Doug Ford

I have the opportunity to buy a 924s LSD for $1100. I was wondering if anyone had any thoughts.
Here is the situation. I currently run a non-lsd in my 87 944na in SCCA. I also have an 88 944 with Non-LSD that I can sell with the 87 non-LSD and probably come up with the money. My questions is!!! Is the 924s LSD a good one? Should I look alse where & Where & how much? How much should I pay for the 924s LSD?

Any and all help would be greatly appreciated!!!!!

BartW

<a href="http://www.clubautosport.co.uk/" target="_blank">http://www.clubautosport.co.uk/</a>
go to this link, they are in England and the prices are in British pounds so &*1.5=$ basically multiply by 1.5 or so. They have the LSD untit and gearbox for $750 or just the unit for $500 and it is from a 944

Danno

For $1100, you can get a brand-new Quaife or Guard torsen diff. Will work much, much better and last longer than the friction-plate variety.

Geo

[quote]Originally posted by Danno:
<strong>For $1100, you can get a brand-new Quaife or Guard torsen diff. Will work much, much better and last longer than the friction-plate variety.</strong><hr></blockquote>

From what I have read, all the Porsche racers who have tried an ATB diff have switched back to the clutch type diffs.

The folks I know doing DEs and racing with FWD love the Quaife, but like I said the folks on the racing list and forum have given the ATB type the thumbs down.

I'm not trying to argue with you Danno. God knows I respect your opinion. I'm just confused now.

Danno

"I'm not trying to argue with you Danno."

Of course not! We've just got our data from difference sources, using different cars, coming to different conclusions...

"all the Porsche racers who have tried an ATB diff have switched back to the clutch type diffs."

These are most likely the 911 folks. One thing the torsen doesn't do is axle lock-up under braking. Having such a function from a friction-plate diff provides a more stable, easier to handle car at the limit of braking. However, I take the tack that a differential is not a suspension member and shouldn't be used to fix another problem from some other part of the car. If you have to use a differential as a suspension member, you really should be asking yourself why you've got most of the weight of your car hanging out behind the rear axle to begin with. This causes a twitchy rear-end at maximum braking as well as threatening to lock up the front easily and throwing all your steering inputs off.

Now when used only for its intended purpose, that is, allowing differential rotation of the rear wheels around corners, nothing can beat the torsen design. It provides super-smooth, linear torque splitting from 50/50 in a straight line, up to about 80/20 in the corners with no sudden lock-up twitches, or stepped splitting of power. In order to get that kind of a torque-split with a friction-plate diff, you have to use a high spring-preload which effectively gives you a locked diff. This causes understeer coming out of corners and in the case where you are able to lay down enough power to overcome the spring-preload, the actual range of limited-slip, friction-plate action is very narrow. It will go from locked-diff 50/50 split through the limited-slip range right through to 80/20 where it becomes an open-diff and spins the inside tire (and wears out those friction-plates quickly too).

If you had the finesse and a corner that was set up just right, you can keep the car, its power and cornering line such that the friction-plate diff is right on the edge of the 50/50 locker-diff mode and the friction-plate slipping mode. An example of this is Turn-2 at Willow Springs up the hill. In this case, you'll notice that the back end of the car will wallow back and forth as the diff. goes between those two modes. A torsen will not exhibit any such rude behavior and be perfectly smooth from 50/50 to 80/20 or whatever bias ratio is dialed into the gearing.

Also someone several months ago posted heat-soak temperatures of the diff. after pulling off the track and the torsen was about 20-degrees lower than the friction-plate diff.

Remember, we don't drive an ancient VW on steroids, there are advanced options out there that don't have to fix mistakes of the past...

Geo

[quote]Originally posted by Danno:
<strong>These are most likely the 911 folks. One thing the torsen doesn't do is axle lock-up under braking. Having such a function from a friction-plate diff provides a more stable, easier to handle car at the limit of braking.
</strong><hr></blockquote>

OK, that makes a lot of sense to me. Yes, I did notice it was all 911 (or derivitives) drivers, but until recently, that was about all I've seen post on the racing forum (with the occasional 951 driver doing DEs).

I can see now how the twitchy 911 would like the greater stability under braking given the peculiar weight bias and handling of that car.

I was just assuming it was a RWD thing since almost all of my serious driving in the last 15 years has been FWD and karts (which reward a pretty agressive style if set up right).

[quote]Originally posted by Danno:
<strong>Remember, we don't drive an ancient VW on steroids, there are advanced options out there that don't have to fix mistakes of the past... </strong><hr></blockquote>

LOL!

Thanks Danno. It certainly opens up my options. Any idea regarding improvements in lap times? I've some friends who have picked up as much as 2 seconds a lap in their SE-Rs on minute, minute-thirty tracks, and the SE-R comes with a VLSD from the factory!

Kevin M

Hey Guys,
I went through this dilema (LSD vs. Torque biasing) about 6 months ago. If you call GT, Guard Transmission) Paul will send you out some great comparative info between the two. GT sells both types of units and will not decide for you. To summarize the info they send out, if you primarily drive on the street or auto x, then torque biasing is probably the way to go. If you are primarily on the track, then LSD. My car is a dedicated track 951. So it was a clear choice for me.
As much as we all value and respect Danno's input, I may have to disagree with him a little bit here. Yes, 911's have their own particular problems, but any thing that allows you to brake a little later and stabilize your car during braking and initial turn in on the track is very useful. Another nice aspect to the GT and I guess all LSD's, is that when one wheel lifts off the ground, eg. running over some curbing or candy stripe on the track, power is sent to the wheel on the ground. With a torque biasing (quaife, gt. etc.) this will cause all power to transfer to the wheel in the air just like an open diff.
A last thought, if you enjoy driviing on the track, get something other than the open diff. It made a tremendous difference in my car as I was able to do a before and after comparison (no LSD from the factory)

Kevin

Geo

[quote]Originally posted by Kevin M:
<strong>To summarize the info they send out, if you primarily drive on the street or auto x, then torque biasing is probably the way to go. If you are primarily on the track, then LSD.
</strong><hr></blockquote>

Since mine is a dedicated race car, o that at least makes the purpose clear.

[quote]Originally posted by Kevin M:
<strong>A last thought, if you enjoy driviing on the track, get something other than the open diff. It made a tremendous difference in my car as I was able to do a before and after comparison (no LSD from the factory)</strong><hr></blockquote>

Kevin, thanks for the input. What lockup % are you running and have you noticed any of the locking/unlocking situation Danno mentioned?

Doug Ford

Ok, it sounds like the factory unit is not the way to go. Thanks <img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" />

Now the discussion goes on & on & on

Thanks for everything and keep it coming!!!!

I have a guy who wants to buy one of my non-lsd for $500. He is going to trade out some work for it as well. <img src="graemlins/jumper.gif" border="0" alt="[jumper]" />

Danno

"is that when one wheel lifts off the ground, eg. running over some curbing or candy stripe on the track, power is sent to the wheel on the ground. With a torque biasing (quaife, gt. etc.) this will cause all power to transfer to the wheel in the air just like an open diff. "

yech, that's yet another rationalization used by the 911 folks to justify their tail-waggy cars. Ignores the following:

- most of the time, you're not laying down maximum power at the apex of a turn when you're going over the berms anyway.
- locking-action from a diff. under braking messes up trail-braking (ANYONE who's fast uses trail-braking)
- an unloaded inside tire that has zero load on it can still be in contact with the ground, yet provides no traction anyway.
- the actual amount of time the tire spends in the air is minimal (milliseconds).
- there is no such thing as a 0/100 friction-plate diff. asided from a welded locked diff

Basically, with a friction-plate diff, it too will spin the inside tire when you lift it off the ground like a torsen unit.

924RACR

Yeah, I don't want the car stabilized under braking, I turn with my left foot!!!

OK, so hey, what's the pricing like on the GT units, LSD vs torsen-type? Anyone know? I'd love to have one, but don't know that it's in the budget for this winter...

Also, what's the install like? I do all my own stuff, have torn down boxes and all, but haven't yet swapped out a diff. Do you just need to bolt in and torque properly, then maybe check the preload on the diff carrier bearings?

I've got 3 cars which could all use diffs...

David Salama

I called GT a while back and I believe the torsen units were about 1200, and the LSD cluch type 1400 approximately. Not a big difference. They said the greatest lock up from their units are 50%. I was looking for an 80% lock up, but have failed miserably in finding one. The 968 differential can be made to 80%, but it is not compatible with my 951. Does anyone have any helpful information. Thanks.

Steve Lavigne

[quote]Originally posted by Danno:
<strong>These are most likely the 911 folks. One thing the torsen doesn't do is axle lock-up under braking. Having such a function from a friction-plate diff provides a more stable, easier to handle car at the limit of braking. However, I take the tack that a differential is not a suspension member and shouldn't be used to fix another problem from some other part of the car.</strong><hr></blockquote>

With that logic, couldn't it also be said that the differential's function is to allow the drive wheels to travel at differing speed through corners and shouldn't be used to solve a weight distribution problem (especially with cars with not enough weight over the drive wheels) under acceleration.

Any car can get unstable under heavy braking if you have enough rear brake bias. If you have something to limit rear lockup, you should be able to trail brake further/harder into the corner.

Seeing as a second brake pedal system tied to the steering gear, as rumored to be used in Mclaren F1 cars, is not a viable option for most people, couldn't something that allows you to trail brake further and harder into a corner without locking up the rear wheels be a good thing.

We often see Schumacher modulating lock on the inside front wheel while trail braking into a corner. Instead of seeing this as an error, this is considered optimal use of the available grip of the loaded outside front tire under trail braking, especially for a qualifying lap or an in lap. Benneton F1 actually tested, or possibly even raced with a front driveshaft with some sort of limited slip differential to discourage front lockup several years ago.