which limited slip setup is best?

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which limited slip setup is best?

Postby lrayner on Fri Jan 11, 2008 6:43 pm

I am looking at limited slip options for my 944 Spec. build. Apparently factory rebuilt tranny's with the factory limited slip are available; Guard transmission has a torque biasing differential as well as a limited slip; and 9M has a limited slip differential I have seen offered on E-Bay.

Does anyone have any experience with any of these? What setup is best for a 944 Spec to be used primarily for AX and DE/TT's?
Thanks
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Postby kurquhart on Fri Jan 11, 2008 7:26 pm

The main performance difference is that a TBD only works during acceleration, while an LSD also works during braking. However, if you are braking in a straight line, an LSD won't help much; it is used more to reduce oversteer when trail braking.

A TBD is cheaper to purchase and won't wear out, while an LSD is more expensive up front and has clutch discs that need to be replaced every few years or so, depending on use.
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Postby martinreinhardt on Sat Jan 12, 2008 12:41 pm

I use a Guard Transmission (GT) torque biasing differential and I am very happy with it. It is very similar to the Quafe, the other popular torque biasing differential.

If you autocross and drive big track events, the TBD is probably a better choice. If you do big tracks only, you should use a LSD for the reason Kris mentioned.

Note: I noticed the gearbox oil temperature is much higher with the TBD, and I had to start using a thick racing gearbox oil.

http://www.guardtransmission.com/lsd_info.htm
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Postby kary on Mon Jan 14, 2008 12:29 pm

You should purchase a Guard LSD, for one they are masterpieces of machined work, to bad we put them into a tranny never to be seen again :) 2) You can order differet lock up percentages for ramp up and ramp down, thisis useful depending upon how aggressive you want to be. 3) the lock up pn braking does great things when braking in a straight line at high speeds, it is not only for trail braking and into corners. if you get a 50/80 (can;t remember mine) lock up when coming down from 145 MPH to 50 MPY in turn three at Fontana, you will notice the car tracks very straight, without it you will get wiggle in the car tha unsettles you. 4) this type of LSD is awesome in corners and high speed sweepers, you feel more confident so you will driver faster, yields better times all around.

A lot of money for a Guard but definitely worth every penny I paid!

Good luck!
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Postby Dan Chambers on Mon Jan 14, 2008 2:54 pm

Leigh:

I pondered this for quite some time before making my choice in my '87 944 ... the white one that caught a few spec-cars in it's day. :lol:

I think for your needs and the amount of small track/AX driving you do you'd be better off in a GT-Torque-Biasing differencial. As a 944 owner, I have direct experience with this (sorry Kary, Kris, Martin .... but you guys are in 911's, and there are some significant differences from 911 vs. 944 with respect to LSD/TBD set-ups).

My reason's for TBD from Guards Trans.:

1) Bullet-proof quality build and function. No worries about fragility.

2) No need to remove/replace diff. clutches. Once the tranny is dropped and the TBD is installed, you're done with it. Poke the tranny back in, and go drive.

3) With all the tight-turning under load at AX's, the stress on the Limited Slip Diff. Clutches at AX's may be a little higher than Big-track driving ... thus a slim possibility of needing to R&R the trans. more often for maintenance (Check with the shops to get opinions on this. It could be crucial to 944 maintenance costs). With the TBD ... just drive it. (Note: if you find yourself leaning toward more Big-track events, I agree with Kary: an LSD may be a better choice because you can "order" the different excell/decell variables, you will have greater stability braking ... esp. trail-braking at high speeds, and you won't be stressing the clutches quite as much on the "more open" corners of a big track.)

4) The TBD doesn't affect the way the car slows under hard braking like an LSD. For some that is good. For me, I always liked a "free" diff under braking. But that's just my personal driving style. Everyone's style is different. I found with a more neutrally balanced set-up, the 944 doesn't appear to have the same "tail-wiggle" under heavy braking as the 911's ... but that is just my observations having driven both the 944 and the 911 on big and small tracks alike. Others may disagree with me.

Martin is spot-on about the heating issue and TBD's. Since you have more gears turning inside, the heat really builds up quickly. I ended up using a mixed-bag of trans. lubes to keep things cool, and functional (Royal Purple/Swepco/Liqui-Moly blended from a viscosity range of 75 to 140). I also found a used 944S trans. that had beefier final-drive bearings for handling the extra heat and torque stresses.

You will experience much better exit speed on cornering with either set-up. Again, it'll boil down to personal driving styles, personal preferences, money, and vehicle use.

Good luck with it. feel free to PM me if you want more details on my old set-up.

Happy New Year!
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Postby lrayner on Fri Jan 18, 2008 1:53 pm

[quote="kary"] if you get a 50/80 (can;t remember mine) lock up when coming down from 145 MPH to 50 MPY in turn three at Fontana, you will notice the car tracks very straight

While I have yet to drive at Fontana, I suspect that the only way I'm going to be approaching 145 MPH in my 924 S is if it's dropped out of a plane :) Ah, well... I appreciate everyones feedback. I think I will go with the torque biasing diff but will probably wait until I need tranny work as the cost to set it up is such that one might as well do the synchros etc while it is apart. Hope to see you all in a week! cheers, Leigh
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Postby Dan Chambers on Fri Jan 18, 2008 7:19 pm

lrayner wrote:
kary wrote: if you get a 50/80 (can;t remember mine) lock up when coming down from 145 MPH to 50 MPY in turn three at Fontana, you will notice the car tracks very straight

While I have yet to drive at Fontana, I suspect that the only way I'm going to be approaching 145 MPH in my 924 S is if it's dropped out of a plane :) Ah, well... I appreciate everyones feedback. I think I will go with the torque biasing diff but will probably wait until I need tranny work as the cost to set it up is such that one might as well do the synchros etc while it is apart. Hope to see you all in a week! cheers, Leigh


Okay, I know you thought you were finished, but ....

A tranny rebuild on a 944 will cost you ... BIG! Some garages won't even do a re-build. I was quotted anywhere from $3K to $4500 for a bearing replacement and general reconditioning ... before shipping it out of state!

Since there are sooooo many used 944 tranny's out there, you may consider a swap with a used trans from any one of several sources, including local dismantlers and guys like Tim Comeau and John Chambers who have heaps of used 944 parts.

As an example; I picked up a used 944S trans (1992) for around $700.00 that had great bearings/syncro's and gearing (A local dismantler in El Cajon :wink: ) . I then bought a Guards TBD thru Mirage for under $1200.00 that included install, and paid an additional $200.00 for the trans R&R. So for less than the re-build I had a really stout trans with TBD.

Shop around and talk to the resources within the club. You have many options.

Good Luck!
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Postby Jad on Sat Jan 19, 2008 8:52 am

Of course you need to be careful or you will be buying a 1992 944S transmission like Dan, and there is no such car so you will have been scammed :nono:

I think a limited slip helps a LITTLE bit at AX, but with the limited HP of the 944, the gains are minimal and a lot of the wheel spin can be cured with an improved suspension. At big tracks I don't think it would help much at all. I never bothered to spend the money for the 8 years I had n/a 944's.
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Postby lrayner on Sat Jan 19, 2008 6:18 pm

Not everyone (as in me) is as smooth as you are Jad. Anyways, I'll be getting a '93 tranny, doing Dan one better!

In addition to all of the good advice here, I thought Joe, at European Motorsports, gave me some sage advice yesterday. Basically he said drive the car for awhile and see if I cann't limit the wheel spin with suspension set up which will be stiffer on the spec car than on my 944 S. The differential should be the last thing to do. And in the meantime, Dan, I'll keep my eye out for one of those less expensive alternatives you mentioned.
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Postby Dan Chambers on Sun Jan 20, 2008 6:27 pm

Jad wrote:Of course you need to be careful or you will be buying a 1992 944S transmission like Dan, and there is no such car so you will have been scammed :nono:



:oops: Duh! Maybe it was a 1991? I can't remember .... to much water on the brain from owning a 944 :lol: :lol: . (Of course ... now I'm an airhead.)

Leigh, you've got the right idea. Looks like you'll have a great season. Good luck with the project. 8)
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Guard Transmission Torque Bias Differential

Postby rblaesi on Tue Feb 05, 2008 4:15 pm

I have a low run time TBD in the box. It came from another racer in SDR. It will fit a 911 with a 915 gearbox. It may also fit a 930 turbo four speed, but I am not sure about that. He paid $1,000 for it. I bought it from him for $500. I never installed it. Please make me an offer! Call Randy at 619-804-0434. :)
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Postby Bob Gagnon on Sat Feb 16, 2008 5:16 pm

My $.02 experience.

My car was delivered with a 20/100 variable lock LSD. This means a 20% lock under acceleration and 100% when braking. This LSD didn't work well for autocross, the 100% lock under braking promoted understeer into tight corners and the 20% lock allowed the inside rear wheel to spin on corner exit.

I changed to a 40%/65% Porsche motorsport LSD and understeer was less, the car is still stable under braking and the car puts down power better exiting corners.

I think for any given autocross track, my original 20/100 LSD made the car slower than an open differential car would have been since understeer was increased. On the other hand, I think my 40/65 LSD, due to its corner exit traction, is probably quicker than an open differential would be.

I think, for best handling and stability under braking, the best LSD for real racing track use is a variable lock plate type with the bias set with relatively more lock % under braking for a 911 than a 944 or 914.

I think the best LSD for autocross, where braking stability is not such an issue, is a torque biasing type, which differentiates well on corner entry and puts down the power in proportion to wheel loading on corner exit well.

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Postby pdy on Mon Feb 18, 2008 7:50 am

My experience is that braking CAN be assisted by a clutch-type LSD at an autocross, but as Tom Tweed mentions in a related thread, it depends on power.
I found that lower powered cars don't need LSD either way. Higher power (or as I turned up the boost in my 924 Turbo), there was difficulty in
applying the power exiting a corner. Any type of LSD would have helped.

Having driven a Quaife recently at an autocross in a slightly higher powered car, I learned that it was welcome exiting
the corner, but the tighter Type-II corners (trail-braking) pronounced the on-to-off throttle transition. This made braking difficult,
and required a LOT more smoothness and sensitivity on my part. When I drove a Factory Clutch-Type LSD in a higher powered car,
braking was a good deal easier at autocrosses, since the LSD took care of the transition more. The factory LSD obviously did not set the braking
percentage too high relative to the tightness of the corners I encountered. I am sure that too severe a braking bias would indeed cause understeer.

It depends on the power, the track AND driver preferences. Hmm... Ideally, we'd have different Diffs tailored to the track (along with gear ratios too?)...
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Postby Bob Gagnon on Fri Feb 22, 2008 3:37 pm

pdy wrote:
It depends on the power, the track AND driver preferences. Hmm... Ideally, we'd have different Diffs tailored to the track (along with gear ratios too?)...


That is to say just like a real racing team would do ;-) I didn't mean to say there was no advantage during the braking in autocross with a high braking lock percentage, just that this has to be balanced against understeer for fastest lap time.

The book by Peter Wright on the 2000 season's F1 Ferrari is an interesting read. The F1 2000 used an active variable lock differential using the same basic design Porsche used in their 1989 Carrera 4's longitudinal and transverse differentials!

This differential design uses an hydraulic ram that is controlled by a microprocessor and the differential locking % actively changes dependent on however it is programmed. Yaw rates, deceleration rates, lateral acceleration, steering angle, throttle angle etc. can all come into play to achieve the best locking percentage for the job at hand.

The comment was made in the book that Ross Brawn and Michael Schumacher thought it one of the most important and difficult parts to set up for a race weekend, and left it until last to control other variables first.

Wouldn't it be cool if Porsche once again (as Ferrari does in their F430 street car's "E-Diff") used their old technology and produced a modern variable lock differential? There would not be problems with the PSD and the ABD would not be needed. It would be great for autocross, street, track and could be quieter and wear would automatically be compensated for.

Good luck, Porsche don't even want to use a passive LSD anymore. Wasn't in great for us gearheads when Porsche was an engineering driven company rather than a sales driven company? :roll:
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