Auto
to 5 Speed Conversion
Personally a 928 is not a sports car with out a manual transmission.
Typically I would not buy a 928 with a automatic - all things being equal.
But as with most of life not all things are equal and since the other two owners
of convertibles that I know were not about to sell me theirs and since the
dealer selling the car was not willing to do the conversion I was left with
almost all of the things I wanted. Hopefully, it will be less work to
convert an automatic to a 5 speed than to convert a coupe to a convertible - but
time will tell.
Compatibility
Before I began the project I started on some research. The information
is not as readily available as it appears and there are a number of trade
offs. Here are some of the things that I found out. (There is a transmission
comparison chart under the technical section)
 | Limited
Slip Differential - The limited slip differential is transferable from
transmission to transmission - even automatics to the five speed. So
if you have an LDS in your automatic save it for the five speed.
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| Flywheel
- The flywheel changed from year to year. It is important to get
the right flywheel for the right year car. The 85-86 US has a timing
pick up for the injection system that was not present on the early
flywheels. This is different from the S4 timing pick up. The
S4 (87 up) is a single clutch. The dual disk clutch under pre 1987
is considered a stronger clutch, but is harder to adjust. DEVEK
makes a retro flywheel for the S4 that will accommodate the dual disk
clutch. The 84 flywheel is a one year only, but has a
diagnostic pick up that is not needed for the car so other pre-85 wheels
can be substituted.
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| Transmission
- The transmission was upgraded to better syncros in 85. The bad
news - starting in 83 Porsche lowered the ratio of the final drive in an
attempt to increase gas mileage. If you can get a Euro transmission
the final drive ratios will be higher giving the car better
response. Unfortunately the pinion gear is part of the transmission
so the final drive ratio can not be changed with a different ring and
pinion set as on the automatic. Porsche only got this one half
right. The transmission and torque tube on the 78 is a single year
only and has to be installed as a pair.
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| Clutch
Linkage - Here is another place that Porsche cut a minor corner and
cost the converter a bunch of time and trouble. The majority of the
brackets for the clutch pedal assemble is part of all cars - automatic and
5 speeds. The bracket to hold the clutch pedal spring is on all
cars. But the bracket to hold the clutch master was only welded into
the bodies that were to get a 5 speed. I tried to buy this piece
from Porsche but they only sell a complete firewall (not cheap), The
whole thing is weird because the firewall insulation has the cutout for
the clutch master. Five minutes worth of spot welding and a dollar
worth of stamped sheet metal - go figure on a $40,000+ car you got to cut
corners at every turn.
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| Cross
Member - The cross member that supports the transmission is different
for the automatic and five speed. Be sure to get this when you buy
parts from the conversion.
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