LSD - plate or torque sensing?

[willdew]

(Repost from the Porsche forum, but no one there seems to be able to help):

I've got a 968 and looking to add a limited slip diff. The car's used both on the road and track, but obviously the LSD purchase is being driven by track use. My symptoms prompting me to think I'd benefit from one is that I struggle to get the power down out of 2nd gear corners, as I spin the inside wheel. Also, I think I'd benefit from added stability under braking, going into corners (plate diff only?).

My question is, what sort of diff to go for? A plate or torque sensing one? I don't know what types of diff I've experienced in other cars (on MX5's, E46 M3s, 200SX's etc). The LSD L220 option which Porsche offered for the 968 could be either a plate or tor-sen diff, depending what they had in stock at the time. Obviously, mine doesn't have this option fitted.

Torque Sensing:
Pro: Does not cause understeer when going into a corner
Con: Am told it can cause a "shuffling" effect when shifting power coming out of a corner

Plate type:
Pro: Smooth power application out of bends
More stability under braking
Cons: Causes understeer under breaking
Marginally more expensive than tor-sen type
I'm told they can be noisy

Are my pros and cons about right? What are other's experiences of adding a diff, and how did you choose what to go for? And if you have any thoughts on the above, any particular manufacturers to recomend/avoid?

Edited by willdew on Friday 11th April 16:51

[andygo]

I used to run a Salisbury plate type diff years ago when I was rallying. When the plates were new the diff would be very 'tight' and creak and groan on low speed tight turns, especially when parking for instance. As the miles clocked up, the plates would wear (IIRC they were called belleville washers)as the tension went out of the slightly coneshaped shims and the lsd action became weaker.

I also rallied an ex works RS1600i wih a viscous diff. Itwas ok-ish until I had it rebuilt by FFdevelopments who manufactured them originally. After that it was a beast, the diff was so tight. After I sold it, the new owner hit a kerb on the Promenade Stages and was still doing 80mph with the n/s front wheel jammed into the wheel arch.


I also used to race/rally a fwd car fitted with a Thorsen diff. Soundless in operation and with nothing to wear out, it was smooth in operation. IMHO the bet bet for a road/track car. Talk to Quaife who make them. I think they can make them to suit your requiremens, but I would have thought with a Porsche it would be a big adantage to have a maintenance free diff.

Edited by andygo on Friday 11th April 16:42

[LocoBlade]

For your purpose Id certainly go with a torque biasing diff. One big pro that you haven't mentioned (and conversely a con of the plate type), is that the ATB diffs are pretty much fit-and-forget and need no regular servicing, whereas plate diffs wear with time and need fairly regular maintenance to keep them performing at their best. For a race car that's not such an issue for many, but for a road / track car thats going to do lots more miles, Id stick with an ATB.

[anonymous-user]

I disagree. Torque sensing diffs are poor on track in RWD, especially where you are kerb hopping and unloading a wheel. Also plated diffs dont need that much maintanence, lots of road cars have these fitted as standard and can go for 100K miles without a rebuild. They have the major benefit if you so wish to alter the preload, which allows you to alter the handling, some plated diffs also allow you to have diferent lock rates (ramp angles) under power and under acceleration, so you can tune the braking and corner entry charicteristics to your driving style or environment.

For a RWD car i would go for a plated diff every time if the choice was mine, as a front diff in AWD/FWD plated can be hard work and more suited to gravel, you are better with a torsen diff at that end of the car.

[LocoBlade]

Which cars have plate diffs as OEM fitment John? Im genuinely interested as although Ive not used a plate diff personally, my uncle does a lot of race prep and he's always said that although the best solution when perfectly set up, plate diffs need fairly regular adjustment to keep them at their best, and do need tweaking to suit the car. Thinking about it though I guess it depends how you use it really, a plate diff in a road car won't actually see any action very often so probably won't wear that quickly in terms of mileage covered.

Agree with the bit about curb hopping but Im not sure many will do that all too regularly on a trackday in a road/track car. Certainly in my Locost BEC, the Quaife ATB transformed the car on track compared to an open diff and Ive not come across any of its potential shortcomings in real life, although that may possibly be due to less suspension travel and less roll in a light Locaterfield I guess.

Chris

Edited by LocoBlade on Friday 11th April 21:52

[Dunit]

My Boxster S had a plated Diff option , Still good after 70K , Great drive out of the hairpin at snetterton. Also this is what Porsche fit to the GT3s.

[Pilky]

Id agree that a plate diff would be best. The ATB needs both feet on the ground to work so to speek. Ive driven out of a stage with a busted driveshaft using a ZF plate diff where as we would have been screwed withan ATB.
Jaguar 4Ha and independant rear ends runs plate diffs I think. They provide a whole host of ratios. One downside is the diff is very heavy but you would be hard pressed to break it.
The alfa 75/gtv transaxle has a plate diff, so to 2.8 capri special.
The obvious rally axles, the atlas (as in the capri), the english, Talbot sunbeam, toyota etc all can get plated diffs.
Im sure some Porshes run LSD's as standard dont they???

[anonymous-user]

Yep, plated diff in a lot of the 911's. A friend of mine had a new box fitted to his GT3 under waranty, they realised after the change it was the normal noise from the plated diff, they all do it. doh.

My STi5 RA has a Helical front diff (torsen), variable clamp load plated electromagnetically controlled centre diff and conventional plated rear diff as standard. You could spec a plated front diff for if you wanted to gravel rally the car. You can buy off the shelf a diff rebuild pack that has various thickness clutch plates which allows you to change the preload to suit. I did an enormous amount of miles on track with my last RA, and didnt need to rebuild the LSD, i wore the CWP out first. That LSD is at over 100K miles now and still used regularly on track.

Once you get into plated diffs, they are quite a simple device to work with, yet very versatile. On the Hewland race boxes i run diferent ramp angles that changes the lock profile for power, deceleration. You can also do tricks like run the bevel washers facing inwards or outwards which alters the preload, and also change the number of active clutch plate faces that are working by the way you stack the output/input plates together, having more or less plates working in friction to control the ultimate lock rate. The diffs in these cars get checked regularly, but they are handling 800lbft with sticky 16" wide slicks and downforce, it's a totally diferent environment to a road/trackday car that require zero maintanence other than regular oil changes in my experience.

You can play about with the lock rate on a road/trackday car with a plated diff just by using a diferent oil grade, i like a tight rear LSD so run on a shockproof plated LSD specific high viscosity oil, running the same diff on a lower viscosity oil slackens the diff off noticably.

[thunderbelmont]

I run a "True Trac" ATB diff in my thundersaloon, mostly because there are not many decent plate diffs that take 600lb/ft up their jacksy and live for very long. It's true, they're not brilliant when kerb hopping, but generally very forgiving.

In my ASCAR, it's got a Detroit Locker. A simple dog engagement diff which locks under power. I drove one at Cadwell today, and expected to have all sorts of problems with it's lock/unlock nature, but nothing at all. A pleasure to drive. Detroit lockers are cheap and reliable too. Not quite fit and forget like the ATB, but less in need of a "look" than a plate diff.

I have a plate diff in my FWD Nova, and it works well. I found it better than an ATB over kerbs. On a wet track, where you keep off the kerbs, there was nothing in it.

[anonymous-user]

Detroit lockers are more likely to wear out than plated diffs IMHO because they rely on dogrings, and the dogs have to jump accross each other with a spring presure against the dog as it does it, which is where you get the judder from whilst they work. When detroit lockers fail they tend to lose you all drive as you lose all the dogs off one side of the diff output assembly, when plated diffs fail they tend to break the bevel washers due to fatigue cracks at the tab points, which just slackens off the diff preload, they still lock up under load so you dont lose drive.

Detroit lockers dont lock under power, they are perminantly in a locked state even with no torque input, rather like a welded diff, as they generate drive from the dogs being engaged with each other. You get an effect of a non welded together diff when you have a diference in rotational rate from one wheel to the other as that makes the dogs jump over each other on the side that is rotating faster. Thats why they are a bh to roll around the padock with any steering lock on, and why they tend to make the tyres drag accross the surface in large radius corners as it takes a fair bit of force to make the dogs jump.

They can be a cracking diff in a powerfull RWD car, but they arnt as subtle as a plated diff and no where near as tunable, they tend to induce more high speed understeer than a plated diff too. They work quite well on low grip tyres like historic treaded tyres, because you get a lot of tyre scrub due to lack of traction from those anyway so you can slide the car into the corner using the heavy locked rear end similar in effect to a lot of trail braking.

[GavinPearson]

I would go for a plate instead of a torsen / Quaife if performance was my first priority.

You do need to make sure you select the correct type and pack the plates correctly, but you should be in a much better position to handle wheels lifting off or high differences in mu with a plate diff.