side cover, and a one-piece bearing retainer. Before getting into the specifics of those two upgrades, there is a third item to consider, which is whether your transmission case material is magnesium or aluminum. The 7:31 was used for the first few years of the 915’s production, during the model years 1972 to 1974, along with a handful of early 1975 911’s. It’s best to swap the 7:31 into an aluminum-cased 915, because of its inherently greater strength; the aluminum 915’s were introduced in the mid-1977 model year. If you have a 1977 transmission (type 915/61), you’ll have to verify the case material of your transmission visually, because Porsche did not change any stampings on the transmissions to indicate the change in case metal (interestingly, my 1977 Carrera 3.0 came from the factory with an aluminum-cased 915, even though its build date was October 1976). The aluminum 915 transmission case, like the aluminum 911 crankcase, is stronger than its magnesium predecessor, although about 14 pounds heavier (12 pounds after figuring in the weight of the later side cover). A magnesium-cased 915 with a 7:31 is still a sufficiently strong transmission for many applications, but the strengthening modifications detailed below become even more vital to its durability.

Strengthening Modifications
Several companies manufacture a one-piece bearing retainer that replaces the stock two-piece retainer (I bought mine from Wevo for $249). The one-piece retainer, which looks somewhat like a figure-8, secures the pinion and main shaft bearings much like the 930 components and more effectively than the stock 915, two-piece factory design, in which the retainers for the pinion and main shaft bearings are independent of each other and thus more prone to movement under load. This modification requires no extra time to install, as the two factory retainers the one-piece part replaces are removed in the course of the ring and pinion swap. The late-model reinforced differential cover plate, which was introduced in the 1979 model year (part number 915.301.311.036) has an annular reinforcement rib cast into it. This differential cover is much stronger than its predecessor and will do a much better job preventing the backlash distortions that wear out the ring and pinion. I bought this part new through my local Porsche dealer for $170 with a PCA discount. The combination of an aluminum case, one-piece bearing retainer, and updated differential cover creates a very strong, durable transmission capable of handling more power than a stock aluminum- or magnesium-cased 915 with a 7:31 ring and pinion.

There is one more modification that can be easily performed in the course of the ring and pinion swap if you think you’ll be doing a fair amount of highway driving: a taller 5th gear. My 915/61 transmission, like most 915’s, came stock with a 28:23 5th gear, which, when mated to the 7:31, creates highway-speed RPM’s higher than I find comfortable for longer road trips. The 1974 915 and 1976 912E gearboxes used a 29:21 5th gear, which is the tallest 5th gear Porsche made for the 915. I sourced one from the well-known transmission builder Gary Fairbanks (you may have to call around) and installed it during the rebuild. It returns the 5th gear RPM-to-MPH ratio back to stock.

The Installation
The first step is of course to find the actual 7:31 ring and pinion set. You can either purchase a core early 915 transmission (this is what I did) or buy the ring and pinion set separate from the transmission. You have to obtain the ring and pinion from the same transmission, as the ring and pinion gears were matched and run-in together at the Porsche factory to assure quiet operation and minimal wear. Obtaining a matched pair is mandatory. It’s important to also acquire the shim stack from the transmission the ring and pinion were removed from (these are the shims that are stacked under the bearing retainers that allow adjustment of the pinion depth). In most cases, you won’t need to change this shim stack, as the transmission case machining tolerances are usually so minimal that pinion depth will not be altered. However, while ordering parts it’s a good idea to pick up a few extra shims, just in case you need to change the pinion depth from its original specs – I bought two .10mm shims for $5 each to go with the two .20mm shims that came out of the donor transmission, to allow a range of shim combinations up to .60mm. Also, if your 7:31 ring and pinion will be installed in a transmission and car with an electronic speedometer, the mechanical speedometer drive will need to be lathed off the end of the pinion shaft. Because the pinion shaft is made of extremely hard steel, this procedure needs to be performed by a machine shop. You shouldn’t be charged much for this – I paid $20.

You will need to buy two items for proper fitment of the ring gear. The first is a 2mm-thick shim that fits between the ring gear and the differential flange that the ring gear is bolted to. The 8:31 differential flange is slightly thinner than the 7:31 flange, and thus the ring gear needs to be spaced out 2mm in order for it to mate properly with the pinion gear. Porsche transmission specialty shops stock this 2mm spacer. The second item is a set of 12 longer ring gear bolts designed for a limited-slip differential (LSD). In order to compensate for the added thickness of the ring gear/shim/differential flange combination, you need the longer LSD bolts so that there is adequate intrusion of the ring gear bolt threads into the ring gear. I performed some measurements on my transmission, and found that the shaft length of a non-LSD (short) ring gear bolt is 16.4mm; the differential flange is 8.1mm thick, and the ring gear shim is 2mm thick. Thus, using the short bolts with the 2mm shim would only allow about 6mm of the ring gear bolt to thread into the ring gear, whereas the LSD bolts will thread into the ring gear several millimeters more. Using the longer LSD bolts is very important, as the ring gear becoming separated from the differential at high RPM’s would catastrophically destroy your transmission, and possibly more. You needn’t buy a new set of the metal bolt-head retainers that you had to chisel off when removing the ring gear – it’s common practice today to use red Loctite on the ring gear bolt threads to secure them.

The pinion depth and backlash setup is a complex procedure requiring great precision and expensive, specialized tools; it would be difficult to adequately explain this part of the ring and pinion swap in this article. In all likelihood a professional shop will be setting up your ring and pinion, so for most people the specifics of this procedure are irrelevant anyway. A shop should charge for 2-3 hours of labor for setting up the ring and pinion, plus the cost of parts. Again, setting the ring and pinion should be done by an expert using the correct tools. Your shop should, however, perform a gear marking paste test to ensure the proper meshing of the ring and pinion gears. In this procedure, after the pinion depth is set and the pinion shaft is assembled onto the transmission, a bright-colored paste is applied to several of the ring gear teeth, and the pinion and ring gears are run together by hand; the impression left in the paste on both the pinion and ring gears will show whether the gears are meshing properly.

The 1976 912E Transmission
Much of what’s detailed above can be simplified if you can find a 1976 912E transmission, which used the 7:31 ring and pinion, but had an electronic speedometer, 29:21 5th gear, and thicker differential flange, making the 2mm shim unnecessary. The 912E transmission is the ideal donor transmission for the 7:31 ring and pinion swap, as many of the conversion parts required for the conversion with an early 915 transmission become unnecessary. But its scarcity will make finding one difficult. It’s a transmission that was only made for one year, for a relatively rare model; the chances of coming across one are slim.

Driving Impressions
The ring and pinion swap is definitely a performance modification that will not go unnoticed the first time you drive your car – the difference is significant, and, not to put too fine a point on it, wonderful. To achieve a commensurate power increase by engine modifications alone, you’d have to spend thousands. I performed this modification in the course of rebuilding my transmission, and could not be more pleased with the difference it makes in the car’s power, and in the usability of the car’s power. To my taste, the stock gearbox’s ratios were too tall; I could hit over 60 MPH in second gear, and get over 100 MPH in third. I found that I could rarely enjoy being able to wind out my engine to redline with the stock gears, because I’d end up going too fast for the public roads. The 7:31 solves both the problems of too-tall gearing, and a too-slow car. In terms of acceleration, the car feels as if it received a jolt of 40 or 50 horsepower, throughout the RPM range. The 7:31 will “wake up” your car like few other modifications, and for much less money.

Parts list:
Red loctite
29:21 5th gear (optional)
Shim stack from ring and pinion donor transmission, a couple of extra shims
Ring and pinion
One-piece bearing retainer
Reinforced differential cover (915.301.311.063), if necessary
2mm ring gear spacer
12 LSD ring gear bolts
Gear Paste