camlob

So it looks like the TBD is good for auto x while the LSD is good for fast tracks. Guards FAQ lists some pros and cons. One of which is that if the car has stiffer suspension and a strong engine, they recommend the LSD. Any comments on this?

I have a 4l engine thats pretty maxed out and stiff shocks with TRG sway bars on the way.

Babalouie

I've never tried a TBD on a 911, but in front-engined cars, they can be really great and fluid in their action. In comparison, a good LSD can be a bit clunky. What I find is that in high power applications where you're likely to get both wheels spinning a little, the TBDs can default into spinning up the inside wheel (ie like an open diff).

This doesn't seem to be an issue at the low power levels, where a TBD can really feel great. TBDs tend to have more of a "free" turn in, and does its thing in a very smooth fashion. But say you turbocharge that same car, and all of a sudden the diff is unpredictable.

I'm not sure how much of this would apply to a 911 where you have this natural weight/traction advantage, but one thing is for sure, which is that an LSD may be crude and clunky but it will always kick in when you want it to. A downside of a tight LSD is that they can cause the car to understeer unless you really are committed.

camlob

Thats the thing, I have a high power application. With stiff shocks.

evoderby

+964

To add:

TBD is maintenance free (fit and forget) whereas friction plates wear, replacement intervals of 30K km on road cars is not unheard of.

TBD is generally heavier in construction......we don't like weight!

As long as my parts say "race" I'm a happy camper..... ;-)

Bill Verburg

TBD are a gear driven design, the gears only transmit power 1 way and so they only work on acceleration and act as an open diff under deceleration, this is important. Besides limiting wheel spin an important function of a LSD is to stabilize the car under braking, TBD does not accomplish this, only a plate type does, the higher the lockup(even up to 100%) the better for braking for acceleration 20-50% lock is usually speced

There are several types of plate type LSD, Guard has one of the best most robust designs, they took a very good Porsche motor sport design and improved it.

early Porsche LSD were supplied by ZF, and used symmetrical ramps w/ 4 plates/side, 2 inner and 2 outer, the one set is splined to the axle the other to the trans. The discs could be stacked one way to achieve 40/40% lock and another to achieve 80/80%. These %'s are just nominal #s giving only a rough indication of the effectiveness of the LSD, setup pre load among other factors would affect the actual torque values for lock and thus the actual feel of the LSd when in use. These diffs could be quite intrusive in action and often in undesirable ways. The 40 much less so than the 80.

Most Porsche street cars were not sold w/ LSD because civilians often complained about how they affected their street car.

an intermediate effort was made to reduce the intrusive nature of LSD by reducing the # of plates and the preload and the plate friction characteristics, this happened in the late '80s-early '90s time frame on street cars.

Also introduced in the early '90s but only used in 964t and 964RS was a 4 plate asymmetric ramp design, these early PMS diffs were 20/100 w/ fairly low preload, they were a big step forward in LSD design, but because of plate material selection wore out early

for the 993 the design was refined, street 993 used 25/65 or 22/40 design, PMS used 40/65 in the RS and RSR cars. These were better than the street designs but suffered from cracked cases in the RSR and GT2 applictions

enter Guard w/ better plate material and much stronger cases, originally they offered only 40/60 but now have several version up to 50/80 (Talk to Matt Monson at Guard),

I have PMS 40/65 in mine find it to be unintrusive but effective in operation and after 50 track days still w/i spec but if I ever had to replace it it would be w/ a Guard 40/60

KaiB

Enter with a few questions, as the answers will help you with the choice. They're obvious, but honest answers will make all the difference in performance and happiness later.

Do you track the car? Hard and often? At what level are you tracking the car? All the power in the world means nothing here, i.e. are you passing GT3s being driven by Level 3 drivers?

Do you AX the car? Hard and often? Same story, how are your times?

Or...do you tend to drive hard on the street (I hate this question, but...).

This is a start towards a choice.

Mark in Baltimore

Awesome write-up, Bill.

For serious track work, LSD is the preferred way to go. The plates wear out, though, and need replacing after a while. The TBD has no plates to wear out, but while I'm sure the gears will wear out at some point in the far future, the service life is far longer than a limited slip's.

camlob

Thanks Bill for the info. As usual, you are a walking wiki

KaiB - I do not auto x the car. I do very spirited fun runs and the occasional trackday.

I understand also if the inner wheel lifts in a corner, the TBD will act as a stock or open diff. Which I would not like. However, my problem really accelerating out of corners where I get a lot wheelspin. Which I think the TBD can help.

I also understand the LSD will cause more understeer. I just read that so I cant confirm that. Maybe Bill can chime in.

I was going to get Jeff's unit but I didnt know the differences till last night

So what do you guys think? I was thinking since I will be spending a lot of cash, might as well add up some more to get the proper part.

Reading the FAQ in Guards website, they say if the car has stiff suspension and has a lot of power, they prefer the LSD. My car is already stiff and I have TRG sway bars on the way.

Bill Verburg

Yes, a clutch type LSD will add to understeer, and add to the heat load in the transmission. Neither is that big a deal, the understeer is easily tuned out.

a 40/60 GT will be pretty transparent in normal use and an eye opener on the track.

I'd only use a TBD if I was into A/X

Mark in Baltimore

You've answered your own question. Get a limited slip diff and be done with it. The understeer is not bad at all and can actually help with trail braking, something that won't make a bit of difference on the street but is very helpful to do on the track. Shoot a PM to Glen Gatlin aka "Glen" and Chris Cervelli aka "Premier Motorsp" as they may have a Porsche Motorsport diff lying around. Might not be a bad idea to call Brian Copans in Ohio (http://www.bmcg-gearbox.com/about.html).

Steve Weiner-Rennsport Systems

JMHO,

TBD's are great on FWD cars (it tamed my VW GTi and made it work FAR better) however, I think they are not the best idea for RWD cars since they lack the trailing throttle lockup that truly stabilizes the car.

For any track use, I'd be using an LSD and adjust the preload and locking factors as needed. This can be done at the time of sale so its all ready for installation. We do these all the time for each customer's need.

camlob

Thanks guys. I just spoke to Matt at Guards and I might use my brothers old LSD from his RS and have Matt rebuild it.

But he gave some good points on the TBD. He asked me how long the tracks were here etc. Our tracks arent too long, meaning the main straight will only get me to around 180kph. And that is only the only section where I can hit that speed. He says the LSD will be very beneficial on 120mph and above speeds. So he said the TBD will be ok for my track over here.

When he found out that I can get my brothers LSD he said it will be the best solution. But adding more lock-up would cost $750 on top of the $1000. The stock 997 RS has a 26:40 lock up which he says could suffice.

So let me think....LOL.

evoderby

Do you steer with the wheel or with the throttle??

mcipseric

Check out the WaveTrack TBD. http://www.wavetrac.net/
Good value, I have one in my 993. I have seen some on the track and the times have been very fast. I cannot attribute the WaveTrac to fast times, but it sure did not seem to hinder performance.

FWIW - I have a freshly rebuilt PMS LSD in my Race car. - Not Cheap......

Good Luck!


Good Luck!

camlob

The wavetrack seems interesting. According to their website, it doesnt act like a TBD. It applies a force to the outer tire that has grip unlike a TBD. Thanks for sharing it.

I have to ship my brothers diff to Guards, which will cost me $200. So total cost will be $1,200. Hmmm....

What's your experience in braking into a corner? Is the car more stable with this diff?