kusee pee

Having previously had my 964 and 964T with G50-version boxes I had at first found the 915 in my 3.2 a little stubborn. However, I now love it and get real enjoyment out of getting the shift right. However, a thought occured to me that you guys may be able to help with. Whilst part of me can understand that there will be mechanical reasons for one gearbox being smoother of shift than others I have no real understanding why the 915 might be smooth as a knife-through-butter on one shift but then a pig on the next. I appreciate that rev-matching etc make things easier but sometimes I expect to get a great shift but then am surprised when it sticks for no real reason. So...my question is this:

Can anyone describe to me exactly what's happening mechanically to make the shift quality variable?

Purely to satisfy my understanding. TIA.

3.2SLANTNOSE

I could only wonder at exactly what is going on, other than a bunch of things going around and another group of things trying to be moved into the moving parts. To be serious now, you will almost get a headache, trying to visualize the 915 at work. Having to visualize somewhat of a mirror image, reverse function, of how the shifters actual relationship to the gear selection is the key. An American, standard 3 speed transmission, the shift pattern is the same, even if it is a column shift or floor shift. It is still a simple "H" pattern for both. This probably does not really answer you question, but... my head is hurting.

Peter Zimmermann

Shift quality is "Variable" due to a number of reasons, but the usual culprit is good old fashioned wear & tear. As the 915 begins to get tired, the wear on synchro rings and sliders eventually begins to cause a synchro to slider mismatch. Imagine a synchro ring, it's thin and about as big around as a coffee cup. Its outer surface is a rough friction material that wears off with use. It does not wear evenly, and because of that neither do the engagement teeth on the inside of the slider. Because of the inconsistent wear those two parts will either mesh well, or be incompatible. Other causes for the problem can be; (1) Poor/incorrect assembly during previously-done repairs, (2) use of sub-standard oil, or (3) poor driving technique. A couple things to remember; (1) every shift must be done in two parts (remove the lever from the gear that you're in and move the lever just to the pressure point of the desired gear, then engage that gear when you feel it's ready), and (2) never exceed about 4500 rpm in first before shifting into second, and never speed shift. The 915 is all about feel, and the driver adapting to the transmission. The 915 can't adapt to the driver, and those who expect it to will have a bad experience.

kusee pee

Cheers guys, that helps. And I feel privileged that Pete has replied to one of my posts (my questions are normally so menial that I feel embarrased posting them!). I am all for smooth shifting and always try to be slow and gentle - I think to myself that the car wants to change gear just as much as I do; we've just got to work together. It's a partnership!

Daniel Dudley

Great summation of shifting technique. Offer up to the next gear. Try to get to the next gear faster than you need to, and feel it in. Shifting too slow can be as bad as shifting too fast, and often a slight blip of the throttle will allow you to offer it up again, but the real trick here is to practice, practice, practice.

Try holding the gear lever with the tips of your fingers, and trat the lever as if it needs to be moved in a precise and delicate way. Move the lever into neutral,and go to the next gear in two steps. If you are shifting into third, move to neutral, move the lever over, and then go up. Press into the next gear gently, and feel if the resistance gets greater or easier. That is how you will develop feel.

Pay attention to the timing of your shifts as well as the feel, for it is the combination of those two things that make for a successful shift. After a while, you will pretty much know when a shift will work or not, and why it did or didn't.

I am just an ordinary guy. From my house it is really hard to tell if you have gearbox problems or not. I know a lot of guys that can drive around syncromesh problems, and the way Peter describes shifting into second really babies the gearbox and would probably help those syncros last forever. It will certainly help you if your syncros are already on the way out.

Here in the Northeast, I often shift from first right into third until the gearbox is good and warm. Swepco is said to be the best lube for our trannies, but it sure does make for a pig of a shift when cold.

3.2SLANTNOSE

That is so very TRUE! The driver has to adapt to the individual 915 that they are driving. Though they are all 915 transmissions, none "feel" the exact same way. But... if you are able to master your own 915, when you get to a different 915, you have the battle half won, and it won't be long to having trouble free shifting, being you will learn the way it feels, and can adapt fairly quickly.