dan_189

Hey all,

Let my partner drive my car over the weekend and whilst she's usually a very very competent manual driver this drive she wasn't.

She neglected to put the clutch all the way in before trying for second on an up shift. Oh the sound, the horror for that split second!

​​​​​Whilst she realised the mistake and stopped trying it got me thinking - how much abuse can this gearbox endure? Do the engineers build some bad habit tolerances into the box?

In my head there are several synchro teeth now missing but I know thats probably not the case.

Car drives fine, just felt like typing out my feelings.

Your synchro grinding stories are also welcome!

HenryPcar

Don't worry ! If the BMW's manual transmission can take it I'm sure Porsche's are no different. Years ago, I taught my daughter to drive a stick on a BMW E30.
It was frustrating and I lost count on the number of times the gearbox went through gear clashes. However, it took her just a little over an hour to get the hang of it and boy was she glad. I could still remember that big grin on her face. I thought it was worth it even if she smoked the transmission. However, the gearbox was fine. The synchros were still as smooth as ever and that car lasted me over 1/2 million miles on the original engine and transmission. Yes, the clutch was replaced once.

dan_189

Ahh nice! Thanks, I knew it was a non issue but the sound of mashing synchos is one I fear!

TBH there aren't too many transmission woe stories, its just the super paranoid 996 driver in me! and something to talk about group therapy perhaps? Haha

TonyTwoBags

I've read about 6spd rebuilds before, but I'm not really convinced it's much of a risk beyond repeated misuse. My condolences on the experience

dan_189

Have read of a few "whine" stories - no pun intended! Usually turns out to be a bearing etc. Was a thread on here where a member did a DIY to replace a worn main bearing, was fascinating.

I've had the fluid changed in it under two years ago and i've treated it well so hopefully that pays off.

Appreciate the support! I'm still shuddering over the sound haha :s

cds72911

It's probably fine. I think you'd know if you drove it after and didn't experience problems.

Also, I don't think it is the sound of syncros mashing, I believe it is the sound of gear teeth mashing. As I recall, the syncros are basically trying to get the speed of the shafts closer to matching so the gears can mesh, when the friction surface wears off the brass synchro, they don't do their job well anymore, so you are trying to mesh the fast spinning gears on one shaft shaft with gears on the other shaft that aren't spinning fast enough yet.

From my what I read, the 2nd gear syncros are a known weak spot on the manual transmission on our 996es. The brass they are made of is intolerant of grinding. The parts prices aren't that exorbitant (for a Porsche part), but the labor is not cheap, and requires a large hydraulic press (100 Ton?) and benefits from special tools and jigs. No personal experience though, just what I gather from reading up on it. Oh, I also read that they replace them with steel synchros in factory motorsport cars (but they're very pricy).

dan_189

Thanks, I read it was synchros that made the noise but im no transmission expert so you might be right.

Gears felt fine afterwards just that 996 paranoia going on... Assuming the gears are made of more solid material than the synchros and can take the very occasional misshap, just as long as it's not a regular thing?

dkraige

It's not gear teeth mashing. The gears are in mesh at all times. When you shift gears you are just selecting which gear ratio is connected to a shaft in the transmission. All the other non-selected gears are still meshed, but are free-spinning on the shafts and thus transmit no power. You aren't bringing gears in and out of mesh. So the sound you're hearing on a crappy gear change is the sound of the teeth on the synchronizer rings crashing while bringing one of the gear sets in lock with a shaft. Really the only way you'll damage the gears themselves is if you shred a synchronizer mechanism enough that there's metal debris suspended in the fluid that then hurts the gears themselves. Otherwise, the damage you'll incur is to just gradually turn your synchronized gearbox into a non-synchronized one, which still functions to move the car, but is less convenient to use

Porschetech3

This is the 2nd gear of the G96 transmission. It has large teeth in contact with the driven gear at all times, the small teeth are the teeth that engage to the syncro slider after the brass ring has sped up the gear to match the speed of the shaft. The small teeth can hold the power of the engine because all 42 teeth are in contact when engaged and handles the power where as with the large teeth has only one tooth at a time in contact to transfer the power.

The small syncro teeth are "v" shaped to help the slider engage, when the grinding occures, you are grinding off the tips of the "v" which will make it harder to engage . If too much abuse to the teeth will become worn back, rounded, or angled and very hard to shift into gear or will pop out of gear.

Second gear seems to get the most abuse due to being the first shift after getting moving and the one most people "grab" when trying to accelerate aggressively.

The good news is that the 2nd gear, syncro, slider ect, can be replaced without a full disassembly. No 50ton press needed. Just remove the trans, remove the rear case.Then the snap ring from the rear bearing, and with a 8 inch 3 jaw 7ton bearing puller, remove the rear bearing,reverse gear/syncro, 1st/2nd gears and syncro all at the same time.
Edit: I forgot to mention you also have to remove 2 of the 5 shift rails/forks/detents, the ones for reverse and 1st/2nd.

I just replace the 2nd gear syncro's in my 99 Cab, and yes with only 70k miles on it , it has been abused many times, and I let my sons drive it on track days, but it was abused before I got it..lol

cds72911

Thank you for the clarification. Sorry for the misinformation.

cds72911

Looks like I was even more wrong. Thanks Porschetech! I guess I should only reply when I have actual knowledge, not just repeat what I've heard, read or been told. Thanks for the info!

dan_189

Thanks everyone, very informative as always.

I'll sleep a little better now!

dan_189

Actually another question, why are there spaces of no teeth in the synchro gears?

Also any tips/rules of thumb when rev matching to up shift? I do it mostly by feel but I'd like to know a bit more.

Also when im coming to a stop im usually not downshifting/using compression, I just coast until stop in the gear I was in then clutch to neutral, thoughts?

So while we're on clutches too im not pressing in the clutch pedal on start up as I read JR advises there is too lunch stress on the trans/engine. RoW car comes out of the box with a no clutch pedal in needed to start.
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dkraige

Not sure why some teeth are missing. Somebody will chime in who's smarter!

Rev matching on UPshift? Do you mean downshift? On the way up things more or less take care of themselves as the revs fall to the right speed for the next higher gear as you let off the gas during the gear change. On a downshift you can read up or find videos on heel-toe or rev-matched downshifts. It's kind of a matter of just practicing a ton to get good at it. I will say it's a lot easier on this car than on the more pedestrian cars I learned to drive on, and it's easier the faster you're driving. Depending on your foot size, shoe style, and how you orient your feet on the pedals to reach the gas and brake at the same time during a downshift, you might find it helpful to extend the gas pedal to the left a little, either with some aftermarket pedals or some custom home-brew parts. BE CAREFUL messing with this though, you don't want to get in a situation where you're hitting the gas accidentally when you're trying to just get the brake.

On the street I'm with you, there's no reason to downshift or use engine braking when rolling up to a stop light. Your car comes equipped with a entire system expressly designed to slow the car down (BRAKES!!!!) so there's no need to re-purpose the engine as a car-slowing device

Porschetech3

1) The size and number of syncro teeth is dependent on designers intended shift performance/use. Normal street syncros have a full teeth circumference, performance syncro's have less teeth, race transmissions have as few as 10 teeth, some clutchless transmissions have 2 or 4 teeth or commonly called "dogs". they are much larger and have enormous backlash clearance.Less teeth and more clearance means quicker shift, but not ideal for street due to a grind when shifting slow and slack on accel/decel due to high clearance.

2) The ideal upshift should be timed to complete the shift as the RPM's drop the nessesary amount for the engine speed and next gear shaft speed to match, saving work on the syncro.1k -2k depending on gear spread.

3) Using downshifts and engine braking when driving normally only saves on brake wear, no other benefits.Performance driving is another story.

4) That is a good habit. I have my clutch switch dissabled so I can start without the clutch depressed, I like to keep as much stress off the crankshaft thrust bearing as possible. I am seeing way to many of the thrust bearing failures.