Limited slip diffs

[Captaintremendous]

Gents.
I'm after some recommendations for a a LSD for my historic tarmac rally car to be, currently nearing the the end of its conversion from track to rally. It's a 1969 2 litre 911. Kaaz seem popular, ZF for the traditionalists but rare etc etc. if you were in my shoes, what would you fit and why? The car will be raced on closed public roads 99% of the time, over here in France.

[hot66]

Hi

Quite a few discussions on this over the years and I'm not sure we ever got a definite answer

Here's a start

viewtopic.php?t=48519

Q

[Mike911scrs]

HI Captaintremendous

as your car is a 1969, which gearbox are you using, and which drive shafts have you on the car,

if you have the early 901 box (ally' case) you will have a choice of Gripper or Quaife.

if you have the later 901/911 (Mag case) you may have the choice of LSD; GT, ZF, Kaaz, Carbonetics, Trackform, but this will depend on the output shaft splines and drive shafts you have on your car, if you have the later gearbox with the later diff' with the larger diff bearings, any of the diffs above will fit, or adjusted to fit.

the other thing to look at, as you will be on the "closed public roads 99% of the time" is a 904 input-shaft and 2nd gear, if you are using 1St gear a lot.

regards mike

[Captaintremendous]

Hi Mike
I have the later case 901/911. 1st won't be getting a lot of use apart from at the off so the dogleg box will be used as intended.
Thanks for your advice, there's a couple there I haven't heard of before. A bit of light internetting required.
Seems to me all plate type diffs are much of a muchness and it comes down to personal preference as opposed to one being far superior to another. A fair comment?
Russ

[Darren65]

I cannot personally comment as I've not used it yet but I went a LSD from Guard Transmissions.... http://www.guardtransmissionllc.com

Mike suggested it would be a great option and it has a number of recommendations on the Early 911S Registry. Some good threads on there discussing the pros and cons of different diffs.....

http://www.early911sregistry.org/forums ... mendations

[Mike911scrs]

HI Russ
you need to look at each diff,
1X for availability
2X price
3X is it adjustable for what I require
4X fit to the gearbox I have, in the box and the out-put shafts, a later 915 diff can be fitted into a 911/915 early box
5X spare parts availability after use

some diffs are adjustable for Pre-load and ramp angles, for take off/cornering and braking, some are pre-set.

my observation on diffs I have fitted;

GT;
fitted a few of these and had good feed back once the diff is bedded in, also I have not had to repair/re-set one,
diff has adjustable ramp angles, the diff comes ready to fit and pre-set for your car, just let GT know your car details
This diff will take STD Porsche drive flanges, both coarse and fine spline

ZF;
Early ZF
have worked on a few of the ZF Porsche diffs, early one's have a longer output shaft to the STD open diff and some times the shafts get changed which damages the face of the diff where the bearing s fit, it is re-repairable, long shafts are hard to find,
Later ZF
the later ZF diff is a better diff with fine splines, parts are still available to repair/re-build them, Pre-load can be adjusted, from 40%-80%
3.2 ZF
this diff is a cast steel diff and has only a small (as in No' of plates) set of plates, has a set pre-load of only 40%

Kaaz;
have fitted this diff, and had no feed back from the owners/drivers, I have fitted one and the gearbox ran for app' 500 miles, came back in for a ratio change, the diff pre-load was re-checked and had dropped off to just short of zero, info is limited and at the time there was no spare parts.
This diff takes STD Porsche drive flanges

wavetrack (had the name wrong on the last post)
customer supplied this diff to fit, it has plates and a gear set in, this had been run for track days and found to work well in a 911, the feed back is encouraging, do not know if spares are available, paper work/info is good.
This diff takes STD Porsche drive flanges

Carbonetics;
only fitted 1or 2 diffs, fit OK and have had no feed back to say that they work OK,
This diff takes STD Porsche flanges, Late fine spline

Gripper;
have fitted this diff to 901 and 915 gear-boxes, they do need work if they come back in the shop, usually lost all pre-load and at slow speed are a open diff, they need re-packing, spares are available, there is a updated version for the 901 box.
This diff for the 915 takes the STD Porsche drive flanges, the 901 needs flanges supplied from Gripper to fit the small early drive shaft CV joints.

Quaife;
this is a torque biasing diff with gears in it, no plates, this diff I have fitted and works well on road/track, I have been told it is not good in a Porsche on the loose, I have had 2 customers run this in stage rally's off road, both with 3.0Lt engines and had no prob's with the car handling.
I have had no diff's stopped working or needing any repair work, as with the GT and ZF the later diff can be fitted into the earlier gearbox OK,
This takes the STD Porsche drive flanges, coarse and fine spline.

hope this my help

one last thing, re-place the gear oil with Castrol B373 oil on all the diff's apart from the Carbonettic diff, as I cannot remember what oil was recommended for running this diff.

Change the oil more due the the diff heat when you are competing

regards mike

[redrobin911]

Chris at Fennlane now has a plate diff for 901 transmissions fitted with Lobro flanges - looks like it good be very good?

Robin

[Captaintremendous]

Mike
Thanks so much for taking the time to post such a comprehensive reply. Plenty of food for thought there.
Feedback that I have gleaned is that Tuthills use Kaaz in all their cars and have done for the last five years without complaint and seem to rate them highly.
From what you say, GT seem to be a good bit of kit. Where can I get hold of one? If it has to come from the states I will get wallet raped by the French!

[fetuhoe]

The KAAZ is readily available but as far as I know only with side gears that suit the 'fine' 43 spline output shafts of the later 915 Transmission so you will need matching output flanges.

The bearing housing is also the correct length to accept the magnetic speedo sensor of later 915s and you may need a spacer to position the bearing in your '911' box correctly.

The KAAZ has a cast end plate and these have been known to break but normally with very hard use.

As Mike says the Wavetrac is a geared Torsen type diff it does have a preload feature but this device doesn't really function in the same manner as the coast ramp on a plate diff so in general terms this diff will only affect traction during acceleration and doesn't help to limit TTO. It does prevent the slight time delay that can occur with a Quaife or other Torsen type diffs after they have been completely unloaded by lifting wheels off the ground.

Grippers do tend to need significant preload due to the type of plates and this can introduce understeer.

There are other designs that use plates with different friction characteristics that allow the use of less preload but they tend to be costly.

OS Giken - 915 Type but again will need 'fine' spline flanges

Drexler Motorsport - very expensive but looks good

Drenth - Netherlands based

Baci Romano - Italy

There is also a Swedish Company making a unit with adjustable preload and I have just requested a detailed spec but again I think it is a 43 spline unit.

[haasad]

No expert here , I can add that Tuthill can supply a fine spline output drive flange , also I agree that a gripper does make, at least my car quite understeery unless you are very committed with direction change , I have learned I like this characteristic.

[Nick Moss]

I have Kaaz with factory supplied settings, it is very tight and makes for understeer unless one is very committed. Softening the diff will lessen the tendency.

[johnM]

My ZF LSD that Mike Bainbridge built and fitted to my 911 gearbox does not understeer, its a joy to have in the car. Well worth the expense.
Kind Regards

[Captaintremendous]

Fetuhoe mentions the Swedish outfit. I don't know anything about them including their name, but the LSD they produce is supposed to be excellent.

[Captaintremendous]

Gents
An update for those that are interested. I eventually went for a Kaaz unit, which is now fitted and the car is on the road again as of lunch time. I have heard various opinions on the Kaaz recommended break in procedure, which involves doing figure eights for half an hour. Kaaz say it is essential, others such as Tuthills say they never bother and just run them in on the twisties using a bit of common sense. Finding somewhere to do figure eights for half an hour is not easy in itself, neither is the prospect of actually doing it. Best have a bucket in the car!
Opinions gents, to do or not to do?

[fetuhoe]

Normal cornering will break it in without any issue.

I have been using Plate Type LSDs on a range of Rally Cars since the late Sixties and never ponced about doing figure of eights or any other break in procedures - you just need a few gentle miles (50 or 60 should be OK) to let the plates bed in and it should be good to go.

Obviously drive it down twisty A or B Roads.