strange 915 LSD number

[jjemg]

Hoping for a little help....I bought a differential for a 915 and I'm not exactly sure what I have. What I was supposed to get was LSD for one of the later 915 trans that came out of an original car. After looking at the number on it and checking it on PET, it is an unknown. The number is 4061806176 516152 S4C 930. Can anybody make anything out of this? Appreciate any help. Jimmy

[svandamme]

have a totally related question :

where can i find the serial of a 915 tranny? ( sprayed all over mine with brake cleaner, still couldn't see a serial number, only part numbers)
how can i determine if i have an LSD or not?

[Grady Clay]

Jjemg,

I think that is a ZF part number. Very few Porsche parts have the actual part number stamped on them.

Since you already have the LSD, some quick investigation is in order. Comparing to your stock 915 differential is easiest. You will need a LARGE micrometer to compare the distance between where the large side tapered roller bearings seat on the housing. That dimension doesn’t have to be exactly (to 0.01 mm) the same because it is shim adjusted. Next you want to measure the diameter where those bearings fit (tight) on the differential housing. Finally you want to confirm the ring gear sits in the same lateral position. You can do this by just cinching it up to the LSD and confirming there is some backlash.

You should also disassemble the LSD and inspect the clutch parts condition and configuration (50%, 100%). While you are in there carefully inspect the spider gears and the torque cam surfaces.

You will want to do the complete ring & pinion set-up drill. I would recommend at least new differential side bearings and a new pinion ball bearing & big roller bearing, others depending on condition, age, miles, use, etc.

What year is your 915? How do you plan to use it?

Best,
Grady

[Steve@Rennsport]

Stijn:

There are no reliable numbers on a gearbox that tells you if it contains an LSD or not. The Type numbers stamped on the bottom center rib tells you something about what year it is and its gearing. It looks like 915/xx and those last 2 numbers tell the tale,...

The best way to tell if you have an LSD is to raise both rear wheels off the ground and place the transmission in neutral. Then, rotate one rear wheel while watching the opposite side. If they rotate in opposite directions, you have an open differential. If they rotate in the same direction, you have an LSD.

[svandamme]

Thanks Steve, looks like i don't have one then
can you tell me where exactly i can find the gearbox serial number?
i've got my chassis Vin, and my engine doesn't match

some other things that my chassis left the factory with aren't there either, so i'm curious to verify my tranny number with the Porsche printout for my vin , but i can't locate the tranny serial anywhere...

it's not a big deal if it doesn't match , but kinda interesting to know

things that didn't match till now :
no front oil cooler
left factory as fushia brown ( only blue under the black paint )
brown interior ( not a trace)
fog lights (missing)
factory sports seats ( i've got recaro's)
engine serial is a 73 targa but not the one that's on the factory printout
car is a euro version, but have miles on the speedo and it was imported from the US at some point
have 16 inch Fuchs, don't think those are stock either ( my spare is a cooky cutter)

tranny can be interesting , to see if there is anything at all stock...

[Grady Clay]

jjemg,

Give us a description of your 911 configuration. How do you intend to use the car? Garage queen full of cool parts? Daily with some DE and auto-X? Track only car?

The LSD clutches are relatively high ware parts for street use if you want to keep the LSD function at the top of its form. These internal LSD parts are not inexpensive by a long shot.

Let me give my opinion and experience with the three types of Porsche differentials; stock, LSD, and locked.

On track the stock differential works great up to the point of power induces tire spin. At that point it limits acceleration out of a corner. The stock diff is lighter than a LSD and doesn’t produce more heat in the transmission. On the street the stock diff is best for tire ware by far. Unless you want to do regular burn-outs at the limit of traction, keep a stock differential.

A LSD is OK on the street, particularly when configured at 50% or possibly 75%. This gives you the lock-up ability under power at tire traction limit yet not too much of the tire dragging in tight turns. Too tight a LSD is very unpleasant in parking lots and tight turns. I would estimate tire ware could increase as much as 30% on the street.

On the track a LSD has some interesting characteristics when at the limit. A LSD set at 50% and with even slightly worn clutch parts will unlock during the transition from trailing throttle deceleration turning into a turn to applying the power just prior to the apex. As power is applied it will progressively re lock. This noticeably changes the handling of a 911 toward increased oversteer just when you don’t want it. A skilled driver can handle it just fine. I prefer one set at 100% and with new parts – just shy of a locked diff. All things considered, a LSD is a good choice for a street & track 911. Like everything else, it is a compromise.

A locked differential isn’t a differential at all, it’s like a go-kart with a solid rear axle. It is great on the track, particularly with a high horsepower 911 (and 935) that you tend to drive with the throttle. It is very predictable and comfortable to race at high speeds and limit of traction. At low speeds and not at the limit of traction it tends to make the car push badly. With hot sticky tires they are very difficult to drive in the paddock other that strait ahead. Immediately we get under and disconnect one inboard CV so the car can be pushed around. Absolutely unacceptable for the street.


There is a good photo of a ZF LSD in the BA book 2nd Ed, p 229.


Steve,

With worn clutches and no torque applied thru the LSD differential, it acts just like a standard differential. The advantage of a LSD is it acts like a locked drive at high torque, like a standard differential an no-to-low torque, and progressively engages/disengages depending on the transmitted torque. The little 4-corner cams on the spider gear shafts are the key. Clever invention.

The only “for-sure” method is to remove the side cover and look.

Best,
Grady

[wevoid]

It is possible to check whether a ZF LSD is installed by looking through the drain hole in the bottom of the final drive housing.

I see this question asked regularly and I have been meaning to do this for a while, so I took these two photos to help illustrate how to do this.

In this first photo - looking through the drain hole, the top of the image is the LHS (as installed) side of the transmission where the final drive cover is bolted.

Your angle of view needs to be skewed, looking towards the side cover to see the parts correctly.

There are a series of three grooves, the narrow groove (top)closest to the final drive cover, is the joint between the main body of the ZF LSD housing and the end cover. These two parts are clamped together by the 12 crown wheel bolts.

The middle of the three grooves is simply that, a groove in the edge of the flange. This is the main body of the ZF differential.

The joint between the crown wheel and the differential housing is obscured in this photo, the third - bottom - groove, is a small groove in the bevelled edge of the crown wheel gear.



In this 2nd photo, view of the same parts, without the visual obstruction of the final drive housing. This time viewed slightly more from the angle of the final drive cover for clarity.

The same features are all visible, plus the joint between the differential housing and the crown wheel gear can be seen.



In this shot you can also see that the two pieces of the differential housing are clamped together by the crown wheel bolts.

If you perform this check you can determine if a ZF LSD is installed. As Grady says above, if a ZF LSD is totally worn, it will behave like an open differential. But you do have the bulk of the parts and can restore that unit to whatever level you choose. This inspection can settle a claim that a ZF unit is installed, but needs rebuilding.

I have worked will both the GT and Quaiffe aftermarket differentials, but do not recall if they have any ID marks on the edge of the flanges as photographed above. Perhaps someone else can contribute to that? Paul G ?

The GT, ZF style aftermarket unit does have the joint between the housing pieces, forming the groove (top) closest to the final drive cover.

The Torsen style differentials, regardless of source, do not have this groove in the flange, that area is solid, one piece - same as the O.E. open diff.

So without removing the flanges, side cover etc, you can do some investigations that will give you enough information to support or refute a claim that a 915 has a ZF or ZF-style differential installed.

It means draining the oil, but with used ZF units running $700 - $1000, it's a useful tool when performing a pre-purchase inspection for a car or transmission.

Regards

Hayden

Thanks to Bill Verburg, I have made these images a little more sano.

[Grady Clay]

Hayden,

You are exactly right. My lame old brain was thinking 901/911/914 and not 915.

Thanks for the good pictures. A micro-light on the end f a wand would aid in that inspection.

In my experience the clutches do not have to be worn out, just normal worn, to act like a normal differential with no torque applied. That being said, I have taken selected parts to assemble the clutches with significant pre-load where it has major friction all the time. Outside of Porsche’s assembly instructions. Unpleasant on the street.

Best,
Grady.

[beepbeep]

Can someone explain to me why factory clutch-type LSD's have so low torque numbers when tested with torque-wrench? We got almost brand new 930 -88 gearbox with factory LSD. It has exactly 1200 miles (distance from Sweden to RUF where car got 5-speed instead) so it shouldn't be too worn.

Torque wrench says 47Nm...factory manual says it's OK...47Nm doesn't sound like very much...

[wevoid]

Grady,

I see as low as 5 ft/lb break away torque before I get too worried. This means being able to turn one wheel by hand (other on the ground, car in neutral) and feels much like a dragging brake pad and not much more.

I have progressively been building diff's with less and less pre-load and less and less stack clearance. It takes a funky combination of parts (O.E. and custom), as I am sure you would appreciate, but the results have been pretty good. Drivers enjoy the way the car will roll through the apex, yet the diff action is very progressive as it is restored very early with throttle application.

Even an agressive build is no higher than 40 ft/lb break away - with a full compliment of friction faces.

One advantage of this, is how long the original performance is maintained after inital build.

Next step for me is to build some with no belvilles at all, just some waffled static plates and very low stack clearance, just like the Hewland versions....

Regards

Hayden

[Grady Clay]

Goran,

My old books say 29-58 ft-lbs for a new or newly rebuilt LSD. My experience is that many have un-detectable torque when moving a tire even as low as 30,000 miles of street driving yet still provide the limited slip function under load. That is what prompts my “high wear” comment.

When testing by the “turn the wheel” method in the car the drag of the rear brake and parking brake on the opposite side can mask the LSD clutch. So can the CVs since they are at extreme lower angle unless the trailing arm is supported.

When the transmission is out of the car and has been sitting, the old lube between the clutches can stick, clueing you to the presence of a LSD even if the clutches are worn sufficiently to have no per-load.

Should it be rebuilt when it gets out of spec? Probably, if you want everything perfect all the time.
Will it continue to function when out of spec? Probably, just not as well as new.

When the LSD is set at 50% only a few of the clutch friction surfaces actually slip, the others just sit there. This causes higher wear on the clutches that rotate and none on the others. I think this leads to the rapid going out of torque spec. The good news is you can disassemble it, move the parts around (sorta like turning a syncro ring), and reassemble at torque spec.

Of course driving style has a great influence. The sharper the turn and more torque, the higher the wear. Its just fun to scoot around!

A LSD is a relatively easy part to service in the car. The parts count is low (the ring gear bolts are the only slightly costly item other than clutch parts) and you can have the peace of mind knowing everything is correct.

Best,
Grady

[pwd72s]

Originally posted by Grady Clay
jjemg,

Give us a description of your 911 configuration. How do you intend to use the car? Garage queen full of cool parts?

Best,
Grady

HEY! I er, represent that remark! My only excuse is that the damned ZF thing was in the car when I bought it, so there it will remain...

[Grady Clay]

Paul,

You are our perfect subject of experiment. Put the “S” on jack stands under the rear shocks and do the “turn the wheel” test. Please report your findings.

The 911 is probably one of few it’s age with an unmolested almost everything.

Best,
Grady

[TimT]

This is a keeper thread.....

should be enshrined in a "thread hall of fame"

Hayden, Grady, and Steve opine on a subject, basically all in agreement...

This is good

[pwd72s]

Originally posted by Grady Clay
Paul,

You are our perfect subject of experiment. Put the “S” on jack stands under the rear shocks and do the “turn the wheel” test. Please report your findings.

The 911 is probably one of few it’s age with an unmolested almost everything.

Best,
Grady

Funny story here...but it will lead up to the results. Shortly after Gordon Ledbetter went through my gearbox, "freshening" things, he changed shops. Grady, I have zero worries when Gordon wrenches on my car. The shop Gordon moved to works primarily on race cars only. My car is obviously not a race car. So one day, I called the shop's owner. I'll admit it, I begged to be let in the shop since I had relied on Gordon, and Gordon now worked for him. So, the owner agreed...my car then on the lift, Gordon working on it, the owner would come out from time to time, looking at her bottom. One thing he did was spin a rear wheel. Both wheels turned in the same direction. Gordon said: "Yes, it has factory installed limited slip. I went through the box a bit ago.". After the work was done, and I'd written the check, I think I received the best compliment on my car I'd had in years. "You know, I think I need to buy your car." So Grady, I can't really say "unmolested" in the gearbox. Gordon went through it a few years ago because of a soft 2nd gear synchro...he fine tuned the ZF at that time. So, today, both wheels turn in the same direction when the car is off the ground, in neutral. I'm kind of weird that way when it comes to this car. If anything goes wrong, I want to fix it or have it fixed...right now. That despite the fact I only drive it, on average, 1,000 miles per year these days. Guess it's becoming a memories thing for me...while younger guys are out there forming memories. For example, one memory I don't regret, but would NEVER do again? Driving past a parked OSP radar cop car on I-5 while pushing redline in 5th...on purpose because CB had told me where he was and that there were no pursuit cars....young & dumb, I once was, no doubt about that.

[wevoid]

If you are lucky enough to have a 915 that was shipped from the factory with a ZF LSD, it will have a unique number on the bottom.

The version number 915/XX is normally followed by a gap and then a 2 digit, one letter or 3 digit # (last of which would be the letter like... 74C) then a space and another 4 digit number.

If that first gap, between the 915/XX and the "74C" has the number 12 stamped in it, then I believe it left the factory with a ZF LSD.

Regards

Hayden

[pwd72s]

Hayden, it's there..it's factory. I have documentation directly from the factory in the form of a letter written in pre "gotta pay for a kardex" days. Silly as it sounds, I wrote them, they wrote back, listing the way the car left the factory. The ZF was among the options. Transmission case numbers? Well, I drove into Sunnyside park near here one day, and they had a HUGE speed bump. As a result, a few numbers in my magnesium transaxle case are missing...If you care to doubt me? PM your snail mail address...I'll send you a photocopy of the letter the factory sent.

[Steve@Rennsport]

Hayden:

Nice pics and a very informative narrative on ID'ing the thing without disassembly. Good work!

I would just add that in my experience, the normal ZF LSD's (4-plate units) will test positive as I described, regardless of wear. You and Grady might have different experiences than mine.

For me, the only ZF units that behave like an open diff (wheels spin opposite directions) are the 2-plate 993 LSD's. This might be due to their extreme rates of wear. I'd be curious to see if one can see anything in the G-50 tranny's drain plug to make a positive ID. I'll be looking at that this week,....

[Grady Clay]

Jimmy,

One thing I forgot to mention when suggesting comparing the LSD to your differential is to see about the speedometer ring. You need to confirm that the LSD will accommodate the speedometer ring, if you need one.

Best,
Grady

[dvkk]

Originally posted by jjemg
Hoping for a little help....I bought a differential for a 915 and I'm not exactly sure what I have. What I was supposed to get was LSD for one of the later 915 trans that came out of an original car. After looking at the number on it and checking it on PET, it is an unknown. The number is 4061806176 516152 S4C 930. Can anybody make anything out of this?

The ZF part number is 4061 806 176.
The diff's serial number is 516152