Hole in Trans! Are 5-spd and 6-spd cases the same?

[911monty]

Have you scanned the DME for stage 2 over revs? Might show something. I saw something similar where an individual thought it was a good idea to leave the trans in first gear while coasting down a hill with the clutch depressed. Can't even imagine the RPM that clutch disk saw before exploding.

[maytag]

Originally Posted by 911monty

Have you scanned the DME for stage 2 over revs? Might show something. I saw something similar where an individual thought it was a good idea to leave the trans in first gear while coasting down a hill with the clutch depressed. Can't even imagine the RPM that clutch disk saw before exploding.

Thanks Monty. There are plenty of over-revs stored in the DME. And heck, I'm guilty of some of them myself, at the track. But a simple over-rev of the motor (what the DME is recording) shouldn't cause this sort of failure. What you are describing, (coasting down the hill with the clutch depressed )wouldn't have registered at all in the DME.
But you got me thinking:
I did this at the end of the front straight at UMC (Outer Loop). I only got two laps before this happened, but in those two laps, I was hitting 121mph on the straight, then braking to around 85mph as I start to turn-in at turn one, but continuing to brake down to about 60mph at the apex. When I went for the brakes and depressed the clutch simultaneously, then tried to go for 3rd gear, it wouldn't go in. I tried again; wouldn't go. So I took my foot off the clutch to coast through the corner, but the pedal didn't come up.
SO: I can only guess at what speed I tried to grab 3rd gear, because I was braking aggressively, and I can't see my hands in the HLT video; but I would guess around 90mph. That should not be an issue for 3rd gear (that's about the MPH where I upshifted to 4th. And on a different track configuration, I hold 3rd to 98mph before braking). Yes, I rev-match as a force of habit (I'm a motorcycle-guy, it's truly a habit, haha).
So let's say I was REALLY fast on that particular lap, and hit maybe 130mph (I doubt it). And let's say I was trying to enter turn one much faster than before (possible, as I had some clear track ahead of me, having passed the corvette I had been waiting on previously). Let's say I hypothetically tried to grab 3rd gear REALLY quickly, at 100mph. I really don't know if this is plausible... I don't have enough data. I find it unlikely, but possible. If the synchros engaged and rapidly spun-up the clutch to the RPM's it would need to engage 3rd gear at 100mph, then I would think this sort of failure is, well, possible. I guess. But I would hope the clutch could take that sort of spin-up, should'nt it?

I just found someone on Rennlist who posted these top-speeds for the 6-speed, with stock tire size:
1st=~42 MPH
2nd=~70 MPH
3rd=~102 MPH
4th=~127 MPH
5th=~154 MPH
6th=~190 MPH

seems about right. I just can't fathom me trying to grab 3rd at 100mph should create this sort of catastrophic failure.

I'm ready to learn if any of y'all have a different outlook on it?

[911monty]

Well Maytag you asked for it! I'll take a stab here (pun intended..) .

Let's see if we can get some responses. Wish I could separate your pics for annotation would make life easier., But looking at the first pic of the carnage this is what I see.

First your pressure plate did a decent job of containment could have been worse.
Now look at the 10 o'clock position. You can see the friction disk that remained together attempted to extrude between the gap of the pressure plate. This was repeated @ 3:00 and 6:00 but successfully ejected

At the 1:00 position you can see the friction material crushed/depressed as it impacted the pressure plate bolt flange. Also repeated in several areas.

Notice how all but one (at 2:30 still has a small piece attached) of the friction disk to the hub attaching points separated. Now look at the 5:00 position and you will see the only attachment point that still has 2 rivets. Notice the puckering in the center? This is from a tension break, It appears they all broke simultaneously at the same instant.

In summary this appears to be a high speed event to me.

[maytag]

Originally Posted by 911monty

In summary this appears to be a high speed event to me.

You are very observant, Monty. Thank you for taking time for me.
I don't think that there's any question it was a high speed event.

The question (to me) is what sort of high speed event should a clutch be able to tolerate? Is this a weak clutch, destined to fail? or did I do something that will result in this same result if I do it again? And if the latter, then what, exactly?
As near as I can tell, it occurred either when I depressed the clutch pedal, or when I poked at 3rd gear, going no faster than 100mph (and probably less than that). Even if I had a brain fart and didn't rev-match (I doubt it, as it's so habitual for me, I even do it on the street) shouldn't the clutch tolerate this spin-up?

Sent from my SM-G930T using Tapatalk

[911monty]

The number one thing that causes a clutch disc to explode like that is grabbing too low of a gear at too high of a speed. When you downshift, as you move the synchronizer, the blocker ring has to speed up the mainshaft gear, the counter gear, the input shaft, and the clutch disc. As an example, Let's say your car would turn 6000 rpm to go 80 mph in third gear. If you downshift into third at 105 mph, just the action of moving the synchronizer spins the clutch disc to 9000 rpm regardless of what you are doing with the clutch or throttle. Consider that with the fact that there is typically no clamping force on the disc at that point........poof. birdsnest in the bellhousing. Skipping gears is also the main cause of the money shift, where the DME sees stage 2 overrevs when the clutch is let out.

PS: The above is only intended as an example to hopefully draw responses so that mine are not the only opinions. Your actual speeds and RPM will probably differ.

[maytag]

Originally Posted by 911monty

....Skipping gears is also the main cause of the money shift, where the DME sees stage 2 overrevs when the clutch is let out.

PS: The above is only intended as an example to hopefully draw responses so that mine are not the only opinions. Your actual speeds and RPM will probably differ.

We're not getting a lot of attention from others on this thread, are we? haha. It's possible that my sense of humor has alienated everyone else. :-) Regardless: thanks for your time, Monty.

So, I DID, in fact, skip a gear. I dropped out of 5th and was aiming for 3rd, but couldn't get to it. So it seems this is where I failed, and caused this failure. I thought that because I was at the bottom of 5th, and shedding speed so fast in the braking zone that I could get to the top of 3rd, rather than bump into 4th in between. This seemed to make sense to me, since middle of 3rd is where I wanted to be as I drove out of T1.

So teach me: why does it matter?
Thinking out loud here: If the motor is turning at 7000RPM and the trans' input shaft is turning at 7000RPM, then we shouldn't care if it's 3rd or 4th, right? EXCEPT, of course, if our speed is wrong for that RPM in that gear.
So what we're REALLY saying, is that I was going too fast to try to get to 3rd. Which I think means I was going over 105-ish. Which suggests I was either downshifting too soon in my braking zone, or I was traveling much faster this lap than I thought I was. (This was the first time I'd reached 5th gear on the straight, but I sort of short-shifted, thinking I could get into the meat of the power. maybe it worked?) If this is the case, then no amount of "Rev-matching" was going to get me there: I was simply traveling too fast for 3rd gear, so when the synchros engaged, with the clutch disengaged (no clamping pressure) the centripetal forces simply overwhelmed the stock-type clutch disc.

Did I make that leap correctly? in which case, I need to brake more before I try to stab at 3rd gear, so I don;t do this again. OR: do I need to bump from 5th, to 4th, to 3rd every time, regardless of speed?

[911monty]

IMHO you are spot on. In 44 years of driving mostly 911s the one thing that is constant is the shifter has always been a vague mechanism. I highly recommend going into each gear while downshifting.

PS I don't know if Sachs or any one else does testing on their clutches to destruction. However you must consider that manufacturers are doing everything they can to reduce clutch disk weight to reduce mass and inertia in order to speed up shift speed and reduce wear on the synchros.