Transmissions, Porsche Transmissions Options

[Randal]

I keep hearing about all the different transmission formats that can be put into a 914 (4 or 6).

One of the guys at Sundays event is building a 914 with a 993 engine with the transmission turned upside down. Now how can that work, i.e., shift patters, etc.

So what are all these different transmission numbers (914/915/?)and what are the differences?

And why are 915 transmissions so expensive?

[ArtechnikA]

One of the guys at Sundays event is building a 914 with a 993 engine with the transmission turned upside down. Now how can that work, i.e., shift patters, etc.

So what are all these different transmission numbers (914/915/?)and what are the differences?

And why are 915 transmissions so expensive?

i'm not the expert here but i can rough out some of it ...

one way an inverted transmission can work is that you can do anything you want with mechanical linkages, including change the direction of twist. another possibility is that the gears have no idea what position they're in on the stacks - on a transmission without a fixed 2nd (like the 901/914) there's nothing to keep you from assembling the clusters in any order you want - all the rotational portion of the linkage does is decide which pair of gears is going to be selected by the fore-aft motion.

i'm not going to write the book on the various transmission types - that's been done, the information is out there. check out Barth's "The Porsche Book" or Frere's 911 Story, or "Excellence Was Expected" for some of the development history.

nowadays 915's aren't -that- much more expensive - but they're bigger, stronger, and able to transmit a lot more torque. they have provisions for a built-in gear oil pump for external cooling.

the cost comes in because they're not set up for mid-engine use - modifications are required. they're enough longer that you have to choose between some kind of custom exhaust or sideshift - you can't tail-shift a 915 in a 914/6 with the original muffler - no room. the linkage bits for side-shift 915's are very low volume parts - basically lots of custom machining.

but they do support lots more ratios than the 901, and for racing purposes, 5th is overhung in the nose cone so it's pretty easy to change - and top is a gear you'd like to be able to change in a hurry...