915 diff failure!

[safe]

Has anyone seen something like this?



Happened during an auto-x start...

How would I prevent this in the future?

[GH85Carrera]

Less horsepower ?

[safe]

Originally Posted by GH85Carrera

Less horsepower ?

That is out of the question!

[Lapkritis]

Only way to minimize if you're going to reassemble with equivalent parts is to ensure when you start that the slack is taken out of the drive line and that you're using a sprung clutch disk.

Heel on the brake, toe on the gas and bring the clutch up where it just starts to push. Expect shorter clutch life but more life from the rest in play.

[JeremyD]

stock side plate?

[safe]

Originally Posted by JeremyD

stock side plate?

Yes, stock but rebuilt, 1977 mag case.

Would a stronger side plate help?

Engine has ~260 hp and ~300 nm.

[safe]

Originally Posted by Lapkritis

Only way to minimize if you're going to reassemble with equivalent parts is to ensure when you start that the slack is taken out of the drive line and that you're using a sprung clutch disk.

Heel on the brake, toe on the gas and bring the clutch up where it just starts to push. Expect shorter clutch life but more life from the rest in play.

No more 4000 rpm and dumping the clutch?

The clutch is new for this year, springed but a much more grippy 6-puck disc. The stock clutch was slipping under load.

[JeremyD]

Yes,

When I was researching my 3.6 upgrade - people smarter than me recommended the following in order of importance:

1. Side billet plate - the stock plate allows too much movement in the bearing carriers - with allow the diff to twist - that's bad. Also while you switch out the side plate - also switch to the 930 version of the bearing carriers - they are more meaty.

2. LSD - you want a Limited slip - if you have one wheel driving (with sticky tires) you get more of that twist that I described above - that's bad.

3. The one piece main bearing plate - or snowman as some refer to it. Usually needs to be installed at a tear down.

4. Cooling - Keep your differential cool - even a small cooler (like the factory euro 915 cooler) can make a huge difference in operating temps - this is mostly for the track though = doubt it would make that much difference on an autocross course.

[safe]

Originally Posted by JeremyD

Yes,

When I was researching my 3.6 upgrade - people smarter than me recommended the following in order of importance:

1. Side billet plate - the stock plate allows too much movement in the bearing carriers - with allow the diff to twist - that's bad. Also while you switch out the side plate - also switch to the 930 version of the bearing carriers - they are more meaty.

2. LSD - you want a Limited slip - if you have one wheel driving (with sticky tires) you get more of that twist that I described above - that's bad.

3. The one piece main bearing plate - or snowman as some refer to it. Usually needs to be installed at a tear down.

4. Cooling - Keep your differential cool - even a small cooler (like the factory euro 915 cooler) can make a huge difference in operating temps - this is mostly for the track though = doubt it would make that much difference on an autocross course.

All things I have planed for the 915 in my 69 coupe.

I do track this 77 targa a lot. Oil cooling would not be a bad thing, 2-3 hours of almost continuous track driving probably cook the box pretty good...
The car is so soft (conforming to the track) that I seldom has any wheel spin, so I skip lsd.

I'll try to get the box to my gearbox guy this week. He reinforces the side plate with a steel plate. That's how it was done in the 70s.

[scarceller]

Fit a G50 in?

Or use the older 930 4 speed?

[arrivederci]

Originally Posted by JeremyD

Yes,

When I was researching my 3.6 upgrade - people smarter than me recommended the following in order of importance:

1. Side billet plate - the stock plate allows too much movement in the bearing carriers - with allow the diff to twist - that's bad. Also while you switch out the side plate - also switch to the 930 version of the bearing carriers - they are more meaty.

2. LSD - you want a Limited slip - if you have one wheel driving (with sticky tires) you get more of that twist that I described above - that's bad.

3. The one piece main bearing plate - or snowman as some refer to it. Usually needs to be installed at a tear down.

4. Cooling - Keep your differential cool - even a small cooler (like the factory euro 915 cooler) can make a huge difference in operating temps - this is mostly for the track though = doubt it would make that much difference on an autocross course.

All great advice and exactly what I've heard. Also, I believe the factory cooler in #4 won't work with the upgraded side plates in #1. So you'd have to go an aftermarket route for cooling.

[sjf911]

Wow, that is something else. Were you having a lot of wheel hop issues? I know I had a lot before putting in my LSD and that really cured it.

[ganun]

The diff carrier broke along the spider gear pins, I don't think the side plate plays a role.

[safe]

Originally Posted by sjf911

Were you having a lot of wheel hop issues? I know I had a lot before putting in my LSD and that really cured it.

Some, when the traction was bad.

[Matt Monson]

Seen it a few times...
It's a 37 year old part being used in motosports. In my experience auto-x is about second in abusiveness only to rally. Lots of shock loading in auto-x.

Hopefully you didn't take out your ring and pinion as well. However, no matter what, I would do a full tear down and make sure your case bearings are still all right.

Sidecovers are a nice upgrade but really won't do anything to prevent this. Nor will a cooler. In fact, many times I find that auto-x guys don't spend proper time warming up their gearbox. You know how they say that the most wear you put on your engine is when you start it? Most wear you put on the gearbox is the first 10-15 minutes until it's temperature goes up. I advise all auto-x racers to spend some time in the morning just tooling the car around the paddock getting the gear oil up to temp.

Get a new differential and you shouldn't have much to worry about. An LSD is nice because if you lift a wheel it keeps driving power to the ground. Downside for auto-x is it locks under braking. Gives a lot of corner entry push right when you are trying to rotate the car early.

TBDs are great for power on traction. They don't lock at all under braking. Downside is they go open if you lift a wheel. Also tend to be a fair bit less expensive than a salisbury style LSD.

PS. I do NOT use LSD and TBD interchangeably. Quiafe does. They say their TBD is an LSD. Not how I define these things from a technical standpoint. a Torsen design, or torque biasing differential is not an LSD in my book. Has no limit under zero traction. It goes open. That's not limited slip.

[safe]

Originally Posted by Matt Monson

Seen it a few times...
It's a 37 year old part being used in motosports. In my experience auto-x is about second in abusiveness only to rally. Lots of shock loading in auto-x.

Hopefully you didn't take out your ring and pinion as well. However, no matter what, I would do a full tear down and make sure your case bearings are still all right.

Sidecovers are a nice upgrade but really won't do anything to prevent this. Nor will a cooler. In fact, many times I find that auto-x guys don't spend proper time warming up their gearbox. You know how they say that the most wear you put on your engine is when you start it? Most wear you put on the gearbox is the first 10-15 minutes until it's temperature goes up. I advise all auto-x racers to spend some time in the morning just tooling the car around the paddock getting the gear oil up to temp.

Get a new differential and you shouldn't have much to worry about. An LSD is nice because if you lift a wheel it keeps driving power to the ground. Downside for auto-x is it locks under braking. Gives a lot of corner entry push right when you are trying to rotate the car early.

TBDs are great for power on traction. They don't lock at all under braking. Downside is they go open if you lift a wheel. Also tend to be a fair bit less expensive than a salisbury style LSD.

PS. I do NOT use LSD and TBD interchangeably. Quiafe does. They say their TBD is an LSD. Not how I define these things from a technical standpoint. a Torsen design, or torque biasing differential is not an LSD in my book. Has no limit under zero traction. It goes open. That's not limited slip.

I agree that it can be abusive and that I'm probably guilty of little warm up....

I'm leaving the gearbox to a guy who has done rally 911s since the 60s, he'll know what to do.

I'm not sure I want an LSD for autox because of the push, maybe a TBD as you say, unfortunately the budget don't allow for that now.

[safe]

OK, the box is back from the builder and with reinforced bearing retainer and side plate.
But, it has a noise... Sort of an grinding noise, like: RRRR...........RRRR...........RRRR...........
The RRRR part a little like when you try to shift without depressing the clutch, but a lot less.

I hear it during coasting in neutral or with the clutch depressed (in any gear). I also hear it when I jack up the rear and turn both wheels at the same time, hopeless to say from what part of the transmission it originates.
Can it be a damage to the pinion or ring gear, how would that sound? The builder said he couldn't find any obvious damage.

[Geary]

I once went into a transaxle immediately after a nasty wheel hop episode, and the only damage I found was bent spider gear pins. I believe wheel hop is responsible for your broken diff body.

[Lapkritis]

Exactly what work was done? Loose lash, bad diff carrier bearings/races. Couple things come to mind as possibilities.

[safe]

Originally Posted by Lapkritis

Exactly what work was done? Loose lash, bad diff carrier bearings/races. Couple things come to mind as possibilities.

Basically the entire box was checked over, lash set and carried bearings was replaced.
Just talked to the builder. The pinion had minor (pinhead sized) damage that was polished away. He said to run it and see what happens, if it gets worse it does...