Ok, here is the deal. I have a 92 RS with a new 385 Stroker. Traction is the problem of the day. I have BFG Drag radials as well. The problem or question is how does the limited slip work. Sometimes when I pull to the line it locks up and then other times it don't. I have been told that it will only engage when the right tire is 100 RPM faster than the left tire. Anyone know how I can be sure it is engaged prior to the launch or even burn out. Last question, is there anyway for me to test the posi unit to make sure it is working.

 

which limited slip are we talking about here, eaton or auburn. i think there was also another one that came factory in some third gens but i can't remember. and how old is this limited slip. what limited slip does is transfers power to the high traction wheel, during tire spin both tires are effectively the high traction wheels and should both get power to them.

 

Sounds like a gov-lock system. Works like a normal differential but when one wheel starts to spin, it locks up. Gov-locks are not very strong and they disengage around 30 mph.

Jack up both wheels. Turn one. The other should spin in the same direction if it's a posi. To see how well a posi works, you need a torque wrench. With only one wheel raised up, a torque wrench is used to turn the wheel. At a specific torque, the clutches will slip simulating going around a corner. Sorry but I don't know the spec.

 

I'm really not sure what set up is in the rear end. I did have new gears installed a couple of years ago. Other than the new gears the rear end is original. 92 RS. How would I determin what type of set up I have and is there a generic method to determine if it is still good.

 

Stephen, Thanks I'll try that method and hunt for the torque specs.

 

The original rear in that car wouldn't have had posi at all, not of any kind.

Maybe that's why it doesn't act normal for a posi?

Only way to know for sure that it's even equipped, is to take off the cover and look. Which is a good idea regardless; if you haven't changed the fluid lately, it probably needs it.

 

Different "posi" units act differently based on design. Personally I don't even like the term "posi" because it's a misnomer. "Positraction" was Chevrolet's term for a limited slip differential using clutch plates. Different divisions of GM and other car makers had their own names for limited slip differentials like "Traction Lock", "Anti-Spin", "No-Spin", "Dual Grip", and "Twin Grip". A lot of import guys are running around with the term "LSD". Normally those guys drive me nuts with their "NOS" and other assorted nonsensical jargon gleaned from "The Fast & the Furious" (hey, I'm no MOPAR guy, but that Charger would have kicked that Supra's @$$ even on 7 cylinders). However, in this case they are right because LSD simply refers to a limited slip differential, which is more accurate than ust applying Chevy jargon to all makes and models. Nothing drives me more insane that reading a classified ad for a '68 Mustang fastback that has a 302, C4 and a posi rear.

Plate and cone style limited slips work on the principle of breakaway torque. There is a preload spring or springs that apply force to a pack of clutches and steel plates on top of the side gears in the differential. The preload provides enoung force to keep both axel shafts spinning at the same rate, and with the same amount of force, until the breakway torque, or the force required to overpower the springs is exceeded. When this happens the clutches slip and allow different wheel speeds, which is very important for smooth cornering, where the outside wheel travels a greater distance than the inside wheel in the same time, meaning that they need to be able to operate at different speeds or a lot of jerking and bucking happens. This is the stock GM design for limited slips, and is alss used by Eaton and Auburn. Most 3rd gens have a limited slip like the one described here.

Another type of limited slip is a Torsen differential made by Zexel or Quaif. This is a gear style differential that is sometimes called a "torque sensing" differential. This differential acts like an open differential until at a predetermined torque, based on design criteria that anticipates wheelspin, a set of worm gears enagage the opposite side gear and a lockup condition is achieved. A lot of drivers don't like these differentials, because they feel that they take time to react, which they do. They are often smoother than a clutch or cone style limited slips though, and are often found in higher end cars like BMWs. Also, a fair number of 3rd gens use a "take-out" Zexel Torsen from a 4th gen car. This is the magic $50 or $100 SLP posi that everyone keeps talking about.

Yet another style differential isn't a limited slip at all: the locker. There are a lot of these out there now, but the old Detroit is still the most widely known. Lockers act like a spool in a striaght line, but under the stress of turning they will unlock, usually with an audible clunk, and then re-lock when the vehicle straightens out again. Essentially a locker works like a limited slip with no clutches. A set of gears rest on top of the side gears, with preload springs between them. The gears mesh side to side, effectively locking the axelshafts together (hence the name). When the torque of the wheels trying to turn at different speeds gets too great the gears dissengage and skip across each other allowing different wheelspeed. These are a bit tough to live with on the street. The off road guys love them though.

The only exception to the locker rule are air lockers, cable lockers, and the new electric lockers. These lockers are totally locked, and will not unlock until you manually disengage them. A lot of off road guys run these in the front, because you can go from locked to unlocked with the push of a button or the flick of a lever. This allows maximum traction in straight lines, but much tighter turning to get around obstacles.

Then there's the spool, which is really no differential at all. The axelshafts are physically tied together with a coupler that's splined on both ends. These couplers are either mini (designed to fit inside the stock differential) or full (mounts the ring gear and bearings and completely replaces the differential). Spools are the hot ticket for straight line racing. They are cheap, simple, and very tough. On the street or autocross track they lead of scary handling and broken axels. Someone will probably try to say that they are fine for street use, and yes there are people that use them in "street" cars, but do you really want to take that chance? Persoanlly I like beign able to parallel park without drawing a crowd.

Ultimately you first need to figure out what kind of differential you have, and as RB said the only way to do that effectively is to pop the cover off the rear. Then you can decide the best course of action to fix or replace the unit. The rather long list of info above gives you an idea of what's out there (is your head spinning yet?).

 

TKO, Thanks, and yes my head is spinning. This should be a sticky, very informative. I am embarrased to say that after 12 years of ownership and numerous setups I have to admit that I have always thought I had a limited slip and have witnessed both tires spinning (on video) at the track, It's a dogleg. I will fix that with a locker based on the information you provided.
Thanks again

 

Yeah, that's a common misconception. A regular open differential can spin both tires. The torque just follows a path of least resistance through the differential to the tire with the least grip. Typically this is the right rear, but not always. Sometimes both tires have equal traction, or the left rear has less traction, and then either they both spin or the left tire spins. This is especially possible when the chassis has been tuned to launch straight at the track. I've seen guys with an air bag in the rigth rear spring spin the left rear tire in a car with an open differential.

If you do decide to go locker I advise you to use the Detroit. I'm running a Lock Right in my car now and it is quite clunky and obtrusive. The Detroit in my Blazer is almost unnoticeable. Be careful the first time you drive it too. They take a bit of getting used to when cornering. Initially the car seems to understeer, so you crank the wheel more, then the rear unlocks and you get some snap oversteer. It can be a little squirley. I've found a better technique is to use the normal amount of steering and feed in the gas. This will overwhelm the springs in the locker sooner, and get it to unlock at a predicatable point, instead of an unpredictable point. The two tire fire is well worth having to modify your driving technique a little, but you may not want to let your wife or a friend drive it without giving a little tutorial first! Having your call in a ditch always sucks.

 

i had a detroit locker installed in my new mosier 9". i also had a pa racing tubular k-member & a-arms w/coil overs and manual steering rack installed at the same time. talk about changing your driving habits all at once! untill i just read your post TKO i thought the oversteer was in the front end, i had no idea that the locker would cause this. i also wondered if the clunking in a straight line whenever you load & unload the rear end was right. it's a pretty decent size clunk almost every time you get on & off the pedal. does this sound normal to you? is there any shiming or adjusting that needs to be done?

 

Yes, all of that stuff is totally normal. My salesman at Randy's actually discussed the fact that there would be some driving adjustments with the locker. He was very up front about it, which is one of the reasons I only deal with Randy's now.

There isn't anything to adjust, and that clunking is a fact of life. I've noticed that it isn't as noticeable in my Blazer, but very apparent in the Camaro. Detroit does make a Soft Locker and a Gearless Locker that are supposed to be much more transparent in operation, but I don't have any direct experience with them. If I had to do the Camaro again I think I'd use an Auburn posi instead. The car now drives like a full on race car, but isn't as fast as a full on race car should be, so it's very frustrating! The problem is compounded by the SPEC stage III clutch that is a bit on the harsh side for the power level my motor makes. If it had DOT slicks on it I'd have a car that felt like I was taking the drag race trailer queen out for the night.

It's good experience though, and when I upgrade to a 9" sometime in the future I think I'll use a posi instead. I was so happy with the Detroit in my Blazer that I thought, hey it'll be just like that in my Camaro. I didn't realize how much the type of vehicle could affect how the locker acts. The Blazer is a fullsize truck weighing in at around 5,000 lbs. It's got 4.88 gears and 38.5" tall tires. The motor is a worked 350 with Edlebrock heads and a Crane roller cam. It's got a 2,400 stall converter and a 700R4 with all the Pro-Built goodies. The locker only rears it's head around very tight turns. In normal driving I don't even realize it's there.

The Camaro by contrast is a car weighing 3,500 lbs. with 3.73 gears and stock height tires (26" maybe?). The motor is very close to stock with headers, full exhaust, and some other minor goodies. The trans is a T56 with the aforementioned clutch. Every time the wheelspeed differs even slightly BANG there goes the locker. It's there all the time, and I know it's there all the time.

 

thanks TKO for the speedy reply!! my ride feels like a trailer queen also. but im running a strong forged 383 with aluminum heads, miniram with 58mm tb 150 shot of nitrous. its a fun ride but not as streetable as i wanted. its a low 11 second car. hopefully with all the things i did this winter it might make it into 10.90s. hope this doesnt sound like i'm bragging but this thursday is track night and i'm real excited! hearing from you has made me feel more at ease, thanks much!! greg

 

Don't appologize! Sounds like a car worth bragging about!

Good luck.

With the way my schedule's been I'll be lucky to make it to the track this year. I've got a fresh clutch in my WRX and I'd like to try and crack my previous best of 14.50 @ 94MPH (totally stock). I'd like to get the Camaro down too and see if I can get a better launch which should get my times down into the 14s. I'm debating slicks, or at least stickier tires, but right now it's just not in the master budget since I bought my house.

I'd really like to build a totally insame turbo small block for my Camaro, but I think over this coming winter I'll swap on a set of SDPC Vortec heads and a roller cam. I'd like to go to aftermarket EFI too, but I'll just have to see. If I could get the car into the 13s I'd be much happier. I figure if I can raise my redline to about 6,000 that shouldn't be a problem. Right now I'm shifting at 5,000 because the motor just runs out of breath with the stock TPI, heads, and cam.

I do have a forged 3.75" crank and a set of 6" I-beam capscrew rods on the shelf though, so a 383 might be in my future as well. I had the 383 planned as an upgrade for my Blazer (heavier vehicle, needs more torque), but who knows. I got a screaming deal on the parts like three years ago so I just bought them and stuck them on the shelf until I could figure out what to do with them. I need a set of pistons (the new Mahle sets look like the best deal going) and a block (I thought I had one, but it turns out my Nova had a 305 in it not a 350, I should kill the guy that sold it to me!). Anyway, I'm rambling now. That must mean it's time to go do some work...

 

you sure know your stuff TKO! and i'm surprised that you said auburn and not eaton, everyone disses auburn b/c the cone clutches are not "totally" rebuildable but fail to realise a cone clutch limited slip will last twice or three times as long as a clutch pack limited slip. and another thing, sometimes when your open differential has a tendency to spin both tires around a turn or on the street it could be that your differential gears are binding together or the gear lube level is too low (in my case it was when i pulled out the stocker to stick in an auburn). honestly i got scared when my open diff started spinning both tires on the street cuz i thought it was gonna blow up on me but the ring, pinion and side and spider gears were all fine. it was just low on lube

 

Yeah, the Auburn design is greatly superior to the typical steel & friction design most limited slip makers have adopted (even GM). The funny thing is that the cone design dates back into the 60s. Pontiac used to make their limited slips this way. They also used positive axel retention instead of c-clips. Then GM ruined it by making all the product lines homogenous, and that meant that the better albeit more expensive to produce Pontiac design fell by the wayside in favor of the Chevrolet style which was cheaper to produce. Sadly, a lot of great designs from the other marks were passed over in favor of Chevrolet designs which were not as good, but more cost effective. Another example is the small block Olds V8. The thing is built like a tank. The bottom end is all forged, it used positive cam retention & shaft rockers. Olds had a 350, but it wasn't a 4.00" bore with a 3.48" stroke. It had a bigger bore and a shorter stroke with a rod that was more than 6" center to center. It also had more deck height. From a durability and performance standpoint it's a way better motor than the typical SBC. It probably cost GM $50 more a motor to produce though, so they started making all engines SBCs and just putting them in all GM's marks. That's the problem with mass production. What looks like a small savings to you and me looks like thousands or millions to a manufacturer that turns out hundreds of thousands of units a year. Damn economies of scale! Okay, that was a bit of a tyrade, but it's true. The cars we buy are built to perform, last, and look good as long as those things don't impact profits negatively.