What was porsche thinking when they built the 915 trans?

[J P Stein]

Originally Posted by Wayne at Pelican Parts

If you start out driving a 914, the 915 transmission seems like a dream!

-Wayne

Which tells me you've never shifted a decent 901 side shifter. I will agree that 90% of the 914, 901s I have driven shift like crap.....but that is not the fault of the trans, more the fault of the people that don't take care of them.....the 924 syndrome applies here I'm afraid.

If you ever get up this way again and the opportunity presents itself, you can take a pass in my tuned 914 with a good 901....which still looses a couple tenths per shift to a Maita. In most cases it quickly gets the time back after the shift is completed.

[Grady Clay]

Wayne,

You are absolutely right on – if you are talking about a ’70-’71 tail shifter. You couldn’t tell if the dog was wagging the tail or the tail was wagging the dog … er shifter. That Rube Goldberg has four EXTRA sloppy linkages. I thought my RS-60 had poor shifting in 1969. I then drove a new ‘70 914 – ugh.

On the other hand a 914-6 (strait shot linkage) and a well maintained side shifter (’72-’76) can shift better than even the best 901/911 and far better than a good 915. Greased lightening, no fumbling around and never a missed shift.

Applying all that has been learned, up-dated and aftermarket new parts and careful inspection and assembly, you can build a 915 that is a pleasure to drive.

Best,
Grady

[RWebb]

Grady,

can you compare the shift feel of the 901/911 with that of a 915?

- all in a 911 car, not the 914 stuff...

Thx!

[javadog]

Originally Posted by RWebb

Grady,

can you compare he shift feel of the 901/911 with that of a 915?

Thx!

I can!

A fresh 901 shifts quite well, maybe better than a 915.

A fresh 901 tail-shifter in a 914/6 shifts well in the first three gears but fourth and fifth are a lottery. Still, it's better than a tail-shifter in a 914/4. A 914/6 with a side-shifter doesn't shift quite as nicely in the first three gears as when it was a tail shifter but fourth and fifth are actually usuable. The linkage is a little tighter than a 911 but a little slower to me, as the planes are a little different.

JR

[Grady Clay]

Randy,

I think the two big issues with a 915 compared to a 901/911 are the dramatic increase in the size of the synchronizing components for 1st and 2nd gears and the 5-to-4 shift.

The larger syncros require more force to operate. When there is significant difference in speed for the syncro to deal with, the servo force is higher again. The good side is that they will synchronize even with large differences in speed and aggressive shifting IF the parts are all new. They (1 & 2) never seem to find the situation where you can just ‘slip’ into gear even matching revs.

The 3rd, 4th and 5th gear syncros are the same as 901/911 and don’t suffer from this issue.

The 5-to-4 downshift is fraught with difficulties. Do you rely on the reverse lockout and just move the lever rearward? Do you let the spring push it to the left risking going into 2nd rather than 4th? With worn or mal-adjusted parts you can move the lever to neutral yet leave 5th engaged and not be able to put it in any other gear. If this happens, when stopped you can select 1st or 2nd and lock up the transmission. All this is due to the manner of selecting the 5-R shift rail. If Porsche had used the selector finger to engage the 5-R rail (as the 901 does with 1-R) this probably wouldn’t be as much an issue. Someone should invent a clever lockout for the 1-2 rail when coming out of the 5-R rail to force you to select the 3-4 rail first.

How does all this translate into ‘feel’? Dealing with 1st and 2nd simply takes more force. The 5-to-4 downshift has ‘uncertainty’ associated with it. I tend to shift down to neutral, stir the shifter slightly to know where I am and then select 4th gear. I have personally never caught 2nd gear but I have been passenger and seen how easily it is done.



As somewhat an aside, there is a progressive nature to clashing the gears when you ‘beat’ the syncros. The ‘grinding the gears’ is not the gear teeth but the engagement dog teeth on the free gear and the sliding sleeve clashing. On the sliding sleeve it simply rounds off the nose of the dog teeth. The dog teeth on the gear have a much bigger problem. Not only does the clash round off the nose of the teeth but some metal is moved toward the syncro ring. This shows up as a little sharp protrusion on the inside of the end of each dog tooth. Every syncro from a set with damaged dog teeth has a small grove worn into the syncro ring from this protrusion. As this happens it slightly compresses the syncro ring to smaller installed diameter, reducing the synchronizing functioning. This causes easier clashes in a vicious cycle.

If you install a new syncro ring under these protrusions, it will not work as well as the old one.
2¢

Best,
Grady

[RWebb]

Thx much - I sometimes hear people describe the "light feel" of a 901 box rel. to a 915. They must be describing the synchronizing force...

I've never driven new or fully renewed versions of either transmission.

[robert walsh]

Sorry, wrong post.

[diamonddh]

Momo 3.2...classic reply.......

mine: '78 SC...needing a Wevo.

[ruf-porsche]

Originally Posted by Formerly Steve Wilkinson

I've always thought the basic reason for the 915 design and its compromises was its light weight and compactness.

I always thought it was what they could scrounge around at the VW warehouse.

[burgermeister]

Originally Posted by ruf-porsche

I always thought it was what they could scrounge around at the VW warehouse.

There is nothing wrong with a beetle transmission. I liked mine a lot, actually. It put up with an incredible amount of abuse.