So you don't think that G-sensor knows it is still in a corner, when the vehicle is in the middle of a "long" corner?

For a PSD system to really fuction properly, the brain needs a bunch more information. The most difficult information to obtain would be the actual "road distance" that each tire is traveling. And the really, really tough thing that the brain needs to know is those distances....ahead of time, so that it can provide the proper lock percentage, before the vehicle is in the corner.

Here's the brutal reality of the whole thing....Porsche took the time to engineer and develop the PSD system and installed the pieces, in the first vehicles, over 22 years ago. Add in some time for development....which means they started on this project over 25 years ago?

I don't think I've ever seen a Porsche Factory race car with a PSD system installed.

And beyond that, the current generation Cup Cars and RSRs still use a pure mechanical limited slip. Granted it has "evolved" about 15 generations from a 928 limited slip, but it is still mechanical.

Tell you anything, at all?

 

I find this hard to believe. Most club racers wouldn't notice that the inside tire is going up in smoke, the engine is bouncing off the rev limiter and they're not making forward progress? I usually notice that when I'm on a track in a car w/o a LSD.

I'd say anyone w/ even a couple track days under their belt would notice. IMO, open diffs are obnoxious to drive on the track.

 

You know, opinions are like a... In my experience the majority of people do not know that they were missing something by not having an LSD until they actually have one. This is part of why so many b-grade LSDs and TBDs (like Quaife) will always be popular. The change in lap times are immediate and measurable. Even a crappy LSD will be better than an open diff. Actually learning to use it and manipulate it to maximize lap times takes years.

Now keep in mind that 90+% of the people I cross paths with drive 911s. The vast majority of 911s left the factory without an M220 optioned LSD because the reality is that the rear engine design gives the car phenominal traction and as the chassis has improved, the "need" for an LSD has diminished over time.

I've driven probably a sum total of 3 hours behind the wheel of a 928 in my life. And none of those hours have been on track. However, I suspect that like many other v8 powered RWD cars they suffer miserably when they lack an LSD. But that's not who was being referenced in the comments about your average run of the mill PCA/POC Club racer. Show up at any Club event and most of the cars there are some form of 911/Carrera. As I have said previously, you 928 guys have a unique chassis and my generalized, 911 biased comments probably don't apply to you guys. With that, I will agree.

 

Matt:

In the "original thread" I pointed out to C3 (before he was totally consumed and screaming that I was picking on him) that the movement of the rear suspension in the 928 model, when accelerating and de-accelerating, was going to "influence/delay" the action of the limited slip.

I think that this will be a very important piece of the puzzle.

 

Isn't the "kine-matic" rear suspension design of 993's a near copy of the Weissach axle? I wonder how that effects the workings of the LSD vs the old school trailing arm suspension?

 

Mike Simard had said that it there was no need to take the comments being made as elitist, but I'm sure you can understand how your comments could be taken that way when all the references you were making about LSD performance related to a 911 variant or a Cayman, both cars which have nothing in common with a 928, and how the advantages of these modern diffs could only be realized at the pro-level of these cars. If that's the case, then why would a 928 owner bother paying a premium for one?



In designing a drop in LSD for the street yes, but for the track, I don't know if you could design for any "standard". I think if you went to a grid full of GT3 or Caymans, you'd find substantially the same chassis with a difference in springs, shocks, and alignment adjustments. You're range of chassis influence on the diff would at least have some kind of range.

If we took all of the known current 928 race cars and gridded them, you would find: stock weissach, pinned weissach, modified lower control arms that no longer have the weissach spring blade, stock bushings, aftermarket bushings, no bushings, altered suspension pick up points, etc. If tuning an LSD's is that precise a science, I don't know how you could zero in on a baseline since there really isn't one.

I think if you would be hard pressed to find any three 928 race cars in a single race that are running a Weissach rear end that is configured the same with just different shocks and springs.

 

I'm not sure what you are trying to say, here.

Are you trying to say that one of the "trailing throttle locking" differentials will be hopelessly complex and won't work in any two different track 928's?

If so, you can relax. Professional race teams are looking for improvements in hundreths of seconds and will tune for endless hours to try and "find" the perfect set-up (which very rarely happens.)

A "generic" GT Gears 45/65 trailing throttle locking differential with "soft" locking and release would be "light years" better than anything we've got to play with, currently. Literally, seconds per lap....with the rear suspension in any configuration....and especially with ABS.

 

BUT I don't have ABS........

 

I don't know about LSDs affect on a race car, but I've heard it sure is a trip

 

Okay, I'm with you now.

So here's an application question, one that probably separates the DE drivers like me from the pro racers.

My S4 has ABS, but I rarely get into it when on the track. With an updated diff, to see these improvements in lap times and take advantage of the newer technology, I'm assuming that my driving style would have to change so that I'm actually relying on the ABS to turn the car while trail braking late in the turn?

 

Adam an interesting thread for sure, there is a few technical facts and actual "reality" facts that should be included, to address your post I need to presume you are not using street tyres. So that presumed a 928 cannot use the factory abs on a race track and take the car to it's limits. If you took the car to it's limits you would have hit the "ice mode".

So if you are not taking the car to the limit in braking, can we presume you are not taking the car to it's limits through the corners? That is a question for yourself and not for me. Of course offense is intended just an answer to your question. If you are not at your at the limit of your current setup you will not benefit from a better LSD.

 

I wish to address all interested readers to this post with my answer and not just the writer "Greg" th my answer.

With regards to knowing "before" the corner, I would say that yes and no, today's race cars have super fast processing with their on board computers which will provide real time reactions, given previously they had predictive (mapped) traction control via GPS in the early 2000s in F1 competition before it was banned.

The differential is manipulated via a complex algorithm, it can include a latitudenal and longitudinal sensors, brake pressor sensor, wheel speed sensors, throttle position, some will also use steering angle although in F1 that is not allowed. Ferrari in their road cars also tie it into the shock absorbers and F1 Trac traction control. I have read that it involves around half a million lines of code and that brings me to the point of why Porsche wouldn't use it currently. It is just too complex and costly. Although in some models Porsche have adopted McLarens active brake control, know in F1 as the third pedal.

There is major benefits to the active or (PSD style) differential. In fact engineers have detailed (via SAE papers) that the active differential has around a 5%+ lateral acceleration advantage through corners when compared to the best LSDs.

It I suppose it can be best summed up by the fact it is easier to control mechanisms via electronics than it is mechanical controls. I suppose it becomes like the complex mechanisms that are built into Swiss watches versus a japanese watch with all the bells and whistles built for a fraction of the price. With the diff it is not cheaper but just much better.

 

Reminder: 928 with three-channel ABS treats the two rear wheels as one. A slipping/dragging rear wheel under hard braking causes brake force to be reduced to both wheels equally, until neither wheel is slipping relative to fronts. The ABS code obviously has some latitude built in to allow for normal cornering, at least until the PSD cars that have the lateral accelerometer. I don't know that the accelerometer is used in any ABS decisions even on those cars, since it's still only three channels.

 

I guess one might read it as elitist is one presumes that I consider 911s and Caymans somehow inherently superior to 928s, which I do not. Read my comments again. I did NOT say that the performance advantages would only be realized at the pro-level. In fact, I said just the opposite. I said that while many people who track their cars wouldn't really feel the difference, the instantaneous change in lap times would be all that the individual would need to become one of the converted. When you put an LSD into a car that previously didn't have one, you get a "free" couple of seconds a lap. The difference between the pro and the amateur is that the pro knows how to get that last additional second out of the LSD, whereas the amateur is now in a position to learn something new about racecraft and spend the next year or two refining their skillset as the figure out how to get that last second that the pro was getting the first time he climbed into the car.

Or maybe people will take our whole business model as elitist. We make parts for racing. It is that simple. There will always be a segment of the enthusiast market that decides to put motorsports parts into their "toy" cars, be that car a back road barn stormer or a DE special. But that's not who we make our parts for. We make our parts for racers. If that makes us an elitist company, then we are guilty as charged. We will never make a product for the least common denominator. The Walmart model of sell as many units you can of whatever you can to make the most profit doesn't jibe in my world.

However, in closing, I will leave you with a question to ponder. If I was an elitist, why would I even consider making an LSD for the often overlooked and frequently unloved 928 knowing full well I am probably only going to sell a couple of them a year? I'll tell you why. It is because like you, I am an enthusiast who loves cars. I personally would like to own a 928 one day. I think it is a very cool piece of engineering and an impressive piece of Porsche's history. Unlike the big boys who don't want to make something unless they can ultimately sell 50 or 100 units or more, I'll make the product because I think it's for a cool car. That's my advantage as a business. I can choose to make whatever the heck I want, even if it doesn't make sound financial sense. A 928 LSD is not a shrewd business decision. It's not one that an elitist looking to maximize profit would make.

 

I think it is important to realize that we are dealing with a very old design vehicle....and certainly each of the systems installed have undergone radical changes/updates. Certainly ABS systems are one of those things that have undergone radical changes. While the "current" high performance ABS systems perform quite differently than the ABS system in our 928's, I'd certainly encourage you to "explore" the benefits of your own ABS system. Some clues....when you "feel" the brake pedal begin to bounce from the ABS system being activated, push harder...do not "release" the brakes.

A current race driver, when driving a vehicle that has a current generation ABS, is certainly using that system, at all times.

As a crew chief/set-up person, I can tell you that unless the rubber chunks trapped inside the wheels are smoking, when the car comes to a stop at the pit wall....you aren't using the brakes enough. That is one of the very quick ways that I use to "judge" the ability of a driver.

Over the next year, I will be "assisting" in the construction of a brand new 928 track vehicle. We certainly will be "updating" the ABS system for higher performance use and will certainly be encouraging the owner/driver to use ABS virtually each time he uses the brakes.