Limited Slip for 901 gearbox, advice

[kandhmfg]

Ran my RS clone with 2.7L RS spec motor (webers vs. mfi) at autocross this weekend. Tranny is a '71 vintage 911 mag box. Car pulls like there is no tomorrow. My unloaded rear wheel was laying a black streak spinning like crazy and making pulling out of the corners interesting when getting on it coming out of turns. Had to really modulate the throtte to keep both tires in grip. Wondering if anyone has any advice on what they have used as far as a LSD. Car will be used in Autocross as well as some track usage in DE events. Looking more for autocross optimium performance with advantages for higher speed cornering at the track as well.

Any thoughts are much appreciated.

Erik

[metalracer]

Paul Guard makes one really sweet LSD, super company to deal with, too!!

[jluetjen]

For tight autocross circuits, I think that a Torque Biasing Diff like a Quaffe is preferred. A plate type LSD might tend to set up too much understeer in tight corners. That's the conventional wisdom at least.

[jhora]

Factory ZF LSD recommends lock up at 40% for autox / street and 80% for track
They work well and use friction disc to change the settings

[jluetjen]

Something to keep in mind, a ZF (plate type) diff generates heat (and robs HP) when it is not locked, and the wheels are spinning at different rates. A TBD generates heat (and robs HP) when it is locked. So consider where you want to be giving up a few HP (and generating a lot of heat), and plan your transaxle cooling appropriately.

[Mad Mike]

[QUOTE=kandhmfg;3652038]Had to really modulate the throtte to keep both tires in grip. [QUOTE]

I would have to concur with this. The front of my right leg was sore yesterday from the active modulation of the throttle. Much more than I was used to in SC's and Carrera's.

Erik, I overheard Steve A. recommending a Quaife for his friends 1974 911. I think he knows what he is talking about.

http://www.autotech.com/quaife/index2.htm

That car still blows my mind. I am ready to go for a drive again!

[Mad Mike]

Perhaps you can see the back unloaded. The rear shots are around 70MPH in a medium turn. This is the 3rd gear shift point. The blurry photo, just means the car is faster then my camera!

[spuggy]

Guards make both torque biasing and friction plate types of LSD. http://www.guardtransmission.com/lsd_info.htm

Torque biasing are not preferred for track use, although they are for tight AutoX. They provide no lockup on deceleration and don't work at all if one rear wheel is in the air.

Lots of good info here http://www.guardtransmission.com/velocity.htm

I think the 40/65 asymmetric LSD I put in my car was the best money I spent on it - and that's for street use. Rear of the car feels really planted on corner entry now. Night and day difference. Amount of extra initial push (which was very slight) was noticable at first, but it's predictable and consistant and you rapidly adjust.

[kandhmfg]

Although I do love autocross, I would be willing to sacrifice a little to the understeer side to have something that was more useful for higher speed track stuff. Car turns in really well right now, even after tightening up the front bar, and the rear bar is at full loose right now. Lots of balancing left.

Maybe the LSD sounds like the more appropriate way to go. Our AZ autocrosses are pretty open most of the time also, we don't like those 914 style tight turns.....

And if I read the earlier post correctly, this is going to add some additional heating to the differential so maybe some additional cooling may be optimal. Will have to use that search function to see what I can find on that.

Thanks for the insight. Mike-thanks for the additional pics.

Erik

[spuggy]

Originally Posted by kandhmfg

And if I read the earlier post correctly, this is going to add some additional heating to the differential so maybe some additional cooling may be optimal. Will have to use that search function to see what I can find on that.

Erik, I noticed that Mike Bainbridge (who posts here as MBEngineering) is developing an early gearbox spray bar kit, according to his signature. I have his 915 internal spraybar setup in my car (he built the box and fitted the LSD).

Once you have the oil pump to drive the spraybars, it should be relatively easy to fit a cooler in the circuit as well.

According to Frere, the factory ran internal spraybars in racing because the drag from a full oil fill cost them HP at full tilt, but the spraybars meant that they could run less oil (no drag) and just put it all exactly where it needed to go...

WEVO do a full-boat spray/cooling kit, but I think it might be for just the 915, don't know how the 901 is different.

[J P Stein]

My car is strictly an autocrosser. I should receive a Guard TB diff by the middle of January.

A clutch type LSD will cause a car to push in your typical AX corner.
The TB will spin the outside tire if it *totally unloads* (lifts off the ground)....which means you have a suspension problem that should be addressed. "914 corners" don't bother me, nor do "911 courses". ........

[Mad Mike]

The car has 31 rear torsion bars. There is a slight bump in this particular corner creating a perfect 3 wheel shot. Great shot none the less, not taken by either of us.

[J P Stein]

I didn't mean to imply that you had a suspension problem, Mike and God knows I left many a black streak while dialing in my set up, but the inside rear tire never lifted off the ground. The TB diff will deal with that.

Some 914s running a rear sway bar will lift the inside rear....I've driven one of those & didn't like that at all. A TB diff will not deal with that and that's what I was referring to.

BTW, heresy perhaps, but occasionally lifting the inside front doesn't bother me at all....... as long as the sumb!tch goes where it is pointed.

[budge96]

Hey excuse me if I missed something but if your inside unloaded wheel is spinning like crazy leaving tar marks isn't it being torqued? Sure you don't have a LSD already?