915 transmission
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Introduction
The type 915 transmission was a four or five speed transmission designed and manufactured by Porsche between 1972 and 1986. The transmission was used in the type 911 car.
History
With the introduction of the 2.4 liter engine to the 911 in 1972, Porsche felt that the increased torque of the 2.4 over the previous 2.2 warranted a stronger transmission. The design of the 915 is based upon, but is not identical to that of the 916 transmission that had been used years before in the 908 racing car. One feature of the 916 that carried over to the 915 is the orientation of 1st through 5th gears. Two differences between this and the earlier 901 transmission is that 5th gear was accessible via the removal of the end cover. One reason for having 5th be so easily accessible is that this gear is a commonly changed gear for individual tracks. The second difference being that with the 915, the portion that holds the differential and makes up the bellhousing was cast separately from the housing of the central gearbox and end cover. This allowed specific material selection such as die cast aluminum for the bell and transmission housing and magnesium for central gearbox section the gearbox end cover.
The 916 used a low sump to keep the gears out of the oil but with the addition of an oil pump and internal oil squirters. The original 915 used a splash lubrication system with no pump or external cooler. Race versions of the 915 used a 916 style oil pump and external cooler while production car versions used a different style oil pump that was incorporated into the differential cover plate. The larger sized gears and bearings needed to increase the torque capacity of the 915 over the 901 meant increasing the spacing between the shaft centers from 68mm to 76mm (the 916 used 77mm). Another significant change was that the shift pattern now was in a more commonly used “H” pattern that covered the first four gears with fifth gear to the right and up and reverse to the right and down. The previous 901 transmission had reverse and first on the left and this made for an awkward shift from first to second gear.
The 4-speed is the same as a 5-speed, except that in place of the fifth gear, a spacer on the shaft is used and the associated shift fork, etc, is omitted. The type 925 transmission is the Sportsmatic version of the 915. The internals from a 915 were used in one of the transmissions used by the type 924 car. A version of the 915 transmission was used in the 916 car. The significant modifications included the flipping of the differential to allow for mid-engine use as well as a "side shift" style shifter. This utlized a custom end cover as well as moving the shift input to the side of the transmission. Existing 915 transmissions can be converted to this configuration with the proper parts.
It is believed that Porsche set a 181 lb/ft maximum torque value for the Aluminum cased version of the 915. However this value most likely represents a torque level that maximized longevity and that while the 915 can handle higher torque values, they would be expected to impact the lifespan of the transmission. It is believed that Porsche set a 275 ft/lbs maximum torque value for the racing versions (using a reinforced final drive as oil pump, cooler and squirters) of the 915 transmission and 239 ft/lbs for the earlier 916 transmission.
Versions
Listed below are the various versions of the 915 that Porsche produced as well as the stock gear sets...
Manual
| Version | Gears | R&P | 1st | 2nd | 3rd | 4th | 5th | Serial Number | Note |
|---|---|---|---|---|---|---|---|---|---|
| 915/00 | 5 | AZ | HX | NT | QP | TM | ? - ? | ||
| 915/01 | 5 | AZ | HX | NT | QP | TM | ? - ? | ||
| 915/02 | 5 | 7.31:1 | AZ | HX | NT | QP | TM | 73 20001 - 73 2????, 73 30001 - 73 3???? | 1972 |
| 915/03 | 5 | AZ | HX | NT | QP | TM | ? - ? | ||
| 915/05 | 5 | ? - ? | 1973-74 911, 911S, Carrera | ||||||
| 915/08 | 5 | AZ | HX | NT | RP | TM | 78 30001 - 78 2???? | 1973 Carrera RS with oil pump | |
| 915/10 | 4 | AZ | HW | OR | SN | ? - ? | |||
| 915/11 | 4 | AZ | HW | OR | SN | ? - ? | |||
| 915/12 | 4 | 7.31:1 | AZ | HW | OR | SN | 72 20001 - 72 2????, 72 30001 - 72 3???? | 1971-72 911T, 911E, 911S | |
| 915/13 | 4 | AZ | HW | OR | SN | ? - ? | |||
| 915/16 | 4 | 7.31:1 | AZ | HW | OR | SN | ? - ? | 1973-74 911, 911S, Carrera | |
| 915/18 | 4 | AZ | HW | OR | SN | ? - ? | 1973 Carrera RS with oil pump | ||
| 915/40 | 5 | 71 5???? - 71 5???? | 1975 911S, US Carrera | ||||||
| 915/43 | 5 | ? - ? | 1974-75 911 | ||||||
| 915/44 | 5 | 8:31 | AZ | HX | NT | SN | 71 6???? - 71 6???? | 1975-76 911, US 911S | |
| 915/45 | 4 | 8:31 | AZ | KW | PR | SN | ? - ? | 1975 911S | |
| 915/48 | 4 | ? - ? | 1974-75 911 | ||||||
| 915/49 | 4 | ? - ? | 1975-76 911 | ||||||
| 915/50 | 4 | ? - ? | 1977 Racing 911 Carrera | ||||||
| 915/61 | 5 | 8:31 | AZ | HX | NT | SN | 71 7???? - 71 7????, 71 8???? - 71 8????, 71 9???? - 71 9???? | 1977-79 911 with clutch assistance | |
| 915/62 | 5 | 8:31 | 73 A???? - 73 A????, 73 B???? - 73 B????, 73 C???? - 73 C???? | 1980-1982 Europe & ROW without oil cooler | |||||
| 915/63 | 5 | 8:31 | AZ | HW | NT | SN | 74 A???? - 74 A????, 74 B???? - 74 B????, 74 C???? - 74 C???? | 1980-1982 USA & Japan - without oil cooler | |
| 915/65 | 4 | ? - ? | 1977 without clutch assistance | ||||||
| 915/66 | 4 | ? - ? | 1977 with clutch assistance | ||||||
| 915/67 | 5 | 8:31 | 73 E00001 - 73 E10000 | 1984 Europe & ROW - with oil cooler | |||||
| 915/68 | 5 | 8:31 | 74 E00001 - 73 E10000 | 1984-1985 USA & Japan - without oil cooler | |||||
| 915/69 | 5 | 8:31 | 73 E10001 - 73 E11000 | 1984 Europe & ROW Turbo-look - with oil cooler | |||||
| 915/70 | 5 | 8:31 | 74 E10001 - 74 E11000 | 1984 USA & Japan Turbo-look - without oil cooler | |||||
| 915/72 | 5 | 73 F00001 - 73 F10000 | 1985 Europe & ROW - with oil cooler | ||||||
| 915/73 | 5 | 74 F00001 - 74 F10000 | 1985-1986 Europe & ROW - without oil cooler | ||||||
| 923/01 | 5 | 1972 916 |
Sportomatic
| Version | Gears | R&P | 1st | 2nd | 3rd | 4th | 5th | Note |
|---|---|---|---|---|---|---|---|---|
| 925/00 | 4 | 76 20001 - 76 2????, 76 30001 - 76 3???? | 1971-72; 911T, 911E | |||||
| 925/01 | 4 | 7:27 | 76 20401 - 76 2???? | 1972; 911S | ||||
| 925/02 | 4 | 7:27 | C | I | Q | Y | 1973-74; 911, 911S, US Carrera | |
| 925/09 | 3 | 1976; 911 (Mech. Speedometer) | ||||||
| 925/10 | 3 | 8:27 | DA | M | V | 1974-75; US 911S, US Carrera | ||
| 925/12 | 3 | 8:27 | C | K | V | 1976; 911, US 911S | ||
| 925/13 | 3 | 1976; Carrera 3.0 | ||||||
| 925/15 | 3 | 1977; 911 | ||||||
| 925/16 | 3 | 1977; Carrera 3.0 | ||||||
| 925/17 | 3 | 8:27 | C | K | V | 1977; US Japan 911S |
Serial Number
Model and Serial Number are stamped on the bottom of the case below the differential.
915/AA BB CD ENNNNN
- A = Type (i.e. 62 = 915/62)
- B = Differential Designation, 12 = ZF LSD, missing = no LSD
- C = Application, 7 = 6 cylinger engine
- D = Gearing, 1 = ?, 2 = ?, 3 = 5 speed Europe and RoW, 4 = 5 speed USA, Japan, 6 = Sportomatic, 8 = ?
- E = Model Year, 2 = 1972, 3 = 1973, 4 = 1974, 5 = 1975, 6 = 1976, 7 = 1977, 8 = 1978, 9 = 1979, A = 1980, B = 1981, C = 1982, D, = 1983, E = 1984, F = 1985, G = 1986
- N = Sequential Serial Number (i.e. 00001, 00002, etc.). May be 4 or 5 digits. Some models did not start with serial number "1"
Gear Sets
| Code | Ratio | Gear |
|---|---|---|
| AZ | 11:35 (3.182) | 1 |
| BY | 12:34 (2.833) | 1 |
| D | 14:37 (2.643) | 1 |
| E | 15:36 (2.4) | 1 |
| DV | 14:31 (2.214) | 1 |
| FZ | 16:35 (2.188) | 1,2 |
| GZ | 17:35 (2.059) | 2 |
| EU | 17:34 (2.0) | 2 |
| ET | 15:29 (1.933) | 2 |
| HY | 18:34 (1.889) | 2 |
| HX | 18:33 (1.833) | 2 |
| HW | 18:32 (1.778) | 2 |
| JW | 19:32 (1.684) | 2 |
| KW | 20:32 (1.6) | 2,3,4 |
| LV | 21:31 (1.476) | 2,3,4 |
| LU | 21:30 (1.429) | 2,3,4 |
| LT | 21:29 (1.381) | 3,4 |
| MT | 22:29 (1.318) | 3,4 |
| NT | 23:29 (1.261) | 3,4 |
| NS | 23:28 (1.217) | 3,4 |
| NR | 23:27 (1.174) | 3,4 |
| OS | 24:28 (1.167) | 3,4 |
| OR | 24:27 (1.125) | 3,4 |
| PR | 25:27 (1.08) | 3,4 |
| PQ | 25:26 (1.04) | 3,4,5 |
| 26:26 (1.0) | 3,4,5 | |
| QP | 26:25 (0.9615) | 3,4,5 |
| RP | 27:25 (0.9259) | 3,4,5 |
| RO | 27:24 (0.8889) | 3,4,5 |
| SO | 28:24 (0.8571) | 3,4,5 |
| SN | 28:23 (0.8214) | 3,4,5 |
| SM | 28:22 (0.7857) | 3,4,5 |
| TM | 29:22 (0.7586) | 3,4,5 |
| TL | 31:22 (0.7097) | 3,4,5 |
Production Changes
Over the life of the transmission various changes were made. While the transmission case was either all Magnesium or a mixture of Aluminum and Magnesium (it is not clear) it was eventually changed to all Aluminum. This was due to a combination of less use of Magnesium and the added strength (at a cost of added weight) of Aluminum. The ring and pinion were changed due to the 8:31 version being more robust as well as it placed less stress on the differential side cover. The ring and pinion and other ongoing gearing changes better matched the changing characteristics of the newer cars. The original version had a design flaw with regards to the serviceability of the input shaft seal. For the earlier versions, the release bearing guide tube needs to be removed to replace this seal and this requires you had to disassemble the transmission. Later improvements allowed this tube and seal to be removed and replaced without any other disassembly of the transmission. Due to later engines producing more torque, transmission oil pump, squirters and coolers were offered on various models. While the later Aluminum cases are overall stronger, the earlier Magnesium cases are better in one area. The main gearbox case bearing retainer area on the Magnesium case has embedded steel inserts to strength the weaker Magnesium. For the Aluminum versions of this part, the steel insert is not included. This allows the Aluminum versions to be actually weaker than the reinforced Magnesium case in this area. The bearing retainer area on Aluminum cases can potentially become out of round which can potentially destroy the transmission if not addressed. If the damage is serious enough the Aluminum case is not easily rebuilt. Additionally there are stronger aftermarket single piece bearing retainers that replace the factory style two piece bearing retainers. The single piece construction ties to the bearing together and puts less stress on the base material (Magnesium or Aluminum)
There is no single "best" version of this transmission as expected usage heavily influences this decision. However if you were to pick and choose parts from various types, you can use one of the later Magnesium cases to avoid the bearing retainer issue as well as reduce weight, use the stronger 8:31 ring and pinion, the stronger later style differential side cover (prevents ring and pinion backlash settings from changing under load on high power engines), an aftermarket single piece (vs. the factory two piece) bearing retainer (help address the bearing retainer issue), and the later style improved mainshaft to build a hybrid 915. Some feel that if you want to use the 7:31 ring an pinion that the use of the strongest differential side cover is a requirement for long life.
- 1972 - Start of production; Magnesium and Aluminum mixture (or all Magnesium?); 7:31 ring and pinion
- 1973 - Change to input shaft seal design to address service issue flaw.
- 1974 - Change (final) to input shaft seal design to address service issue flaw. Transmission disassembely no longer required.
- 1975 - Mid year change from 7:31 to 8:31 ring and pinion
- 1975 - Change to shift sleeves?
- 1976 - Upgraded mainshaft (1st not on the shaft?)
- 1976 - Change to speedo output from mechanical to magnetic sensor to support new electronic speedometer.
- 1977 - Change to Aluminum final drive case with Magnesium for remander of case (some, but not all)
- 1977 - Change to 1st and 2nd sycros?
- 1978 - Change to all Aluminum Case
- 1978 - Reinforced differential side cover (addition of extra ribs)
- 1979 - All models 5 speed now
- 19?? - Improvements to shifter and syncros?
- 1984 - Addition of oil pump and cooler for some models
- 1986 - End of production?
914 Relevance
The 915 is a popular upgrade choice when installing a larger displacement engine in a 914. Engines that are 3.0 liters and beyond (especially if used for track duty) may benefit from a 915 transmission. Some people with engines in this same range have no problems with the original 901 based transmission, but ultimately the benefits of a 915 depends upon the amount of torque your engine produces, how aggressively you drive and the desired lifespan of the transmission. The largest factors in dealing with a 915 upgrade is the higher cost of the transmission itself, conversion for mid engine use, and purchase or fabrication of the required shift linkage. Common solutions for the shift linkage include using replicating the factory solution used on the 916 car, the George Vellios setup (which is much like the factory solution) or the WEVO setup (which solves the problem differently than the factory did).
Misc
- There is an "early" and "late" version of the torque bias LSD (40%/80%). Is the early vs. late split in 1978 when the case changed?
