limited slip on a '73- is it worthwhile?

[RSBob]

Have been cruizing the archives and have found lots of info but not exactly what I am looking for.

After upgrading to a 2.7RS spec, with an rebuilt 915, I have been toying with the idea of adding a LSD. The car is primarily for street, but also for occasional DEs. What I am looking for is a 'save' for my poor driving skills, to help keep the back end from stepping out and losing it on fast corners and wet. Question one, is there a unit that works well for DE and street or are they mutually exclusive - what are the draw backs? I read about increased tire wear, but that's not a concern. I also see it takes more power to drive a LSD out of a corner, so would my motor have enough grunt to make a LSD worthwhile or would it defeat itself? On tight 20 MPH and 30 MPH corners would I want a unit that works well at a DE? I think I am asking to much of a single unit, but but want to know your experience. Here are a couple of good quotes I found:

RDANE WROTE: I also have a 80% ZF and drive in the rain all the time both street and track. I live in Seattle after all. Car pushes some but they all do. The 80% ZF is my second transmission, the first being no L/S. I do notice the improvement on the track and in the rain and the additional push. I also don't break a wheel loose on every wet, street corner, which happened before with no L/S. Everyday street driving hasn't changed but it has become more enjoyable with a L/S.


The new generation of differentials that change the locking effect were
created as a direct result of the early European Cup Races. They were not
(repeat not) used to "get around the understeer issue I mentioned earlier."
When you drive a 911 into the corners real deep (Which ABS and trail braking
techniques allow you to do), the rear end on a 911 tries to step out.
(Oversteer) The increased locking effect of the "trailing throttle lockers",
which is what the factory called these units in the beginning, increases the
locking effect and drives both rear tires at the same rate. This keeps the
rear end going in a straight line and helps with the oversteer under braking
problem. Of course, the more the locking effect, the more horsepower needed
when accelerating through a corner. (Anyone tried to push a race car with a
locked rear end with the wheels turned?) So, as you get back on the
throttle, the locking effect decreases, which is what is desired.

[ttweed]

Rick-

Are you having problems with excessive wheelspin on corner exit, or too much oversteer while trailbraking on corner entry? If you are, than an LSD could help. Otherwise, it's money down the drain (except for the "cool" factor.) If you get seriously into DEs or time trialing and start pushing your car to the limit, it might be worthwhile. If you drive in wet or snowy conditions a lot, it might be worthwhile. If you want to autox the car, consider a Torsen type LSD, as the ZF clutch type WILL induce understeer which will make tight corners more difficult.

TT

[Jack Olsen]

I agree with Tom. 'Cool factor' is the only thing it would offer, in your circumstances.

It's not going to make the car any more forgiving in street or DE driving. It will make the car more dangerous in low-speed, tight turns in the wet.

It's a fine-tuning type of tool for track driving.

[beepbeep]

I believe Torsen/Quaife would be best choice in your case. It won't make car "push" in the corners and will move traction to wheel that has best grip. It's also maintenance-free and costs slightly less than clutch LSD.

It will open up when braking though...

[RSBob]

Thanks Tom, Jack, and Goran. I always appreciate advice from the board's more senior members. In this case, I think I may have to lean more with Tom and Jack. (Sorry Goran;-). 3 grand is a pretty steep price to pay for 'cool factor' - if there is no dramatic benefit. I could better use the money on other items which just miraculously keep popping up (whether truly needed of not).

Jack, now that BB2 is pretty well sorted out in the build dept, are there any other projects in the hopper? I was hoping you would take on a 914 and build BB2s personality into one.

[rdane]

Rick, for where we drive and our weather conditions I do think a L/S has several advantages.

I had both the stock gear box and later the limited slip on my 3.0. No one encouraged me to buy one. I found the addition of my ZF limited slip made my car more enjoyable in everyday driving.

Much of the push has been dialed out with bigger sways and 225/245 tires on the car which is how the car was set when you drove it as a 3.4.

[RSBob]

Dane - you trouble maker you! ;-) I certainly was impressed by your grip in the twisties even in wet. I have the bigger sways already. I noticed when you were driving, you were feeding progressive amounts of input into the steering versus what I would call a smooth turn. Was that a result of the LSD or you feeling the tires start to break?

[rdane]

Rick,
Last run it was me breaking the wheels loose with the throttle then playing catch up with the wheel. Well that is my version of it...could be I am just a bad driver. Anyway breaking the rear end loose was intentional. It isn't the l/s causing trouble.

Either way what I really noticed with l/s is that on any corner in the wet I am either hooked up or spinning both wheels at the direction of my right foot and the throttle. With no l/s I would get serious one wheel spin in the 3.0 and I wasn't hooked up, just wasting power until dropping the throttle down hooked up both wheels.

My shop really tried to convinvce me a l/s was a waste of time and money. Many of the guys here said the 80% ZF was going to be terrible to drive as well. I bought my tranny set up as is, short gears and l/s. Great set up for fun driving. It also allowed me to run down a few similar cars on the track with better drivers and race rubber but no l/s.

You drove my car and it was wet that day. Did you notice anything weird ?

I have also been driving a Torsen type LSD last couple of weeks and I can tell the differences in several places. Not sure what the differenced are yet but it is different. My take is not as well planted..or consistant. But I'm not knowledgable enough to say much past my feel for the car. And I hate other people's "feel" as feed back.

All we do here is drive in the rain so I have no problem telling anyone that a ZF isn't a big deal. Craig 911 is now running a 80% ZF as well in his car maybe he'll chime in.

The only down side I can see to the ZF is maintance, cost and some added weight. But I won't own another pcar without a l/s. In the grand scheme of things it is a cheap date.

Bottom line? I think my car is more fun because of a ZF l/s set at 80%. I can find several posts here by guys who know a lot more than I suggesting just the opposite.

[jluetjen]

Yeah, but $3K? If he's having problems with traction and control, I think $3 would be better spent on professional track driving school (one that teaches you how to drive at the limits rather then just how to safely drive at the track). Chances are that he'll still have enough left over to buy a few parts or do a few DE's.

We always seem to have this bias that a 911 has fundimentally flawed handling and that by buying this or that do-dad that we can make it into an awsome handling machine. The reality is that stock 911's can be safely driven far faster then most of us will ever go. It's not the car that's holding most of us back, but the driver. The first step to getting better performance from a 911 is to understand how much un-tapped performance it already has!

[rdane]

I don't disagee with any of that thought process..the cars are awesome as is. But a l/s was also a factory opton available on the cars

$3K?

Where does that number come from?

[ted]

Cars do funny things without out the LS diff.
The frustration and fear of having to lift mid corner while trying to accelerate and control the inside rear wheel from free wheeling makes an LS sensible to me.
If you have not experienced this then skip it until you push the car to the limits of the open diff,
then reconsider it.
You'll know when you need it, and then the $$ will make sense too.

[Craig 930 RS]

RSBob - what is the weight of your car, the HP/TQ and what track use will your car see?

I need to add some things here:
---------------------------------------------------------------------
*I bought my rebuilt ZF 80% LSD for $750
*I have discovered NO downsides to a LSD - none. As in zip.
*It is noticeable in s l o w parking lot manuevers - a slight 'chuk' 'chuk' sound as one side is turning and the other almost not at all and the LSD tries to do its job.
* I am putting 280HP at the wheels, so my case may be different than yours............but I have noticed not a hint more push at the track, and we have several slow & tight corners at the local go fast place.
*The back end hooks up well, all the time, no drama, no 'upsetting the car one side wheel spin' etc.

Dane & I both have lighter weight cars with lots of power - and use them at the track and aggressively on the street.

You must "find what your mission is" for use & couple that with the HP/TQ & weight of your car to see if you *need* an LSD.

Personally I would not use a torque biasing unit unless you are an avid autocroser.

If you get a diff, use an 80% ZF.

[Craig 930 RS]

Trying to post some nice pics of LSD install, but Pelican isn't allowing it right now (?)

[jluetjen]

Dane;
I was making reference to this statement by RSBob who started this thread.

3 grand is a pretty steep price to pay for 'cool factor' - if there is no dramatic benefit.

Personally I thought that Limited Slips were selling for about $1200 - $1500. As I've been settling on drive train spec for my car as it evolves into an limited budget SCCA race car (as opposed to DE or Track car), I've settled on a welded up diff to start as giving the best performance bang for the buck. HP target is 200+ and weight will be 2050 on 7x15 inch wheels and slicks. The "spool" will help to keep the car straight when braking at the expense of pushing mid-corner. I figure given the car's rear weight bias and the free choices for spring and sway bar tuning, I should be able to take care of a lot of that mid-corner push. Having done some karting over the last few years since I last raced, I'm fairly confident with being able to handle the locked rear end aspect of the car. I've got a couple of open diff's in reserve.

I figure that I'll do better right now spending that $1500 elsewhere on the car.

[Craig 930 RS]

I bought mine from an independent guy called Knight Racing in the deep south - cannot find the # or addy -

[lateapex911]

Lots of folks think an LSD will help reduce oversteer....

not really.....it WILL help BOTH tires drive out of a corner, rather than allowing the inside wheel to spin when unweighted. However, when the traction point is reached, then the rear will drift wide in power oversteer.

That said, 911s are pretty benign and well behaved without an LSD as the inside wheel takes a bit of work to spin. If you are encountering that situation (inside wheelspin out of a corner) then and LSD is needed, although certain setup chnges can help.

If lap times are the goal, then a Torsen is likely a good place to start the research.

[Craig 930 RS]

Lots of folks think an LSD will help reduce oversteer...not really.....it WILL help BOTH tires drive out of a corner, rather than allowing the inside wheel to spin when unweighted. However, when the traction point is reached, then the rear will drift wide in power oversteer.


Exactly my experience with the 3.6. So contollable.
I'd recommend it to anyone. Hell, even for autocross it is nice!

[rdane]

Hey even I could find a better place to spend $3K more wisely.
I had missed that number in the previous posts.

Rick somewhere under $1500 is a more common number I think.

I am also looking at going to a fully locked diff. It wouldn't hurt the way I like to drive or my use of the car and the plus of no maintance.

[RSBob]

Weight: 2200.
Modified RS Spec
HP: 222
Torque: 192

Style of driving - A fast recreational driver who enjoys hammering the twisties. Am giving up AX for DEs to improve technique as well at increase seat time. This car will never be dedicated to track only. Had a very sure footed Audi Coupe Quattro Turbo (not a Sport Quattro) and that car cornered on rails, but could be forced to break lose if driven like an idiot. I know the 911 will not approach the level of the 13 year newer technically advanced Audi but would like a little more margin for catching the car.

Dane makes a strong case for a ZF.

[rdane]

The thing that I don't get is most of the regulars here run a ZF l/s in various percentages of lock up but you don't see a lot of guys suggestiong their use.

I am thinking for how I like to drive and where we drive...and you spent the day in the car driving with me so you know...a fully locked dif would be just the ticket and cheap.

You have plenty of power and torque in your light weight (Craig should be more careful calling his car a light weight at the current 2400# ) a l/s will just add to the fun.

We'll have to do another drive as soon as my ride is out again (three weeks or less) and you can revisit the idea then. How much longer on the new paint?