User 52121

Yes that was the parts price.

I had a local shop do the install. Porsche specialist who I met through the local PCA. Not sure if he gives PCA discounts but my total bill was under $800 and I had him do a couple other things to the car while it was in there (installed some rebuilt brake calipers, ran some fuel system checks for me, installed new carrier bearings.)

My guess is that $6-700 is probably pretty fair for the diff install. To check pinion clearances, etc. you have to disassemble the trans - and usually you have to assemble/disassemble 3-4 times to get it right. (Remove trans, remove old diff, install new diff, check clearance, uninstall to add shims, reinstall, recheck clearance, uninstall to re-shim if necessary, etc.) So figure a couple hours labor at least.

chrenan

Thanks Omni, good info. I've rebuilt engines before, and would do that again with no hesitation, but a transmission might as well be a magic black box to me, so its good to know what the costs are to have it done.

333pg333

"I drive FLAT OUT!"

...so put me in the fast group....

slap929

How bad is it to rebuild the AOR limited slip trans? How much would it run to send it out for a rebuild?

samluke

The LSD is completely seperate and independant of the gearbox section. Factory LSD's are very easy to rebuild. A low breakaway torque does not indicate that the LSD is not working. There is a lot of misunderstanding on how these work and what the factory toque measurement/check means. With the "hot" setup, there is no breakaway torque, it would test the same as an open diff, but provides an 80% locking ratio.

I have tracked for many years with AOR tranny's I haven't managed to wear one out yet.

DivineE

Ok. I'm confused how does that work? I thought the standard factory unit was a very simple design with friction plates like a clutch. Holds together like a solid axle up to a point so there is a resistance to slip when one wheel is spinning which forces 40% of the drive torque to the other wheel.

Surely with that simple set up if you lifted one wheel off the ground (with the car in gear) and tried to turn it you would feel the resistance of the friction plates trying to stop the wheel turning? Is there something more going on?

Oddjob

You will need to look at an exploded diagram to get a better understanding.

The static test torque just measures a preload on the assembled pack induced by the bevelled spring washers on each end of the stacked up guts. I am not sure it indicates much, other than to confirm that you didnt forget to put some parts back in.

As mentioned above, this preload is very small, 6-10 ft-lbs. This small amount of internal friction would not limit wheel spin on a motor making 300 ft-lbs at the flywheel, some 3500 ft-lbs through the differential.

The internal spider gear shafts are suspended by cutouts in the pressure cups. These cutouts are angled and what you hear referred to as the ramps (the angle of the cutout is important to the lockup).

When there is an input into the diff (by the driveline) the spider gear shafts are pushed into the pressure cup ramps, which drives the cups outward into the friction discs. The stronger the input, the more force on the friction discs, creating more lock.

The "40%" lock rate is not a percentage of actual torque transferred to the other wheel. My recollection is that its more of a designation, identifying the friction surfaces. 2 friction discs have 4 friction surfaces, so the 40% figure comes from 4 friction surfaces.

LSDs can be modified by adding additional friction discs, which increases the # of friction surfaces, increasing lock rate. Another mofication that is used by the aftermarket LSD manufacturers is to change the ramp angles to increase or decrease lockup under loaded or unloaded conditions.

User 52121

No problem. I'm the same way. I have no hesitations about pulling a motor apart, doing electrical or electronic work, etc. Son of a mechanic, grew up working on cars. But what happens between the far end of the torque tube and the rear wheel might as well just be magic as far as I'm concerned. Diff install, with all the shimming and precise pinion depth requirements, etc... I figured I'd leave that to a professional.

Re: clutch type diffs, stock LSDs, etc... I read several posts both here and on Pelican where guys who autocross regularly tend to cook the clutch-type diffs in about a season since you really work them hard with high power and tight turns. Hence why I went with a torsen style - I primarily autocross my car. I didn't want another part that would require maintenance.

ninefiveone

Actually it is both. A 40% lock rate will send 40% of the torque over to the other wheel. A lot of LSD's can change their lock rate with different combinations of friction disks and in some each additional disk translates into 10% more lockup.

Bill

I spoke to Paul Guard (Guard Transmissions) and he absolutely recommended his clutch pack lsd over his torque biasing diff for our cars. He said that his clutch pack was superior, end of story.

Oddjob

The big disadvantage of an open diff, is that it will allow the inside wheel to spin powering through a corner. Both an LSD and TBD eliminate the inside wheel spin and allow throttle/power to be applied sooner; both are a huge improvement over an open diff.

There are pro's and con's of the clutch type LSDs compared to the geared TBDs. The TBD's are fine for street use, auto-x (probably preferred) and track use. The dedicated track/race cars tend to use the clutch type as there are benefits of having some unloaded lock to help straight line braking.


Can you explain how the friction coupling in a clutch type LSD would be capable of transfering a set % of torque from one output shaft to the other?

ninefiveone

Is your question about precision? e.g. how does it transfer exactly 40%?

or is your question how does it work at all?

If the former, yes, it's clutch packs and it's not a precise science particularly given friction surface wear.

If the latter, there's plenty via google explaining how LSD's work and the transfer rates.

Oddjob

My question is for you to explain your claim, because it appears you have some misunderstanding.

Do you believe an 80/80 transfers 80% of the torque to one wheel, leaving 20% at the other wheel? Or a 50/80 balances 50% of the available torque between each wheel?

ninefiveone

I see... ok.. if we want to get into it.

It's actually torque bias. The math is torque at the high friction wheel divided by torque at the low friction wheel. It's easier to just think of it as transfer rate, which while not technically accurate, is easier for people to understand and is close enough.

It's certainly not that a 40% lock rate identifies how many friction surfaces there are. Your description of how an LSD works is, of course, accurate. But your definition of bias ratio is not. You can achieve any bias ratio with any number and combination of friction plates depending on the differential.

We're also ignoring the differences between 1-way, 2-way, and 1.5-way setups but that's mostly irrelevant for this conversation as well.

If the misunderstanding is that you thought I was confusing the way a TBD works compared to an LSD... I'm not.

But really...why the hostility?

TonyG

No way.

The BIG benefit of the LSD is the ability to hard core trail brake without spinning.

To really get the benefit you have to have a LOT of locking % under decel.

The difference is HUGE on the track.

This is where the GUARD LSD shines.....

With the Guard LSD you can have a lot of lock under decel and a lot less lock under accel. Which means you get the huge trail brake benefits while not inducing a lot of push under accel

Yes it's done with the ramp angles. And it's not rocket science. However you simply can't do it with a stock LSD. Furthermore, the stock LSD carrier is somewhat weak where it's welded together (unlike the Guard LSD which is one solid piece with no welds). And yes I broke a stock LSD diff carrier... which destroyed the entire transmission along the way BTW.

TonyG