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Again, probably not the synchro. Helical gears are cut that way to ease shifting and to reduce gear noise. Due to their shape, physics causes them to naturally repel each other--meaning they don't like to stay engaged as well as straight-cut gears.
Your observation that the gear pops out when you give the car gas makes me think you've got a bearing issue or a linkage issue. In the former, the addition of torque to the equation can cause the meshing gears to push against each other more strongly, which moves the shaft enough to overcome the retaining force of the springs that hold the gears in engagement. In the latter, the added torque causes the drivetrain to move just enough to allow the shift linkages to move and pull the gears out of engagement. For illustration, have someone rev your engine and watch the tail pipes. They move because the engine is trying to rotate on its mounts. For your sake, I hope it's the latter.
I am not familiar with the guts of this transmission. What other gear(s) share one or more of these shafts? I would think that worn shaft bearings would result in gear pop-out for any other gear that shared the shaft that was held by the worn bearings.
All that being said, I would certainly start with the shifter cables. Rebuilding transmissions is expensive. Shifter cables are not (relatively speaking, of course--this is a Porsche we're talking about).
Our modern manual transmissions, are called synchromesh (or constant mesh) because all the helical drive gears are constantly engaged. The drive shaft(s) spin freely in the center gear holes. The way the gear is engaged, is not by moving gears, but engaging the driving gear to the shaft... not gear to gear. This shaft engagement is done through syncro rings... with teeth and slots ... also known as "dogs".
The synchros in my 2000 Boxster Ss MT (same one in the 911) had nubs on the dog teeth. These nubs allowed for the rings to "lock" into the corresponding slots. You could feel the engagement of these nubs ... if you shifted real slow, you could feel a two-stage clicking through the shift lever. This is how a knowledgeable source described what was happening to my car... something many complained about at that time.
A common reason for popping out of gear is these nubs (or whatever locking mechanism/tapering they call it) wear away. My Boxster would pop out of 2nd when ambient temps were near or below circa 20F until warm. I had to make sure I felt the second click... else only the dog teeth tips were engaged... the car would move ahead until I applied some torque and pop.... back into neutral.
Anywhoo... you probably will have to get your transmission dropped unless they can find something obvious with the cable linkage. A little test you can do is shift slower and look for the feel of the double click....... and pull a bit harder on the lever as you go. Compare this feel with the other gear engagement.
BTW, The gears are cut helical (angled) for noise reduction. Did you notice how some cars make whiney noise when in reverse? That is because the manufacture is saving money by not helicaly cutting the reverse gears.
The synchro only facilitates the engagement of the gears. It regulates the speed of the dog clutch (or shift sleeve or whatever Porsche calls the part that slides along the shaft and locks the gear to the shaft) to the spinning helical gear and allows the teeth of the dog clutch to engage with the corresponding teeth of the spinning gear.
Once the gear is engaged, however, the synchro doesn't hold the gear in engagement, the physical connection between the dog clutch and the helical gear does. At that point, the synchro ring is just along for the ride.
Play in the shaft bearings allows the gears and synchros to wobble on the shaft, which is exacerbated by the tendency for helical cut gears to force each other apart. This wobble bumps or pushes the dog clutch off of the gear and causes pop-out. The overlap of the teeth on the dog clutch with the teeth on the helical gears is surprisingly small.
At least, that's how I understand it to work. It is also my understanding that the misalignment caused by worn shaft bearings leads to the synchros getting worn, which exacerbates, but does not actually cause the problem. A bad synchro results in missed gears and bad noises, but if you can get into gear just fine but have pop out after you're in gear, the synchro isn't the causal issue.
This video is surprisingly informative and not meant to be patronizing at all--I am a visual learner so I found it to be really illustrative:
Of course, German engineers love unnecessary complexity and I reserve the right to back off of any of the statements made above if it turns out that the guys at Weissach designed these transmissions to work in a completely different way than every other synchromesh transmission.
Well got the car back from the shop and no easy fix i´m afraid. They checked all the external linkage and tried to adjust but it still pops out from 6th gear. Got a estimated quote of around 2500 USD based on what they believe is a bad sync. Sigh....
I'm sorry to hear that. I really hoped that it would turn out to be a bad linkage or really worn motor/transmission mounts allowing movement in the driveline.
FWIW, $2,500 isn't particularly expensive for a major repair on these cars.
I'm sorry to hear that. I really hoped that it would turn out to be a bad linkage or really worn motor/transmission mounts allowing movement in the driveline.
FWIW, $2,500 isn't particularly expensive for a major repair on these cars.
Just trying to put it into perspective for you.
Thanks! Yea i know its not that expensive - i´m happy that it was the previous owner (with one month left on his engine insurance) that got bore scoring - THAT would have hurt bad.
Well got the car back from the shop and no easy fix i´m afraid. They checked all the external linkage and tried to adjust but it still pops out from 6th gear. Got a estimated quote of around 2500 USD based on what they believe is a bad sync. Sigh....
Silly question: did you add any "friction modifiers" or something that made the gears extra slippery? There are additives that are sold to extend gear life..... bogus as you actually need a bit of friction so the dogs grab the cones (a type of clutch) in the transmission.
Silly question: did you add any "friction modifiers" or something that made the gears extra slippery? There are additives that are sold to extend gear life..... bogus as you actually need a bit of friction so the dogs grab the cones (a type of clutch) in the transmission.
If so, then maybe just a flush may clear it.
Peace
Bruce in Philly
No, i have not put anything in for that purpose. Of course there is a possibility that the previous owner did that but i doubt it. I actually talked to a guy that specialises in gearbox re-builds and he did not think that its a bad sync but rather the "gearstick "fork"?? (did not quite get the exact name) that has gotten a wear due to previous miss-use, ie having your hand resting on the gearbox stick when in 6th gear and thus putting strain on the "fork". Im going to see him today and if all good plan for a repair in the near future.
Ok so just a quick update for those who helped me out with tips and ideas or if anyone has the same problem. I spoke to the mechanic earlier today and after opening the gearbox up he concluded the following needs replaced.
6th gear sprocket kit (really not sure if this is the correct word in english)
5-6th gear shift fork.
Cost for parts and repair here in Sweden at a very reputable Porsche mechanic - just under 2500 USD.
Ok so just a quick update for those who helped me out with tips and ideas or if anyone has the same problem. I spoke to the mechanic earlier today and after opening the gearbox up he concluded the following needs replaced.
6th gear sprocket kit (really not sure if this is the correct word in english)
5-6th gear shift fork.
Cost for parts and repair here in Sweden at a very reputable Porsche mechanic - just under 2500 USD.
Glad you have resolution....... How about asking for the worn parts? I do as they make great desk sculptures. Also we would love to see the gory guts and what actually wore out.
Thanx for posting.
Peace
Bruce in Philly
Last edited by Bruce In Philly; 08-30-2017 at 03:06 PM.
Glad you have resolution....... How about asking for the worn parts? I do as they make great desk scuptures. Also we would love to see the gory guts and what actually wore out.
I have always been told that gear pop-out is caused by a bad synchro, but in my admittedly limited knowledge of how a manual transmission works, I can't figure that one out. The synchro doesn't hold the helical gears together. In most manuals, the gears are held together by the shafts they ride on and sometimes by fairly weak springs once you're in gear. Once a gear is selected, the synchro is out of play. It's usually play in the bearings that hold the shafts that allows movement of the shaft(s) and that allows the gears to pop out.
I can only assume that the synchro idea comes from the fact that once the shaft bearings are worn and allowing movement, the synchros are probably pretty worn out, too.
I'm just thinking out loud here. Again, I'm not a transmission expert. I've rebuilt a couple of Toploaders but replacing the guts of a '60's transmission with all new parts is very different from being able to diagnose issues with a modern transmission. I would be interested in knowing how worn synchros could cause gear pop out because I've heard that for years but can't get my head around how it actually happens.
If the inside cone of the synchro is so worn out that even when it is pressed on the cone part of the gear and the synchro gap is small, it could freely spin on the gear. This would be coupled with grinding but since the syncrho can't friction grab the gear, it would aid in the problem of the gear popping out.
At first i thought it was me being sloppy with putting the gear in correctly but this is not the case, it has happened even if i really make sure its in the 6th gear.
It happens at cruising speed on highway doing around 80 mph, sometimes just after shifting in to 6th but sometimes it would stay in gear for a longer period 20 minutes or so. Accel or slowing down doesnt seem to have an impact.
I get the feeling that its more prone to pop out when the car is warmed up and only happens in 6th gear. I once noticed how the gearstick kind of "wobbled/shaked" before poping out.
Thanks for taking time
FWIW I am having the EXACT same problem. Only happens on the highway in 6th and only happens once the car is WARMED up. I am documenting my fix on my YouTube channel. It's been a process....
Ok so just a quick update for those who helped me out with tips and ideas or if anyone has the same problem. I spoke to the mechanic earlier today and after opening the gearbox up he concluded the following needs replaced.
6th gear sprocket kit (really not sure if this is the correct word in english)
5-6th gear shift fork.
Cost for parts and repair here in Sweden at a very reputable Porsche mechanic - just under 2500 USD.
For others reading this, he likely had his 6th gear set replaced. Porsche doesn't just sell the free spinning 6th gear but as a set so that old wear on one gear doesn't result in new wear on the new gear. The two gears are the 6th gear that freely spins on the shaft going to the engine and the sister 6th gear that lies fixed on the shaft that goes to the differential.
08-01-2017, 04:20 PM
