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Smart quod bastardus
 
fredmeister's Avatar
 
Join Date: Oct 2002
Location: Baltimore, MD, USA
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Garage
garage method of checking lsd function

Anyone know of a quick and dirty way of checking the condition of the OEM lsd in a 930 4 speed box? Trying to gage if mine is still functioning or not.
I thought there might be a way of jacking the car up on blocks and measuring the forces to turn one wheel while the other is blocked from rotating.
Is this a logical approach, and if so how much force approx. should be expected to turn the unblocked wheel by hand as long as the diff is in good shape?
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1979 930 Turbo....3.4L, 7.5to1 comp, SC cams, B&B intercooler, Snow Perf water/meth injection, Rarlyl8 headers, Garret GTX turbo, 36mm ported intakes, Innovate Auxbox/LM-1, custom Manually Adjustable wastegate housing (0.8-1.1bar),--running 0.7bar max
---"When you're racing it's life! Anything else either before or after, is just waiting"
Old 12-30-2009, 09:10 AM
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There is a specification for that and it can be measured with an inch pound torque wrench turning one wheel hub to measure the resistance while the other is not allowed to turn.
The clutch pack can be shimmed up or 4 new clutch discs installed to raise the limited slip percentage or bring it back to origonal spec if worn. You can also remachine some parts to fit 6 clutch discs and plates - 3 on each side in there if wanted.

I don't know the turning resistance spec but you could find it somewhere or from someone that rebuilds 930 LSD's.

The way the LSD is designed the pressure applied squeezing the 4 LSD clutch discs and plates increases with engine torque.
The spider gearsets teeth are cut at an angle in a way to push away from each other under load while the LSD carrier surrounding them and the clutch discs limits how far thay can push away from each other and thats how the pressure on the clutch discs is increased under load.

It's hard to describe how the ZF LSD used in Porches and BMW's works without referencing to an exploded diagram or having one apart on a bench in front of you.
Old 12-30-2009, 09:34 AM
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...and while on the topic, I also have LSD (but not the Timothy Leery kind). I've got a weird symptom that has been there since the day I bought my car, and I think at one time I read a diagnosis.

When backing up out of my gargage and turning 90 degrees at the same time, I'll get this little knick-knock sound...only when turning while backing up. Probably a clutch on one side or the other. My knowlege is zipppp on differentials.

Jim, your thoughts....just live with it, or worry about it (or maybe fix it some day)?
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Old 12-30-2009, 12:41 PM
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Fredmeister,,,,,,,,,,,,,,,,,Why not give it a
seat of the pants road test. I own both w/wo LSD. On one without LSD in the lower gears you can definately feel it push the rear of the car to the right ( also lift the left front somewhat) without. On a higher than factory split you can hear a slight knock and feel a tugging when slow sharp turning most noticed in reverse, unless near the upper limit of split, then a knocking noise / rear wheel tug forward or reverse. On a long curve under power with a high split 80/20 the car will push straight unless you break the wheels loose so you must get your entrance speed right as it is difficult to raise it once into the apex of a corner. This is a general fun way I think to decide if you want more posi effect or if it is still working.
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Anthony @ Voitureltd Bayside WI.

Last edited by voitureltd; 12-30-2009 at 01:26 PM..
Old 12-30-2009, 01:08 PM
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Quote:
Originally Posted by mark houghton View Post
...and while on the topic, I also have LSD (but not the Timothy Leery kind). I've got a weird symptom that has been there since the day I bought my car, and I think at one time I read a diagnosis.

When backing up out of my gargage and turning 90 degrees at the same time, I'll get this little knick-knock sound...only when turning while backing up. Probably a clutch on one side or the other. My knowlege is zipppp on differentials.

Jim, your thoughts....just live with it, or worry about it (or maybe fix it some day)?
I've never heard a noise like that from my car's LSD while backing up and turning so I don't know what to say on that.
The LSD internals look like they would work the same way in forward or reverse. Rotational torque would just be working on the oposite side of the helical cut ring and pinion gear teeth, and the small spider gears straight cut tapered teeth.

Sure it isn't a CV joint making the noise?
Old 12-30-2009, 01:19 PM
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Thanks Jim. I guess it could be a CV joint, but they both seemed to be fine last time I had the motor out. Again, it's only when turning while backing up. I thought someone once mentioned that when turning and backing, the wheels are turning at different rpms (which would make sense because of the radius of the turn being different from the inner wheel to the outer wheel) and can result in such a noise if all isn't quite perfect within.
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Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.
Old 12-30-2009, 01:26 PM
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Quote:
Originally Posted by voitureltd View Post
Fredmeister,,,,,,,,,,,,,,,,,Why not give it a
seat of the pants road test. I own both w/wo LSD. On one without LSD in the lower gears you can definately feel it push the rear of the car to the right ( also lift the left front somewhat) without. On a higher than factory split you can hear a slight knock and feel a tugging when slow sharp turning most noticed in reverse, unless near the upper limit of split, then a knocking noise / rear wheel tug forward or reverse. .
So Tony, are you saying that some noises when turning are normal? Or did a previous owner outfit my car with a higher than factory split?
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Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, stock everything else. The result of a massive Pelicanite good will fire recovery effort. Truely an open book, ready for the slippery slopes to modification.
Old 12-30-2009, 01:40 PM
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Quote:
Originally Posted by mark houghton View Post
So Tony, are you saying that some noises when turning are normal? Or did a previous owner outfit my car with a higher than factory split?
Mark,,,,,. I have 2 that I know of that have a higher than factory setting and one with a Quaife. My experience is the more split ratio you go the more noticeable it gets. My guess is yours has been tweeked a bit if the rest of the deiveline is trouble free. Most all 930s I've had do have LSD and sport seats (fussy sort of a thing with me) and if stock settings,they do not have the aforementioned characteristics.
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Last edited by voitureltd; 12-30-2009 at 03:00 PM..
Old 12-30-2009, 02:09 PM
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Quote:
Originally Posted by mark houghton View Post
Thanks Jim. I guess it could be a CV joint, but they both seemed to be fine last time I had the motor out. Again, it's only when turning while backing up. I thought someone once mentioned that when turning and backing, the wheels are turning at different rpms (which would make sense because of the radius of the turn being different from the inner wheel to the outer wheel) and can result in such a noise if all isn't quite perfect within.
There's no way of knowing if a CV joint cage is cracked or deep grooves have worn in the inner and outer slots or races the balls run in without taking it apart and cleaning all the grease out with solvent. If there is a crack in the cage or a deep groove worn in the forward moving direction, then the ball can make a click or clunk while passing over it when the CV is torque loaded in the reverse moving direction.

The wheels turn at differnent speeds on all cars in forward or reverse unless there is a locked diferential in it.
In that case the inner tire will be forced to rotate faster over the pavement it's going over and it will skip and and make short skidding noises as it does it, especially if there is some dirt, sand, or gravel on the pavement.

Try pushing a race car with foot wide race slicks and a locked dif while turning the steering wheel... it takes several guys pushing on the car and it's difficult because the tires want to rotate at different speeds but the locker won't let them.

++++++++++++++++++++++++++++++++++++++++++++++

Try pushing your car backwards in nuetral while the steering wheel is turned. That way the CV's are not loaded with torque to drive the car and the balls would be less likely to click or clunk if they roll over an unsmooth worn internal surface and the differential is just going along for the ride as the 4 wet clutch discs slip in 90 weight gear oil.

I don't know why anyone would bother with the expense or labor increasing the limited slip percentage in a street driven 930 LSD.
It makes no sense at all in a street car. It would increase or cause annoying chattering of the 4 clutch discs when making sharp turns slowly like from a stop sign and would just wear them out faster.

The ZF LSD slip rate raises considerably from static idle state with torque under acceleration and deceleration.
Old 12-30-2009, 02:41 PM
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