The Type 915 Transmission
One of the most significant changes to the 1972 911 series is a transmission similar in
concept, but different from earlier Porsche designs. It is distinguished to the casual
observer by a revised shift pattern (conventional 4-speed “H” pattern, and
optional 5th off to upper right), and from under the car it looks larger than the earlier
series. In fact, it is substantially larger to the extent that the rear body section was
recontoured to accommodate it, and in the process structurally much improved, an added
bonus for ‘72.
The 915 does not have a tunnel type case, but a 3-piece design which is a departure
from longstanding Porsche tradition. The pressure cast light alloy housing is divided into
a differential housing, a transmission housing, and a shift housing. The whole assembly
has increased torque capacity, and should provide adequate capacity for future years.
There are numerous detail changes from previous designs, most of which allow for easier
adjustment and assembly of the internal parts. First and second gear synchronizers are
larger than 3rd, 4th, & 5th, and larger than in earlier designs. The input shaft
splines have been reduced from 24 to 20 and are an SAE specification.
The traditional ring and pinion ratio of 7:31 is retained, with 7:27 (Sportomatic) and
7:37 as optional. Although the 2.4 engines establish a new high for torque and
flexibility, it is interesting to note that the ‘72 standard ratios are shorter than
before. Although the 4-speed is recommended as adequate, there is substantial
justification for the 5-speed, for an enthusiast.
Figure 1, below, lists the standard production 2.4 ratios for all T, E & S models,
with previous 2.2 ratios shown for comparison. Overall ratios are in parenthesis.
| The diagrams for the 915 4-speed and 915 5- speed
transmissions are Figures 2 and 3, respectively. The type 925 designation is given to
the new Sportomatic counterpart of the 915. Any enthusiast who has not driven a 925
Sportomatic should do it! It is super-smooth, quick, flexible and a joy in traffic, and a
winner for autocrossing and gymkanas. Figure 4 is the 925 diagram.
Figure 5 shows all the standard and optional ratios
for the 915 transmission and their application to 1st thru 5th speed positions.
The non-standard ratio gears are available from the Porsche factory racing department
and not yet available in optional production transmissions, as in previous years. As of
March 1972 the only first gear sets available are 14/37 and 14/31 at a cost of 540 DM per
set, ex-factory; all other ratios are available at a cost of 290 DM per set, ex- factory.
(Approximately 3.1 DM = $1.00 + duty 15% and freight) |
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Non-standard 915 gear sets are identified by teeth ratio and gear position (1st, 2nd,
3rd, 4th, 5th) rather than by part number, as of this writing.
| Figure 6 is a sectional drawing of the 5-speed type 915 transmission and
shows the position of the five gear sets. In the previous 911 and 912 5-speed
transmissions (type 901, 902) second gear was integral with the main (input) shaft, and
necessitated changing this shaft to the 904 type shaft before the second gear ratio could
be changed. This has been changed in the type 915 so that first gear is integral with the
main shaft, thus making second thru fifth ratios easily changed. If a 915 first gear is
changed, the shaft is part of the gear set, thus explaining the relatively higher cost. |
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A 915 4-speed is the same as a 5-speed, except that a spacer on the shaft is
substituted for fifth gear, and the associated shift fork, linkage, etc, is omitted.
Experience to date has provided the following, worthy of note:
- When replacing a speedometer cable, care must be used to obtain perfect engagement
before tightening, or the drive gear will be easily forced past its retaining snap ring
and back into the transmission (a nightmare!)
- As with all Porsche transmissions, do not overfill; a 1/2” below the filler plug is
best. Overfilling will cause oil seal failure, loss of oil and resultant transmission
failure.
- The front main oil seal is installed as per Figures 6 & 7. This is backwards from
intuition and requires transmission disassembly for replacement (This is to be changed in
future production).
- The snap ring retaining the pinion bearing outer race has been replaced by a shoulder
machined into the differential housing, as of about March ‘72 production. Any owner
experiencing difficulty in shifting into or especially out of 4th and 5th gear should
immediately have this checked. It is understood that there are only 257 cars with this
possible difficulty. Inspection can be easily accomplished by removing the differential
side cover.
C. A. STODDARD
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