 |
||||||||
 |
||||||||
915 Series Porsche TransmissionIntroductionThe 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 and type 916 cars. HistoryWith the introduction of the 2.4 L engine to the 911 in 1972, Porsche felt that the increased torque of the 2.4 L over the previous 2.2 L 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. There are a number of similarities as well as differences between the 915 transmission and the 901 series transmission that it replaced. Similarities include...
The 915 shift pattern uses 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. Opinions as to why Porsche made this change vary depending upon who you ask. One theory is that customers complained about the pattern used in the 901 series so it was changed in the 915. Another theory is that having 5th gear and reverse in the end cover was so that it would be easy to change the ratio for 5th gear to suit specific tracks for the racing versions of the 915 transmission. Maybe both factored into the decision. Like the 901 series, the 915 case consist of three main parts. However instead of using an intermediate plate like in the 901 series, the 915 is roughly broken up into three section with the first housing the differential and bell housing, the middle housing the main gear stack, and the last housing 5th gear and reverse as well as the transmission mounting structure. While the earlier 915 transmission were cast using magnesium housing, Porsche eventually transitioned to aluminum. The racing type 916 transmission used a low sump to keep the gears out of the oil and relised upon an oil pump and internal oil squirters for lubrication. 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 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 type 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 utilized 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. The larger sized gears and bearings, and associated increase in the spacing between the shaft centers from 68mm to 76mm (the 916 used 77mm), allowed for an increase the torque capacity of the 915 over the 901 series. 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 racing transmission. Typically these transmissions were provided with a standard “open” differential. However some examples were provided from the factory with a ZF clutch type Limited Slip Differential (LSD). The 915 series transmission was eventually replaced by the G50 series transmissions. Production ChangesThe basic design changed very little over the production life. Main transmission case material changed from magnesium to aluminum. The speedometer changed from a mechanical to an electronic mechanism. The first gear synchronizer system changed from a symmetric to asymmetric dog tooth design. 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 original version had a design flaw with regards to the serviceability of the input shaft seal. For the earlier versions, the input shaft seal is hidden behind a non-removable clutch throwout bearing guide tube. Replacement of this seal in earlier transmissions requires disassembly of 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. A number of other various internal items changed during production. As you would expect with the transmission being used in various cars there are design features specific to those cars. Those used for mid-engine placement have the differential “flipped”. Listed below is a more specific history of production changes...
Known IssuesWhen used within its design parameters the 915 series transmissions are well built and tough. However those that are pushed beyond these design limits, have high mileage and/or are poorly maintained can expose the weak spots in the design.
VersionsListed below is a chart of the various versions that Porsche produced. Currently the chart is incomplete. Manual
Sportomatic
Gearing and DifferentialDifferential915 transmissions were either delivered with a standard open differential, or an optional ZF clutch type limited slip unit. Final DriveThe final drive consists of a pinion and ring gear. As this is a transaxle design, the pinion gear is on the end of (part of) the output shaft. The ring gear is bolted to the differential. The final drive ratio is the ratio of the teeth count between the pinion and ring gears. For example a typical 915 final drive has a ratio of 8:31 (3.875). This translates to 8 pinion teeth and 31 ring teeth.
Main GearsEach gear set generally consists of two gears. One for the input shaft and one for the output shaft. There are some slight exceptions. As while reverse has two gears (one of which is the "idler") it also has it's own shafts. On most 915 series transmissions the input shaft includes 1st gear however a special input shaft that does not include 1st gear does exists. So most 1st gear sets would include the input shaft. The gear ratio for a gear set is the ratio of the teeth count between the input and output shaft gears. For example an 915 “A” 1st gear has a ratio of 11:34 (3.091). This translates to 11 teeth on the input gear and 34 teeth on the output gear. While all 915 series gears can be described via their teeth count (and associated ratio), to make it easier to identify specific ratios, Porsche used an A through Z based naming system. With “AZ” being a low 11:35 1st gear and “TL” being a high 31:22 5th gear.
In a perfect world, any of these gear sets could be used in any location to provide ultimate flexibility with respect to gearing. However due to the basic design of the transmission (internal case space, shift fork locations, input/output shaft design, etc.), specific gears can typically only be used in specific locations.
Serial NumbersThe type and serial number are stamped on the bottom of the case below the differential near the drain plug. Unfortunately area is also the lowest part of the transmission and it is not uncommon for the stamping to be inadvertently ground off. 915/AA BB CD ENNNNN
Links
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||