Flott Leben

Kind of a basic question, but can somone please educate me and elaborate on the advantages and disadvantages of each type of differential: Free vs. 40% LSD vs. PSD, as far as street use and track use are concerned? Thank you.

MikeN

Not an expert by any means but....

Free or open diff: Means only one wheel moving the car forward at any given time. The other is pretty much along for the ride. Advantages?
cheaper, lighter, and less complex then the others.......pretty sorry advantages though. Disadvantages?, only one rear wheel moving the car forward at any time, bad for traction on slipery surfaces, bad for putting down power at the track. Some will never notice any of these disadvantages during normal street driving, some will.

40%LSD. Free rear wheel will lock at 40% driving force of the other (I believe that is correct) when it loses traction. So if anytime during normal driving the main rear drive wheel loses traction, the other will lock at 40% and help propel the car forward. Great for slipery surfaces, and putting power down at the track. Disadvantages?, more expensive, more complicated, more to break, but most wouldn't want a 928 without it
or the PSD.

PSD.......electronic controlled version of the locking differential. Offers 0-100% lock of the free rear wheel when needed. Uses various sensors to detect wheel spin and adjusts diff. lock by a computer controlled hydraulic pump. Pretty advanced concept in its day, taken from the 959. Adavantages?, very nice for street driving......can't imagine anything better except all wheel drive. Disadvantages?, somewhat complicated and expensive if it breaks, weight, more electronics, etc. For the track, most would probably prefer the 40% non-electronic, mechanical lock. Cheaper, much less breakable, more predictable results maybe.

Others will have more.

Jim bailey - 928 International

The early limited slip can be stacked up to an 80 % lock up , the USA 83-89 slip got reduced to only 40% ; rumor is a lawsuit in which a 930 (911 turbo ) driver happened to be a lady crashed the car because of the "racecar " like handling of the high percentage limited slip ................ The late PSD computer controlled slip does much more than just limit wheel spin it will in certain high speed corners adjust the slip to "drag" a wheel around the corner modifying the rear steer action somewhat .

Flott Leben

MikeN, Thanks. That is very helpful.

Jim,

I was hoping you would chime in. So, if you had an '87 with an open diff. which LSD would you put in? (and do you have any in stock?) Thanks!

Bill Ball

The standard or "open" differential does not provide power to only one wheel. The open diferential provides the same torque to both wheels. The problem is what that torque does if one wheel loses traction. As long as you're not doing burnouts, stunt driving on 2 wheels or going into dirt, ice or snow, both rear wheels will get good power with a standard diff. As long as torque does not exceed traction for one wheel, the standard diff works fine. Hence you can drive a standard diff car on snow, dirt and ice if you apply gas slowly and don't get the torque too high and cause one wheel to slip. Backing off a little will get both wheels moving again. If power only went to one wheel you'd be unable to do so.

Flott Leben

Thanks, Bill!

Bill Ball

I'm no expert either, but I don't think a limited slip offers much advantage to anyone except the most advanced racers. If you get a tire in the dirt on the track, you shouldn't be applying power and an LSD driving the wheel with traction will just hasten your exit if you do try to power you way out of that mess. Perhaps if you like to do a lot of oversteer drifting it would make it easier to break both tires free. I bought an LSD and was about to put it in my 89 that I use for open road racing, but I decided it would not actually do anything useful for me. Sorry, sold it already.

SharkSkin

I see you're from New York. Also note that LSD-equipped cars can be nearly undriveable on icy roads. Where an open diff will allow one wheel to spin, the opposite wheel remains planted on the road. With LSD on an icy road in the same situation both rear wheels will want to spin, probably not ideal. It is MUCH easier to get an LSD-equipped car to swap ends on wet/icy roads. The idea that the LSD setup is better in poor traction conditions is based on a number of assumptions that don't always apply.

Vilhuer

SharkSkin, have to disagree you here. LSD is very useful in slippery conditions. It does make rearend more nervous but same time guarantees some traction. When both wheels spin they spin at same rate. On really slippery surfaces it's possible to get stuck on level ground with open diff when only one wheel spins. Have to really really try to manage that with LSD.

Front wheel drive cars are generally considered much better winter cars. LSD levels the field considerably. I had BMW 320i some years ago with ZF 25% LSD. Basically same design as 928's 40%. On several occasions front wheel drives got stuck on uphill traffic lights and such where BMW didn't have any problems because of LSD. Also owned similar 320i except it had open diff. It was hopeless in same situations.

SharkSkin

Erkka, I wasn't trying to say that LSD is not useful in slippery conditions. I was simply trying to point out that the car will behave different, "more nervous" to borrow your phrase, and that in some situations the LSD can be worse in slippery conditions. I've had a number of cars with and without LSD and I agree with your observations. Like so many things, it comes down to driver skill/preference.

To cite a clearer example: Pick a corner. Drive around that corner in wet conditions in a RWD car with an open diff. In the middle of the corner, punch the throttle. Most likely you will just spin the inside tire, and continue on around the corner. Now equip that car with LSD and repeat the experiment. That car will now most likely spin out.

I guess that the real point I was making is that under certain conditions(e.g. applying power in a slippery corner) LSD will not necessarily perform better or be safer... which runs counter to a common misconception held by many people who have not driven LSD-equipped vehicles in foul conditions so I thought it worth mentioning.

Flott Leben

Thanks for all the input. I am getting a little confused over which is better in which conditions. Anyone else have some opinions??

MikeN

I think it has a lot to do with driving style and the 928 in general. If you are very careful with the throttle, LSD is great in ice, snow, and rain, but
as SharkSkin said if you happen to get both wheels spinning you'll probably be in a little trouble. I know that breaking both tires loose in a 928 is fairly hard compared to other cars with higher gearing. That said, once you do get both of them moving it can start being a handful. That's were the PSD shines in my opinion, it is such a nice controlled lock of various amounts with some electronics to watch over it.

Cameron

I was surprised at how active the PSD system was when I did my DE. During the wet session, it was 'helping' me on almost every corner and on many straights under accel. Even during the dry runs, it appeared at times that I would not have expected. For example, the right hand third gear sweeper under full acceleration while running up from about 50 mph to about 90 mph. Also, there is a third to fourth gear change there at about 90 mph. I swear that the PSD light flashed when I put in the clutch doing that gear change on the sweeper at about 90 mph.

.......Cameron
'91 Euro GT

Jim bailey - 928 International

Yes Cameron the PSD was helping you turn , it gets input from a lateral acceleration meter under the drivers seat (shared with the airbag system) .

Gretch

I have an opinion, don't know how valueable it is or not, I would say that for me, the importance of "posi" in a car is proportioned to the amount of power it has. In a lightly powered car it does not matter, in a heavily powered car the engine will easily over power the traction of one wheel in various conditions, causing it to spin with a loss of overall motive traction. "Posi" increases the amount of applied power required before there is a loss of motive traction, by as much as 100%, under certain circumstances.

Given the option I would install a PSD, and "pin" it to a preferred level of slip like Mark Anderson has in his car. Second best for me is LSD because of the simplicity and lower weight.

But that is just my opinion