LSD Question: R U Experienced?

[Por_sha911]

OK, I can't stand it any longer. My car came stock with LSD. (Sorry but every time I see "LSD", I think of my college days in the 70's. But I digress...)
Anyway, here's the question: How do I know which one do I have? I'm seeing that there are different ratios (if I understand some posts correctly). A little edumecation would be appreciated.

[Grady Clay]

Joe,

Do a search on Pelican. We have hashed out some good stuff you will like.

Best,
Grady

[Por_sha911]

OK, I did the search thing and I'm not getting the answer. Maybe I just don't know the question. I know WHAT LSD is. What I want to know is how do I know if I have 45/65 vs. 50/80 vs. ZF??? Also, what does the higher numbers mean?

[Geary]

Hi Joe

The numbers signify "percentage of anti-slip effectiveness" (Porsche's description). We call it "lock-up percentage", although these are just relative numbers. The first number is lock-up percentage on acceleration, while the second number is lock-up percentage on deceleration.

Your car likely came with a 40% limited-slip, or 40/40. These normally require servicing each 30,000 miles to maintain their effectiveness.

[rcwaldo]

This may help explain the numbers to ZF LSD's. Like Geary mentioned, the numbers describe the angle of ramps inside the diff that dictate how agressive the lock-up will be, when loaded. 45/65 is basically a 45 degree ramp surface for accel, where 65 is 65 degrees of ramp for decel. The lower the angle, the more aggressive the lock-up of the diff..

Hope this helps..

Chris

[Geary]

This is a popular misconception. The numbers correspond to locking percentages only, with the higher percentages obtained with LOWER ramp degrees. For instance, in the GT3R-style LSD, a 40% ramp has a 50-degree angle, while an 80% ramp has a 30-degree angle.

[Wil Ferch]

Geary:
Just what sort of 30k mile maintenance is required? Where is that in "the manual" ???

---Wil

[jluetjen]

The numbers correspond to locking percentages only, with the higher percentages obtained with LOWER ramp degrees. For instance, in the GT3R-style LSD, a 40% ramp has a 50-degree angle, while an 80% ramp has a 30-degree angle.

Whenever I've seen limited slip's refered to as ##/##, that refers to ramp angles and is the way that most of the world describes limited slips. Porsche on the other hand has always refered to limited slips in terms of ##%'s. I suspect that they did this because generally their limited slips were symetrical at something like 45/45 and they varied the % of slip by rearranging the clutch order and spring pressure.

Anyhow, I doubt that you could draw a tight correlation between ramp angle and % of lock-up because it ignores the spring pressure and clutch surface area that both affect the diff's performance.

As far as I know, you really can't tell what you have without taking it out since even if the diff were a 60% dif, it may have worn down to the level of a 30% (or worse) dif because of wear on the part of the clutches.

[nostatic]

In 1938, Dr. Albert Hoffman, a Swiss chemist working for Sandoz Laboratories in Basel, was the first person to synthesize lysergic acid diethylamide. He developed the drug accidentally during an attempt to isolate an analeptic compound in ergot, a rye fungus with many medicinal alkaloids. LSD was the twenty fifth in a series of ergot derivatives he created while searching for the elusive circulatory stimulant. He quickly lost interest in the drug when it failed to produce significant results in preliminary studies on laboratory animals. (Lee & Shalin, xiv)


Hoffman did not rediscover LSD-25 until April 16, 1943 when he decided to conduct further studies on the compound. In the process of synthesizing a new batch, Hoffman absorbed a small dose of LSD through his fingertips. He described what happened next in a report he sent to one of his colleagues.


Last Friday, April 16, 1943, I was forced to interrupt my work at the laboratory in the middle of the afternoon and proceed home, being affected by a remarkable restlessness, combined with a slight dizziness. At home I lay down and sank into a not unpleasant intoxicated-like condition, characterized by an extremely stimulated imagination. In a dreamlike state, with eyes closed (I found the daylight to be unpleasantly glaring), I perceived an uninterrupted stream of fantastic pictures, extraordinary shapes with intense, kaleidoscopic play of colors. After some two hours this condition fades away. (Hoffman, 15)


Three days after his accidental ingestion of LSD-25, Albert Hoffman began to research the compound in earnest. On April 19, 1943 he swallowed 250 micrograms. In his book, LSDMy Problem Child, Hoffman discusses the implications of his second, more intense LSD experience. First, to his knowledge, lysergic acid was the only known substance that could evoke such extreme psychic effects in such low doses. Hoffman had not expected the 250 microgram dosage to inebriate him. In fact, the possibility seemed so unlikely that Hoffman's colleagues would not believe he had measured the dosage correctly until they had ingested similar amounts of the compound. (Hoffman, 20)

[rcwaldo]

Originally posted by jluetjen
Whenever I've seen limited slip's refered to as ##/##, that refers to ramp angles and is the way that most of the world describes limited slips. Porsche on the other hand has always refered to limited slips in terms of ##%'s. I suspect that they did this because generally their limited slips were symetrical at something like 45/45 and they varied the % of slip by rearranging the clutch order and spring pressure.

Anyhow, I doubt that you could draw a tight correlation between ramp angle and % of lock-up because it ignores the spring pressure and clutch surface area that both affect the diff's performance.

As far as I know, you really can't tell what you have without taking it out since even if the diff were a 60% dif, it may have worn down to the level of a 30% (or worse) dif because of wear on the part of the clutches.

John, yes this is a good point. Friction surface area does play a huge roll in LSD lock-up characteristics, as does plate pre-load or free play.

Chris

[targa911S]

Nostatic...that was priceless. Now I know who was responsible for my twisted life. By the way I just checked out nostatic.fm and realized you too are a bass player. That splains a lot! 40 years of thumpin here.

[Geary]

As I've tried to point out, the lower the ramp angle, the higher the locking percentage. From this you can ascertain that manufacturers are NOT speaking of the actual angle that the ramps are cut at (which would be meaningless to us), but rather the relative locking factors of on throttle and off-throttle of a NEW OR REBUILT LSD. There are a number of ways that various LSD manufacturers refer to their different locking factors, but the Porsche method makes a LOT more sense than others I’ve seen.

Knowing the exact locking percentage of your differential isn’t possible. The best that can be done is to set up the differential as per factory specifications, and refer to that set-up whenever queried by your buddies. (Even the best Porsche drivers in ALMS couldn't tell you for certain if there were 40/60 or 50/80 ramps installed in the test car that they just drove.)

With Porsche LSDs:

Pre-G50 LSDs all came with 50-degree ramp angles, which were adjusted at 40% -- having 4 friction surfaces working (2 friction discs, both sides working; or 4 friction discs, 2 back-to-back on each side) -- or 80% with 8 friction surfaces working (4 friction discs separated by plain plates, so that both sides are working). As you will see in the next paragraph, these figures are merely an attempt to put some sort of meaningful label on how the LSD is set up.

While there is a long list of "econo" versions of Porsche LSDs that typically came in street cars, GENERALLY SPEAKING, G50 & later Motorsports model LSDs came with varying ramp angles, and had 8 working friction surfaces (4 friction discs). But in 2004, things changed a bit. This year, Weissach has doubled the number of plates in their LSDs. Does this now mean that the 2004 Porsche LSD with the same 40/60 ramps as 2003 now operates at 80/120 (an impossible number), if applying the logic of doubling the friction surfaces? No. But you'll have to argue with the engineers at Weissach about how this further frustrates our ability to pinpoint our locking percentages.

Wil, I don't have "the manual" in front of me which states 30,000 miles. For some reason I assumed you had a 915 transmission, and was merely quoting the factory-recommended service interval for a street-driven 915 or 930 40% LSD. The interval could be 1,000 miles or less for a track-dedicated 930. If it's a 993 that you have, I would tell you that your 993 LSD lost most of its bite within 3,000 STREET miles, and is now operating more like an open diff than an LSD.

[Bill Verburg]

Geary,
Thanks for the good info!!

[Wil Ferch]

Geary:
I do have a 915 transaxle...with ZF 40% lockup limited slip...factory option. 1985 Carrera...yep, wheels both move in the same direction on jackstands when you hand-spin one side....
Never saw or heard of a routine maintenance item because of this....
---Wil

[Bill Verburg]

Will,
the friction disks are a wear item. The 993 has half as many as the 915s with more power and weight , they wear out very quickly.

[Wil Ferch]

I understand that...so what are you supposed to do every 30K miles?....rebuild your g'box and add new disks at 30K intervals?

I presume the alternative would be to run superior lubrication, and I run Swepco at the moment. I'm just asking what sort of servicing is intended or written up to do. "Servicing" is like....preventive maintenance, no?

---Wil

[Bill Verburg]

Mine are noticeable worn w/ less than 10k

[Geary]

Wil, your LSD may not need rebuilding, especially since it's passing your jackstand test. Those earlier LSDs had the good plasma-sprayed friction material, which takes a pretty long time to wear out.

On the other hand, the discs that came in the 993s --- even those equipped with 4 friction discs (rather than the econo version which had only 2) --- are made from some sort of fibrous material which literally wears out in just a few thousand miles, or a single track weekend. These horse puckey discs can easily be replaced with the plasma-sprayed variety for FAR longer life, and FAR more bite. If it's the econo version LSD, additional discs can also be installed, making it a "who knows what" locking percentage LSD.

[Por_sha911]

Thanks to everyone for the education. Nostatic: I knew I could count on you to give the facts.

Is there no place on the car that will tell me what the car had setup from the factory?

[Wil Ferch]

Joe:
I am 90-95% certain that the G50 3.2's were much like the 915 3.2's in that a ZF type lsd of 40% locking ( both directions...accel / decel) was used.
I, or someone else may check the pocket spec books or the factory manuals. I am almost as sure that un-equal accel /decel lockup was first used on AWD 964's ......and ontinued to be used later as more and more elctrronic governing controls were used in our cars.

---Wil