PFR

Hopefully I don't lose everyone at the title.

I know there is a ton of information already on the two styles of diffs and I have done my fair share of reading. I also own non pcars with both styles of diffs and drive both in competitive race series. My question is for those people who race their 951s looking for the last few tenths on the knife edged limit, why do they run the diff that they do?

The majority of track enthusiasts seem to use a GT with a 50/80. This has to be one of the few front engine cars that I have seen utilize such an aggressive lock-up under decel. I can understand the added stability benefits inherent to a clutch diff locking under decel, but is the 951 so prone to corner entry (trail brake induced?) oversteer that such aggressive lock-up is necessary? In my turbocharged miata track car I run a LSD with an aggressive lock-up under accel with limited engagement on decel. Even on this car with it's great turn in the LSDs decel engagement does exhibit a touch of understeer.

I will be running 18x10 CCW C14s with 285/30/18 Hoosier R6s all round an a track valved coilover system and upgraded swaybars with no ABS. I don't forsee the car being unstable under corner entry even with an aggressive trail brake and I can tune the suspension to keep the wheels planted enough for a torque biasing diff to be effective (I'd rather run and LSD, but don't like the 80% decel lock-up). Is there something about these cars I am missing that makes the 50/80 the better option?

Any feedback would be greatly appreciated.

Thanks.

88BlueTSiQuest

It's funny how often someone will post for you to search, and you find the answer was many years ago.

However, this was discussed not even a month ago... In the 944 Turbo forum IIRC.

lowside67

Jesus, dont any of you new users ever search for anything? The answer is obviously quaife.

BAN.



Disclaimer - Jared is my buddy, all in good fun.

PFR

Thanks for the reply. It's possible I'm not the best searcher, but I have read through a bunch of threads on the 951 turbo forum. Lots of info about using a Quaife for autox and a GT LSD for track, but nothing that really explains why so much lock-up on decel is necessary or any discussion on the flip side about how a non-lsd car rotates TOO much. As for a Quaife unloading and going to open on the track most of that can be tuned out with the suspension and I don't aggressively climb the curbs at any of the tracks in NWR, so I'm thinking that I want the advantage of good corner entry rotation or hoping someone will tell me why that is not a good idea?

Oddjob

They do seem to be a very good product, but I dont think that most track 944s are using the Guard LSDs. Cost of the LSD unit itself, and the cost of the install/rebuild is relatively prohibitive for the average DE/track 944 owner. I believe that Factory LSDs will be by far the most common, then various brands of Torque Biasing (mostly quaife) will be the next most common type of locking diff.

I am well aware of the arguments for/against LSDs vs torque biasing units for track versus autocross, and also the various loaded/unloaded locking rate options. But how all that corresponds to actual lap time improvement on a dedicated track car is a little more up in the air.

Also, consider when reading/listening to LSD discussion for porsche racing applications, most of that information is centered around 911 platforms. All the factory racing diffs and all the aftermarket diffs that are tested/used for professional racing have been in various types of 911s. And the weight bias difference with a 911 does give some different handling characteristics that do not apply with a 944 - so optimum setup of the diff will also be different.

As far as getting real world feedback from the 944 world on the different diff types: most guys are replacing an open diff with some sort of LSD or TBD, and notice a dramatic improvement over the open diff. The reality is, very few people will have run a quaife versus a guard LSD back to back, to compare.

Ive owned/tracked two 944s with factory open diffs, one with a quaife TBD, two with factory 40/40 LSDs, one with a custom/modified factory LSD that has a higher locking rate. I also have a spare trans that has a fully locked diff, but have not installed that one yet. But I have never done a specific back to back comparison in the same car, having swapped only the diff. Most of the cars had a progression of development, and it would be hard to attribute specific results/improvement to the diff only. But even so, honestly, the big difference is between the open diffs and any of the locking diffs. I have not noticed a dramatic difference in feel or performance between the quaife, the factory, or my modified unit.

The more loaded lock you have, the more the car will push under power. The more unloaded lock you have, the more the car will push on turn-in (if you trail brake). Much of those tendancies can be controlled by messing with other parameters in the suspension setup (especially when using adjustable shock valving).

Optimum diff setup will depend on the car, the driver, the track, etc. None of us have the money, the time, or the knowledge to swap in different lock rate LSDs depending on what track we are running. So we pick a typical setup, and drive it. Most of us dont have the experience, seat time, or talent to know if the diff is causing a handling quirk or not. If the car pushes a little too much here or there, we screw around with spring rates, shock valving, sway bar settings etc, and leave the diff. alone.

Having said all that, as far as recommending lock rates: I dont think I would go too far away from the factory 40/40, without having a very specific and founded reason for doing so. A little more lock loaded and or unloaded wont be dramatically different, so I dont think even the Guard 50/80 will make the car handle completely different. But I would not go with someting like 20/100 or 100/0 without having good reason to do so.

PFR

Thank you for the very informative response, I really appreciate hearing your experiences with the different setups available. You make a great point that the majority of the information out there is centered on the 911s where the 50/80s and even more aggressive differentials follow conventional race car engineering for that platform. That being said and having driven both a 944 and 911 at speed they are quite different beasts, both capable but requiring different areas of attention to improve them.

I suppose the best thing for me to do at this point is take the car out on the big wheels and tires and new suspension and see how I feel about the corner entry with the open diff; if I can handle having the aggressive rotation then the torque biasing diff would seem acceptable. If not, then I either need a nice ramped 40/40 diff or a factory option. Unfortunately after speaking with Mike at GT it doesn't sound like I can opt for a 80/50-30 which is what I think I would have built if given the choice.

I certainly don't mind following the crowd with something that works, but I would sort of like to know why . Thanks again for the information. I hope to be out setting up the new suspension by mid-April and will report back with my findings and eventually a pre and post diff install review.

vt951

Sounds like the thread iniator's question was answered very well. Hope you don't mind if I hijack it now for a different question about torque biasing diffs. I recently bought a Guard TB diff, and I haven't installed it yet. I was talking with a mechanic about it, and he mentioned that torque bias diffs (he said Quaife) are easily broken if you mistakenly put a small spare tire (donut) on one of the driven wheels. Obviously, this is pretty easily avoidable unless someone who doesn't know gets a flat when driving your car, but it still concerns me that the TB diff would be so "fragile". Is any of this really true?

joes

Putting a smaller wheel on any kind of LDS or TBD causes the Diff to fight the faster/ slower turning wheels 100% of driving time, a sure way to ruin any one of them in time.

Oddjob

Yes, but it will take a long time. The only time the wheels on a car are turning at identical speeds are if the tire diameter is exactly the same and if the car is going exactly straight, which is nearly a practical impossibility. So in reality, the diff is slipping to varying degrees all the time the car is moving.

The diff is reacting to different wheel speeds, side to side. It doesnt know or care why the wheels are spinning at different speeds - whether its because one wheel is smaller diameter, or that the car is turning a corner and the inside is spinning slower than the outside wheel.

Sure, wear can be accelerated by use: track use, driving in circles for days on end, using a space saver spare for a road trip to LA and back, etc...

Carrera 4s recommend that front to rear tire diameters be within a certain % to reduce center differential wear over the life of the car. Audi quattros recommend to use the same size tires front and rear (they have a center and rear quaife type diff - and a reputation of having a very reliable driveline). But the diff's wont explode if you put on a spare tire to drive to Tires Plus. On those cars, there is more issue with the traction control systems identifying different wheel speeds and falsely thinking its a slip condition (tripping the ABS), than actual wear/failure of the differentials.

FRporscheman

My dad's C4 has major awd system issues due to the PO putting on gigantic 18" wheels and stretched tires to fit in the fenders. The center diff makes noise and kicks. I have no idea how long it was run with those wheels or what else the PO might have done to it to cause the damage, so I don't want to cause paranoia, I'm just confirming that as far as our C4 is concerned, wheel size mismatch can lead to issues over time.

Cory M

there is a good article about differentials in this months Velocity (Porsche Owner's Club mag), you can see it on the web here:

http://www.porscheownersclub.org/pdf...ty_Vol53-1.pdf