Differential (or LSD)

[superfans]

Stupid Question - What is the purpose of a limited slip differential (or LSD)? Does the new 997 with PASM has it? What happens to a car if it does not have any differential (or LSD)?

[geza]

Just about every car on the planet has a differential of some sort, but a few have a limited slip. I don't think Porsche sells the 997 with PSM and LSD - but I could be wrong, since our 04 40th 911 has both.

When you take a left turn in a non LSD car, the left driving wheel can start to spin as you power out of the turn (vehicle weight starts to transfer to the right side of the car, making the process of spinning the l/h wheel easier). The driving wheel with the least traction will slip/spin as you apply a lot of power. The driving wheel with the most traction (r/h side) won't get any power applied at all, as a standard differential wants to direct all the power down the path of least resistance/traction - in this case, the left wheel. The opposite takes place when you turn right, under a lot of power.

A limited slip diff does the opposite - it locks the rear wheels when slip of one is detected - and that means when you apply power exiting a turn, the differential will limit the amount of slip that any driving wheel could experience - locking both together past a certain point. This enables you to apply full power out of a turn. A big help when trying to move very quickly.

The only reason a differential has to be there in the first place is to enable the inside wheel to turn slower than the outside wheel when making a turn. Without this feature, the car would tend to "hop" through a corner - much like the racing cars of old that ran 'Detroit Lockers' - these were either locked, or not - with a sudden transition between the two modes.

Edited October 12, 2005 by geza

[superfans]

It seems that the regular 997S with PASM use the "Active Brake Differential" and only the sport suspension with -20mm ride height use the LSD. What is the difference between the Active Brake Differential and LSD? Thanks in advance.

[geza]

Active brake differential is where the PSM system on the car detects slippage of the wheels under power or braking and modulates each individual wheel brake if necessary to render control and directional stability where the driver may not be able to react quickly enough - nothing to do with the differential that is distributing drive power from the gearbox to the wheels.

Edited October 13, 2005 by geza

[JimB]

It looks like LSD is becoming a thing of the past which is really unfortunate. The downside on street cars in that locking your differential while still turning can induce a whole bunch of understeer. Light front tires because you are on the gas and two big back tires trying to go straight can be a little scary. In a race car you are used to it and know when to expect it. I would guess the lawyers are all over this.

When I updated my 99 to a 3.6 X51 I intentionally updated it to an 04 because that's the only year Porsche offered their best engine with LSD. In stock PCS club racing you have to run combos that were available from Porsche. LSD make a huge difference on the track.

Jim