Limited slip additives not required but OK to use?
03-22-2018, 07:49 PM
#1
Burning Brakes
Thread Starter
Limited slip additives not required but OK to use?
Asking for my ‘87 S4 which has an LSD, looking at last discussions it appears that an additive is not required button would a 75-90w GL5 from Valovine for LSD (specifically) be OK to use?
03-22-2018, 08:34 PM
#2
Chronic Tool Dropper
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Hi Tom --
"LSD Additives" are friction modifiers you can use to "tune" a clutch-type LSD. Non LSD-specific oils often cause too much grab in a new LSD, so a dose of "friction modifier" is added to relieve some of the chattering that might bother you when going around slow corners.
Clutch LSD sets are typically "tuned" for a certain breakaway torque. That is, the amount of torque you might apply to the axle on one side raised, gearbox warm and in neutral, opposite wheel on the ground, to get the LSD to release a little and allow the raised wheel to turn. Opinions vary on what it needs to be. For street cars, a breakaway value between 20 and 30 lbs/ft is just right for me. In an online discussion regarding rebuilding LSD packs, Greg Brown concurred. He may have updated info to share, as the discussion was a while ago and he's undoubtedly done a few since then.
Meanwhile, back at aging reality, the clutch-style LSD packs in most of our 100k+ cars are pretty worn. I can safely say that for most cars, especially if they are refilled and run with straight non-LSD oil on schedule, owners will be lucky to have the target 25 lbs/ft of breakaway torque available even before any special oil or additive is used.
What to do... Refill with regular oil, go drive the car some to get the oil through everything. Next time it's convenient, test the breakaway torque with your torque wrench, and see if it's greater than 30 lbs/ft. If it is, drain some of the oil out and replace it with LSD-specific oil. Rinse, repeat, until the number you want clicks on the wrench.
The LSD in my car has been swimming in a 50/50 blend of Redline oils since it came to me with about 20k on it. The LSD has consistently shown about 20 lbs/ft of breakaway torque required. Last time I checked it was about 20k ago now, now with 5k to go before the next service. I may throw the dice and put the 2.20 one-legged box in there to see how it does, at which time the diff oil blend will be moot. The car is sitting on stands at home now, not exactly convenient for a quick check to verify condition today.
---
Greg is the most diligent LSD rebuilder/tuner I know. With luck he'll dive in here and share more wisdom.
"LSD Additives" are friction modifiers you can use to "tune" a clutch-type LSD. Non LSD-specific oils often cause too much grab in a new LSD, so a dose of "friction modifier" is added to relieve some of the chattering that might bother you when going around slow corners.
Clutch LSD sets are typically "tuned" for a certain breakaway torque. That is, the amount of torque you might apply to the axle on one side raised, gearbox warm and in neutral, opposite wheel on the ground, to get the LSD to release a little and allow the raised wheel to turn. Opinions vary on what it needs to be. For street cars, a breakaway value between 20 and 30 lbs/ft is just right for me. In an online discussion regarding rebuilding LSD packs, Greg Brown concurred. He may have updated info to share, as the discussion was a while ago and he's undoubtedly done a few since then.
Meanwhile, back at aging reality, the clutch-style LSD packs in most of our 100k+ cars are pretty worn. I can safely say that for most cars, especially if they are refilled and run with straight non-LSD oil on schedule, owners will be lucky to have the target 25 lbs/ft of breakaway torque available even before any special oil or additive is used.
What to do... Refill with regular oil, go drive the car some to get the oil through everything. Next time it's convenient, test the breakaway torque with your torque wrench, and see if it's greater than 30 lbs/ft. If it is, drain some of the oil out and replace it with LSD-specific oil. Rinse, repeat, until the number you want clicks on the wrench.
The LSD in my car has been swimming in a 50/50 blend of Redline oils since it came to me with about 20k on it. The LSD has consistently shown about 20 lbs/ft of breakaway torque required. Last time I checked it was about 20k ago now, now with 5k to go before the next service. I may throw the dice and put the 2.20 one-legged box in there to see how it does, at which time the diff oil blend will be moot. The car is sitting on stands at home now, not exactly convenient for a quick check to verify condition today.
---
Greg is the most diligent LSD rebuilder/tuner I know. With luck he'll dive in here and share more wisdom.
03-24-2018, 02:43 AM
#4
Former Sponsor
This is a complex problem....and finding the "perfect" gear oil, combination of gear oils, or friction modifier is like searching for an honest politician.
There's a couple of things to realize. Starting in 1985, the transmissions have synchros that have a friction surface that is a sprayed on moly coating....exactly the same as the surface for the friction discs in the limited slip.
The trick becomes to find the gear oil that allows the synchros to properly "grab" onto the cones on the gears without excessive slip. AND be slippery enough to not have the limited slip chatter. AND not be too slippery, which doesn't allow the synchos to generate enough friction to "grab" onto the cones enough. And not grab on so hard that the moly surface "smears" (which can also happen if the synchro doesn't grab enough....causing excessive heat and thus "smearing".)
The absolute best gear oil I've found for use in the limited slips is hands down Mobil One 70-90 LS. The trade-off is that it is just a bit too slippery for "worn" synchros....there will be a tiny bit of "crunchy" feeling if the synchros are nearing the end of their life. This usually shows up in the more used gears....1st through 3rd. Note that this gear oil works extremely well on new synchros.
Some people swear by some magic "combination" of two different Redline gear oils...and I've got a couple of older GTs thst have a couple of crunchy synchros (if you don't drive them perfectly) that I'm going to try this "combination" on.
Unfortunately, the use of Redline Gear Oil seems to be limited to use on transmissions with "worn in" seal surfaces. It will leak out of freshly replaced seals....to the point where I tell people that they can certainly use it in my rebuilt transmissuons, but to skip the phone calls (to me) about the seals leaking.
There's also a huge problem with the "Borg Warner" design synchros (1985-1995 928's) getting enough "cool" gear oil to work properly when driven hard....but that is an entirely different discussion with an entirely different solution.
There's a couple of things to realize. Starting in 1985, the transmissions have synchros that have a friction surface that is a sprayed on moly coating....exactly the same as the surface for the friction discs in the limited slip.
The trick becomes to find the gear oil that allows the synchros to properly "grab" onto the cones on the gears without excessive slip. AND be slippery enough to not have the limited slip chatter. AND not be too slippery, which doesn't allow the synchos to generate enough friction to "grab" onto the cones enough. And not grab on so hard that the moly surface "smears" (which can also happen if the synchro doesn't grab enough....causing excessive heat and thus "smearing".)
The absolute best gear oil I've found for use in the limited slips is hands down Mobil One 70-90 LS. The trade-off is that it is just a bit too slippery for "worn" synchros....there will be a tiny bit of "crunchy" feeling if the synchros are nearing the end of their life. This usually shows up in the more used gears....1st through 3rd. Note that this gear oil works extremely well on new synchros.
Some people swear by some magic "combination" of two different Redline gear oils...and I've got a couple of older GTs thst have a couple of crunchy synchros (if you don't drive them perfectly) that I'm going to try this "combination" on.
Unfortunately, the use of Redline Gear Oil seems to be limited to use on transmissions with "worn in" seal surfaces. It will leak out of freshly replaced seals....to the point where I tell people that they can certainly use it in my rebuilt transmissuons, but to skip the phone calls (to me) about the seals leaking.
There's also a huge problem with the "Borg Warner" design synchros (1985-1995 928's) getting enough "cool" gear oil to work properly when driven hard....but that is an entirely different discussion with an entirely different solution.
03-24-2018, 12:20 PM
#6
Chronic Tool Dropper
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Readers should remember that the concern about LSD oil and the synchro coatings are limited to 5-speed cars. For those of us blessed with automatic cars, the gear oil in question lubricates only the differential.
Another reminder is that it takes a good while for the effects of any blended oil to show up in differential breakaway torque values. It isn't something you can just pour some in, and test/tune the blend while the car is still raised for the oil swap. It needs to be driven enough to exercise/exorcise the old oil that's packed in the LSD clutch surfaces. As mentioned above, most of our cars will be hard pressed to make the 20-30 lbs/ft breakaway torque number with straight GL5 oil, much less a blend with a friction modifier.
Another reminder is that it takes a good while for the effects of any blended oil to show up in differential breakaway torque values. It isn't something you can just pour some in, and test/tune the blend while the car is still raised for the oil swap. It needs to be driven enough to exercise/exorcise the old oil that's packed in the LSD clutch surfaces. As mentioned above, most of our cars will be hard pressed to make the 20-30 lbs/ft breakaway torque number with straight GL5 oil, much less a blend with a friction modifier.
03-24-2018, 12:20 PM
#7
Former Sponsor
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03-24-2018, 12:59 PM
#9
Former Sponsor
Readers should remember that the concern about LSD oil and the synchro coatings are limited to 5-speed cars. For those of us blessed with automatic cars, the gear oil in question lubricates only the differential.
Another reminder is that it takes a good while for the effects of any blended oil to show up in differential breakaway torque values. It isn't something you can just pour some in, and test/tune the blend while the car is still raised for the oil swap. It needs to be driven enough to exercise/exorcise the old oil that's packed in the LSD clutch surfaces. As mentioned above, most of our cars will be hard pressed to make the 20-30 lbs/ft breakaway torque number with straight GL5 oil, much less a blend with a friction modifier.
Another reminder is that it takes a good while for the effects of any blended oil to show up in differential breakaway torque values. It isn't something you can just pour some in, and test/tune the blend while the car is still raised for the oil swap. It needs to be driven enough to exercise/exorcise the old oil that's packed in the LSD clutch surfaces. As mentioned above, most of our cars will be hard pressed to make the 20-30 lbs/ft breakaway torque number with straight GL5 oil, much less a blend with a friction modifier.
Pour in Mobil One 75-90 LS. Change it every 30,000 miles. No "magic" needed.
.
Dr. Bob is 100% correct. It takes several miles and lots of turning to flush out gear oil that causes chatter. The design of the limited slip units makes cooling and lubrication of the friction surfaces difficult.
BTW....it is pretty rare to have one of the "snow" limited slips in the 85-95 vehicles have 30 ft.lbs. of breakaway torque. New specification is 7 to 20 ft.lbs. I guess it is possible for the friction discs to "flatten down" the peaks of the Moly surface and have enough friction surface area that the release torque increases to 30 ft.lbs, however it is rare.
03-24-2018, 01:10 PM
#10
Former Sponsor
Yes. And Porsche filled the factory Cup Cars with that gear oil...and recommended that it be used. Those cars obviously had limited slips (very aggressive limited slips with a design that allowed better gear oil exchange between the friction discs...quite possibly by accident.)
Over here, it is very difficult (impossible?) to find Mobil One 75-90 without the "LS" designation.
I believe the two are one in the same.
Over here, it is very difficult (impossible?) to find Mobil One 75-90 without the "LS" designation.
I believe the two are one in the same.
03-24-2018, 03:09 PM
#11
Rennlist Member
https://www.ebay.de/itm/Mobil-Mobilu...kAAOSwWxNY13lw
Thx Greg,
I think this is the oil to use.
The limited slip diffs in the race cars are more like a claw clutch ! ;-)
Thx Greg,
I think this is the oil to use.
The limited slip diffs in the race cars are more like a claw clutch ! ;-)