what in it's design makes the 915 notchy?
 

I have set up my 915 to the best I could, and it's going fine. The whole mechanical link from shifter to gearbox has been fitted with new bearings, and the coupler been delicately adjusted spline after spline till best response. Every 10th or so downshift from 3rd to 2nd asks for a double clutch. Apart from that, it's going fine.
However, it rarely feels like 'butter', and any gear change needs some more force than I am used to in other cars. No grinding or weird noises however, just some notchy feel as if it needs to be pushed over some cam. Hence my question: what parts/gears/syncros/sliders/ etc in the gearbox are causing this resistance ?

Like Porsche is a two syllable word, the the 901/915/930 transmissions are a two syllable shift, due to the balk ring syncro design. There needs to be a split second of time allowed for the shift to be accepted. You should not over-rule that moment by forcing a shift. As syncros and shift sleeves rub on each other over the years, the rough friction surface on the syncro wears off to a polished surface and the sleeve, can't slow or speed up the free-wheeling gear fast enough for a nice, reasonably quick engagement.

Yup this....as John said...

http://forums.pelicanparts.com/uploa...1604090167.jpg

i always told my clients between any gear shifting, one second delay

One thing I've found is many 915's have been converted with the factory short shift lever. That makes the shifting much more notchy. I always convert back to the regular lever, longer throw, but so much smoother.

As John said, it's a two step motion to shift these. At first resistance of gear engagement, pause, then finish your shift motion. It becomes natural fairly quick.

Even better learn to double clutch; gives required pause and makes changes really smooth when you get it right. Downshifting needs blip of throttle and then there's heel and toeing for braking and downshifting when approaching a corner. Getting it right adds a whole new level to the fun.

There's nothing you can adjust or replace in the linkage that's going to overcome an issue in the gearbox. I've owned many 911 3.2 G50's, but this most recent was my first 915 car. My car has pretty low miles (mid 40's) and it shifted more or less as you describe yours does; occasionally balky and needed a double-clutch downshift, occasional noise going into 1st unless at idle, a little vague in feel, etc. I installed a Wevo shift coupler and dialed it in carefully after much trial and error, and it was a little better (less vague) and just got used to the character.

I had a very knowledgeable air-cooled tech drive the car for an opinion, and he immediately said it has some broken dog teeth in at least two gears. I decided to address it immediately rather than waiting for further issues. When he pulled the motor and opened up the gearbox, it turned out to be exactly what he thought, along with a cracked pressure plate. He replaced all synchros, dogs, and sliders, etc. I haven't picked it up yet, but I'm really excited to see how much better it drives. I guess the moral is that I assumed that mine was just normal based on everything I've read about the way the 915's shift, and obviously that's not the case. I'd bet that is most of these are in need of the kind of freshening that mine needed, and if you think yours has an issue then it probably does, and new bushings, couplers, and fluid isn't going to fix it. The upside is that if the gears themselves are fine (mine were) that this isn't' a very expensive job (probably under $5k).

915 transaxles shift just fine. You should never have to double clutch a downshift. The balk ring design just means you have move the lever a little slower.

If it doesn't shift properly, there is something wrong inside.

I think you’re right, just be precise and slow with changes. Be it the 915 box in my 72S or the G50 in my 993 Cup RSR, change precisely

I picked up my first Porsche 1972 911-T at the factory in September 1971. I kept it for seven years and it had just over 42,000 miles when sold. I never had the problems shifting as described in this thread.
I have a 1980 SC and I have the same problems as the O/P describes. I do not know the exact mileage because the 85 MPH speedometer was replaced but paper work I have indicates it may have over 60,000 miles.
I have replaced all of the easy to get to parts that have been mentioned in other threads on this matter including the clutch.
So as Winders suggests its best just to slow down the shifting until I can rebuild a worn 915 transmission.