Proper way of shifting the 915 gearbox

[gduke]

You can rapidly upshift the 915 tranny. It's downshifting that you need to be most concerned. There's much more stress on the gears down shifting than upshifting. It's important that you match speeds of the rotating masses to prevent the gears from slapping while downshifting, eg. blipping the throttle in nuetral and shifting into the lower gear, then adding gas as you release the clutch. The tranny teeth are more likely to break downshifting than they are upshifting. When downshifting you prevent the gears from slapping which can break teeth.

[flyingplatypus]

i get a bad grind when shifting into first from second... is that normal, or do i need to double cluth, or what? sometimes i get a little noise when going to 1st from a stop too, time for a rebuild?

[island911]

Originally posted by wholberg
Explain to the skeptical "friend" how syncros work.

Syncronizing gears accelerate or decelerate the clutch disk to match speed with the pressure plate. They do this through friction. Pausing betweeen shifts or applying light pressure to the next gear until it drops in extends the life of the syncro gear because maintains a better oil film, resulting in less metal to metal contact as that portion of the trannie catches up with the next gear. Ramming it into gear forces the surfaces together with more force, resulting in increased wear. Even the non-Porsche owning friend should understand this.

Just because his trannie is not worn out yet, does not mean it is not experienceing premature wear due to hard shifting.

Good write-up. Though the "friend" was talking about shifting quickly . . not shifting "hard".

For sure a 915 can be shifted quickly, (without detecting any grinding or resistance) And, then no damage or premature wear will result from that style of shifting.

The "pause" should be only as long as it takes for the rpm to drop to match the speed of the next gear -- usually pretty damn quick, unless you're slow to pull your right foot off the throttle.

[911pcars]

Originally posted by wholberg
Explain to the skeptical "friend" how syncros work.

Synchronizing gears accelerate or decelerate the clutch disk to match speed with the pressure plate. They do this through friction. "

Uh... not quite. You have just described the tail (synchros) wagging the dog (clutch disk). It's the other way around. Two things affect the rotational speed of the transaxle gears/shafts and thus synchro speeds; (1) the engine, when the clutch is engaged (pedal up) and (2) the rear wheels via the differential unit and the gearbox output shaft.

Because there's a network of gear clusters between engine and rear wheels, there must be a device (or series of devices) called synchromesh assemblies that cause to-be-meshed gear and shafts to rotate at the same speed before they can mate up and deliver torque.

In practice, one shaft/gear is driven by the engine via the flywheel, pressure plate and clutch plate (splined to the gearbox input shaft). The input shaft and whatever is meshed to it rotates at a different rate from the other gears/shafts that rotate from their connection to the rear wheels through the differential unit.

"Pausing betweeen shifts or applying light pressure to the next gear until it drops in extends the life of the syncro gear because maintains a better oil film, resulting in less metal to metal contact as that portion of the trannie catches up with the next gear.""

Anytime the synchromesh assembly attempts to match these different rotation speeds, friction surfaces are involved and thus wear. Synchros work their magic by creating friction. If the surfaces are as slick as engine bearings, there would be no synchro action. Moving the synchro slowly or quickly to match rotation speeds still involves wear of the assembly, so the thought that downshifting slowly saves the synchros is not correct. The synchro must still match different rotating speeds.

The ONLY way to minimize synchro wear and shift quicker is to match the rotation speeds of to-be-meshed gears and shafts, and the only way to accomplish this when downshifting is to double-clutch properly. Reving the engine with the clutch disengaged is not double-clutching and does nothing to rotate the now un-connected input shaft. Reving the engine while the clutch is disengaged can only rev-match the clutch disc (input shaft) with the flywheel (pressure plate and engine) and that's all. Notice the shift is already completed during this maneuver.

However, with the transmission in neutral and the clutch engaged (pedal up), the engine is connected to the input shaft. The input shaft speed is thus controlled by the driver via the throttle.

For example, if the car is in 4th gear at 3000 rpm ready for the shift to 3rd, one only has to rev the engine in neutral (clutch engaged), so the engine (and input shaft) is at the same rpm it would be while in 3rd. In real-world practice, one revs the engine a little higher in neutral to compensate for the time it takes to de-clutch into neutral, engage clutch and rev-match, then declutch again for the shift into 3rd. In that time period, the driver increases engine revs to the ideal rev-matching speed. If done correctly, you will only hear two engine speeds; the prior revs in 4th and the engine revs as it would be in 3rd.

Gear ratio charts show the exact rpm difference between each gear position. This speed differential is constant no matter what the engine speed. If the rev difference between 3rd and 4th gear is 900 rpm, then it doesn't make any difference if one shifts from 4th to 3rd at 3500 or 4500. The revs after shifting to 3rd will be 4400 and 5400 rpm, respectively. Knowing the rev difference between all gears allows you to rev the engine to that differential before downshifting. Of course if one inadvertantly downshifts to 2nd gear instead of 3rd at 6000 rpm, then you might develop other problems not of a gearbox nature.

Double-clutching is usually NOT required when upshifting. The normal progression is higher rpm to lower rpm when upshifting. As engine speed DROPS when upshifting from 1st to 2nd, etc, the speed difference decreases between the gradually slower rotating input shaft (engine speed) and the gradually increasing speed of the gearbox (rear wheels). The usual pause period required with the 915 allows the slow-acting synchros to mesh and lock the shafts/gears together. Rushing the upshift also places load on the synchro as it attempts to match the different rotating speeds of the input and output shafts/gears. When to-be-meshed gears/shafts are at the exact same rotating speed, the gearbox shift will be slicker than gorilla snot (to coin a phrase).

Hope this helps understanding rather than adding confusion.

Sherwood

[evill]

Sherwood nailed it.

Good explanation and write up.

Ed

BTW- My 915 seems to be harder to downshift on Tuesdays.

[deoxford]

So I am doing it right after all... I been tell my wife that,wait wrong topic

[no substitute]

Originally posted by flyingplatypus
i get a bad grind when shifting into first from second... is that normal, or do i need to double cluth, or what? sometimes i get a little noise when going to 1st from a stop too, time for a rebuild?

Bad first gear synchro perhaps. Double clutch down helps smooth it out in both cases. I wouldn't rebuild it just for this, however.

[frankc]

What about shifting into first from a stop? Sometimes my 915 puts up a fight. This is especially annoying when at a red light, and then it turns green and you can't get it into first. The harder you try, the more difficult it seems (of course, there's alway an impatient driver behind you).

I did have the 1st gear synchro replaced not too long ago, and it is definitely better, but there are still times it will not go. My mechanic told me one trick with the 915 is to "bump" 3rd before shifting into 1st when starting off. That is, don't shift all the way into 3rd, just apply pressure as if you were (there is a little notch you can usually feel). This trick does work. It seems to go into 1st effortlessy every time after doing this. Has anyone else heard of this?

[tshih]

frankc, the way to get into 1st easily at stoplights is to first bump it slightly into 2nd and the shaft speed will make for easy shifting into 1st. To get reverse smoothly first go into 5th (at stop!!! not on highway mind you!!!).

[no substitute]

I shift into first as I stop. There is the T/O bearing issue with this method, so you can get it into first as you stop and then take it out of gear while you wait at a long light. This seems to help the second gear bump method when it's time to get moving again. It's also possible a tiny adjustment of the pinch bolt (under the cover, under the carpet, in front of the rear seat) would help.

[frankc]

This is where I need some clarification. So the synchro's are there to match the input and output shaft RPM's for the selected gear ratio. However, when at a stop, since the output shaft RPM is zero, what the synchros are really trying to do is *stop* the input shaft from spinning so that it can mesh with the output shaft, correct?

So, the question is (and maybe 911pcars can help me here), with the input shaft only spinning at idle speed when dis-engaging the clutch, it seems like it shouldn't take long for the synchros to stop the input shaft when attempting to shift into 1st. Certainly (what seems like) several seconds and several attempts should do it. So, there must be some other explanation as to why it won't go into 1st from a stop at times. Regardless of how long or how many times I try, it won't go. I've noticed that if I engage the clutch for a second in neutral and then dis-engage and try 1st again, it will go. It's almost as if where the two shafts stopped spinning, they won't mesh and I have to spin the input shaft to try meshing at a different spot.

Does that make sense?

[nixter]

Interesting thread... I wasn't aware that so many people shared so many of the exact same issues with this tranny! This latest issue with the getting into first from a stop is something I've dealt with constantly. A while ago it became near impossible to get into first from a stop. Turned out I had a cracked fork somewhere down the line. I had that replaced and my plate ground (to eliminate shudder) and things are much better. There's still the 1/2 shift issue and the "1st gear at stop" issue but they are managable and just part of the charm I guess!

N

[911pcars]

"....synchro's are there to match the input and output shaft RPM's for the selected gear ratio. However, when at a stop, since the output shaft RPM is zero, what the synchros are really trying to do is *stop* the input shaft from spinning so that it can mesh with the output shaft, correct?"

Not exactly. When at a stop the rear wheels are not rotating and thus the output shaft is not as well. However, with the clutch disengaged (pedal down), the input shaft should also stop rotating (it is connected to the clutch disc which is not clamped by the pressure plate).

What I was trying to say was that, due to not completely disengaging the clutch (clutch needs adjustment), the input shaft is still rotating. That, along with a worn synchro, could increase the difficulty in shifting into first (or any gear with a worn synchro). If the gears grind while shifting into reverse with the car at rest, this indicates the input shaft is still rotating and not yet fully disengaged.

Of course, there are other factors that could make the shift more difficult, including worn or bent parts.

Sherwood

[nabilious]

I have to say all this talk on syncros prompted me to do some experimentation since we had a nice spring weekend. I can make my 915 NOT grind when I upshift to thrid by "faking for 4th" then going to 3rd. even if I take my time or try a quick shift up to 3rd it always grinds, I won't pretend to know why that works but it makes the "zzzziippp!!" alot less frequent.
lemme know what you think.

[Zeke]

The difficulty in selecting 1st from neutral while at a dead stop can be equated with trying to start many cars with the key with pressure against the steering lock. Move the wheel a bit and the key turns effortlessly. I assure you this analogy is not anything but, because there are no similarities between the two; a transmissinon and the steering lock. But, you will notice that if the car rolls a tiny bit, the stars in the universe all line up and the lever goes into 1st with little effort.

Or, look at it this way. When the car is parked, not running, and you want to put it into gear w/o using the clutch, you roll the car a bit and the gear is there. Sometimes when you pull up to a stop, you just aren't in alignment to make 1st gear. What can you do? Well, if you're on an incline, let it roll and it will drop in. Or, put it in 2nd, if it goes easy, and let the clutch out enough to initiate movement. Immediatley clutch and hit 1st.

Or just hit 1st as you come to the stop and let the TO bearing wear. Any force on the gear lever to find 1st while stopped is just hurting all the components from the shifter bushings to the forks and to the synchros. You should never have to use any more than light pressure to operate the shifter. Find a way to avoid that each and every time.

Shoot, I even back off the throttle on my automatic Chevy truck when it's going to shift and let it do so while saving the bands in the tranny. I got 160,000 out of my original tranny, unheard of in a Chevy 700R-4.

The point is, treat everything nice...........

[air-cool-me]

i find that if it wont go into first at a stop you can just put it in seccond...


then it seems to go into first really easy after that...


even without moving...

[no substitute]

Well said Zeke.

[MAS]

OK, so ... what exactly is it about the design of the 915 gearbox that makes the 1-2 upshift slow? (when the other upshifts are relatively smooth).


-MAS

[Zeke]

Originally posted by MAS
OK, so ... what exactly is it about the design of the 915 gearbox that makes the 1-2 upshift slow? (when the other upshifts are relatively smooth).


-MAS

You just need a 901 for awhile to appreciate the 1-2 915 shift. I do like my 901 H pattern once rolling, but a downshift into 1st in the real tight twisties is a b!tch. Especially considering that if you need 1st, you are busy with a tight turn which you will be out of in the next instant. It's almost worth lugging the motor out of there.

However, the cam profile of my S motor leaves the car dead at low RPM in 2nd or any higher gear. I'm plaaning on installing a Seine Systems gate as I just scored a 915 shifter from Rufblackbird (Jeff) over in HI. That might help.

[MACHINEMON]

Help, I just got my car street legal and spent a few days on the road with it. I am having hard shifts going up from second to third. Grinding. Coming down from fourth to third a snappy double-clutching makes a perfect shift. Is it the syncro going south? Also, I read somewhere here about a mod to raise the fifth gear to overcome highway running with 15"'s on an older car. 1975 s. Thanks Joel