Limited Slip Diff Questions

jpachard · 05-30-2002, 05:44 AM

All, I have an '87 Carrera I'm prepping for the track and want to install a Limited Slip. I have contacted PMNA( Porsche Motorsport) and they have a 40%/60% lockup one they recommend for my application. Does anyone have any experience with these or any insight regarding these Diffs??

Cheers, James

gumba · 05-30-2002, 06:33 AM

Everyone I talked with suggested a limited slip w/80 % lock up. I suggest you call a few places that specialize in transes, gears, diffs etc. such as PowerHaus II, Guard Transmission to name a few. They both advertise in Excellence.

ErVikingo · 05-30-2002, 06:46 AM

There are several types of LSD's; mainly clutch type (ZF) and torque bias (torsen type). Clutch type require rebuilds (I think they last long though) and torsen (as in GT and Quaife) do not.

Personally for a Carrera, I would use a GT or Quaife, just install it and forget it. This type is friendlier for street use than the clutch type.

We had a ZF rebuilt once and made so tight that the engine would die when you tried to park the car, you waould have to rev it real high to keep it going. We took it out the next day...

Sources, GT (Guard Transmission), Quaife (see Excellence or Pano), Carquip, .....

jpachard · 05-30-2002, 08:37 AM

All, Thanks for the input. The one PMNA has is a ZF type. I gusee I'll keep poking around and get some info on the Gaurd. I have heard of problems with the Quaife!!??

James

John Rogers · 05-30-2002, 08:48 AM

I have a Quaife limited slip in our 930 and in the close ratio gear box in my race car. Both have been very reliable and trouble free. I had a standard ratio gear box built a couple of months ago and had trouble getting a Quaife at the time so we used one of the GT (Guard) units. The machine work and finish was much better than the Quaife and has worked well in the two races since. The other aftermarket unit is one that PowerHaus II is selling that is the same as a GT and price is about the same. The cost for the unit was about $1250 if I remember correctly? Good luck.

Bill Verburg · Bill Verburg's Garage

The 915 lsd was available with locking factors of 40%-80% adjustable via shims which would preload the friction clutch packs against a large spring washer. I think that I remember 40% as being rpo. The newer trans(G50 & derivatives) are available with locking factors which are different under acceleration and braking(40% on acceleration and 60% on deceleration in your case), this is possible because the ramps on the pressure rings can also have different angles. . This is an asset for a track car and is used by most of the pros in their cars (fyi it is standard GT2 fare).
For more details talk to Paul Guard at Guard Transmissions 888-89 GEARS(gears@gte.net)

jluetjen · jluetjen's Garage

Quote:

...and they have a 40%/60% lockup one...

I think we need to be careful with terms here so that the discussion doesn't get confused.

For some reason, Porsche LSD's are always described in %'s. I spoke with Steve at RennSport about diff's and he said that this is based on the pre-loading and clutch arrangement in the diff. I'm not aware of any way to confirm if a diff is 40% or 80% without taking it apart. Never the less, 40% diff's are softer and have less preload then 80%. A 100% locked diff would be a spool. From a drivers perspective, a 40% diff will have a "softer" lock-up then a 80% diff. As a rule, the more powerful the car, the higher percentage lock-up you want. Also note that a lot of the stock Porsche diff's are symetrical (see below).

The rest of the world describes LSD's (at least plate or Salsbury LSD's) according to ramp angle. A 40/60 LSD has 40 degree acceleration ramps and 60 degree overrun ramps. There are lots of different versions 30/60, 60/60, 30/30, 45/45, etc. If the two numbers are the same than it is a "symetrical" diff. The ramp angle determines how much force is applied to the clutch pack and can often be thought of as how abruptly the diff will lock. If you have a 40 degree acceleration ramp, under acceleration picture the clutch pack being wedged into a 40 degree angle. It clamps pretty quickly and it clamps HARD. It basically will lock under acceleration. On overrun, the clutch pack is being wedged by a 60 degree angle, which is closer to vertical wall, so the compression forces are not so great. So there will be some differential affect to the rear wheels.

The locking percentage that Porsche uses to describe their diff's is generally determined by the way that the user assembles the springs and clutch stack-up. So you will rarely see a non-Porsche defined by its locking percentage.

Other style diff's such as Torsion and Quaffe's (which are similar in application, but not design) are basically one size fits all. They tend to prevent the lightly loaded tire from spinning while under acceleration and do nothing under deacceleration. There is no adjustability.

If you are a novice, the Quaffe can be a good place to start. If you are competing at a more serious level, then Plate style dif's are the way to go because of their adjustability. How you adjust them is a "black art" and often depends on the driver and the car. Some examples that I've heard...

1) A lot of lock-up on acceleration will cause SIGNIFICANT understeer as soon as you pick up the throttle. If you car is already pushing, adding a LSD will make it worse. If you car is loose when you pick up the throttle, then it will help. Basically the improved traction from the inside rear tire tends to push the front end in a straight line rather then allow it turn by rolling on a smaller radias turn.

2) I've heard that a properally set up diff can be worth up to 1.5 second per lap on tracks with tight corners, more so in the rain. On tracks with open sweepers, the impact will be less.

3) Having a lot of lock-up on overrun can often be a good thing (and never possible with a Quaffe) because it causes the rear tires to brake evenly and helps the car to brake in a straight line. It also may not because it can cause a LOT of corner entry understeer. 956/962's had this as a result of their spools, so the driver had to make up for it by using the throttle to get the car around the corner. Not an easy thing to learn, but effective none the less.

Carroll Smith wrote a whole booklet on the different types of diff's which came out with one of the first Racecar Engineering issues. I doubt that it is available via back issues any longer. The good news is that he repeated a lot of it in his "Drive to Win" book. There is also an excellent discussion of plate type diff's in the Strategy Guide that comes with the Grand Prix Legends computer game (available at EBay's near you!).

Anyhow, if you are looking at a 40/60 diff, I'm pretty sure that they are talking about the ramp angles and not the percentage lock-up force. The 40/60 is a popular combination. You might want to ask around about the rebuilding options so that you have the clutch set-up adjusted to your preference.

speeder · speeder's Garage

I always understood 40% ltd. slip,(this would be stock on older 911's?), as refering to the amount of slip allowed, i.e. the maximum difference in speed between right and left drive wheel, 60% of power going to one, 40% to the other. Is this correct?

A couple of other questions since we have the attention of people in the know; is there an easy way to check slip% on bench? I have a 915 ltd. slip that I plan on installing in my car, do not know if it is stock since it is rebuilt. Have heard that one can check it w/ a torque wrench? Also, I have asked this before, but will a stock 911 ltd. slip in good condition(1982 trans and car), be noisier than non-lsd on the street? TIA for replies.

Randy W · Randy W's Garage

John makes a good argument for ZF style LSDs. I would agree that for track work where the circuit is permanent and you can customize your LSD to your car on that track, the ZF style is the best choice. However for autocross, where the course is always different and a variety of surfaces are encountered, and most importantly where the ability to induce oversteer is not necessarily a bad thing, the Quaife type is widely regarded as the way to go. This is the best type of limited slip differential for street or autocross use because it allows you to quickly get around a tight corner and get the power down quickly. It is not adjustable because it is infinitely variable in its action.

jpachard · 05-30-2002, 06:39 PM

All, Once again, thank you for the input. I think I have my mind set now on the ZF type that Porsche Motorsports has. I have always leaned towards factory parts when it comes to the engine and transmission. I plan on Club Racing the car starting next season so now will be a god time to get it in the car and "rebuild" the driver so I can take advantage of it when I start racing. Thanks for all the input and if anyone else has any other wisdom, please feel free. I haven't purchased it yet.

Cheers, James