I am new to the whole GM thing and such. So I was wondering how you can tell if a rear end is posi or not and what vehicles were they on. Thanks

 

pull the cover and look is only 100% sure way to know

 

Ohh ok I wasn't sure if it had something printed on the rear end somewhere. I know my buddies 86' Police Interceptor Mustang has an L between the gear ratio to say it has a posi.

 

peel out and look to see if u laid 2 tracks or not. IF it lays 1 track that usually means it is a non posi, but u might just want to test the rear end on a slippery surface too, like a painted shop floor. (cone type differntials wear out fast but may still hook on a slippery surface because the force required to lock up both wheels is less than that of pavement or black top)

 

yeah I was going to test that theroy when I get it running, which should be in a short amount of time. I will get back to you if I discover something unless someone might have some other info that might help. Thanks

 

this is a good indicator of LSD...

 

Not trying to be rude, but if Roehm doesn't know how to tell if he has posi, he probably won't recognize a posi carrier when he pulls the differential cover. It doesn't say POSI in big red letters.

And don't believe any of that horse$hit you read on here about jacking up the rear and spinning one tire, and if the other blah blah blah. That's not true (at least for the take-off posi). I tried it when I swapped in my Zexel carrier. The tires still spun opposite directions, and I know it was working.

I vote for the burn-out. It's the only valid reason I can think of to spin the tires.

 

the peel out method doesn't always work

my rx7 has an open diff not a lsd
it spins both tires when going straight
my 78 celica would spin both tires in a straight line I doubt that was an lsd either


but then again if you have a limited slip diff you might only get one tire mark also if it is worn out so that test is kinda useless


best way to check as was stated was to take the rear cover off and take a sneak peak


another way that MIGHT help is to have a friend hold a rear tire and you try to spin the other one if it spins prolly open but this would only work for a clutch style lsd IF it works . it might work but then again still not foolproof only way to know for sure is to take that cover off

as for you G-wiz you have a torsen style lsd right? that thing shouldn't lock up the tires like a clutch unit

 

ok i have a quick question also. the rear-end in my car now is a 3.73 limited slip and i need some better highway gears for it so i can offord to drive it a little better and i'm wondering if they have rear-ends that are like the mustang rear-ends that are full time posi i guess you could say. i want it to be posi even when i turn the wheels and not only when i'm going straight. i'm wondering if any of the gm vehicles came with these in a ratio of like 3.23 or so. my car runs like 5000-5500 rpms at like 120 mph and i want it to try to achieve like 150-160 mph with the car, what ratio would this require if i'm turnin like 5500 at 160(speaking theoretically)? so pretty much what i want is a rear-end that i can do 160 in and still have full time positive traction? and it has to hold up to the 383 stroker that'll be roughly in the mid 400's....

 

Nice username ( i have a similar one on another messageboard)

with my TH400 and 3.23 LSD (also have 275.60.15 tires on the back) 40 MPH is about 1500 RPM.

i'd go check RPM's at a higher speed but i just broke another speedo cable tonight


dunno if my LSD is worn out or it's true posi but even on dry ground if i take a 90* turn the outer tire squeels and it's extremely easy to swing the back end out..


Anyone know how i can find out exactly what rear end is in mine? it's got the "Limited slip lube only" tag.. was there a true Posi unit avaliable in '82?

 

awesome thanks Purple83TA. so if i did calculations correctly, 1500 rpm at 40 mph, thats roughly 37.5 rpm's per mph so i could theoretically go 160 mph at 6000 rpm's...right? i also am running 275/60/15's so thats perfect so now i guess i need to look for a 3.23 full time posi rear-end then.... also one quick question for ya with the 400 tranny. i have now idea what the tranny was originally out of but i was curious as to what they usually have the speedo gearing in those set for? are they usually set for a 3.23 rear-end or would they be lower geared like 3.42s or 3.73s? i just bought a mechanical 160 mph speedo from autometer and since i spent all that money on it i'd like it to be correct so i'm curious as to what the ratio usually is in the 400 trannys....thanks!

 

well, the first one i got was out of an early 80's suburban and the speedo gears were pretty accurate, i had the stock rims on back then and it was barrely off. the 2nd TH400 was out of a late 70's truck and it was AFU..

this one came out of a 3/4 ton early 80's truck with (just guessing) 3.23's or something around there. it was pretty accurate (give or take a few MPH).

my 2nd tranny did have some kind of extra gearbox screwed on... not sure what it did though. the current one is screwed straight into the tranny so i'm guessing 3.23.



I just looked and my speedo cable fell down and layed on the collector flange, that's why it died.. and it's only a few months old too

 

hey check out the equations calulator on the left side of the sites homepage. pop in ratios and play with it until u find the ratio that gives u your 160 mph at whatever rpm

 

Hey WhiteLightnin83,

just curious, where and how did you run your speedo cable?


no matter what mine keep getting bent at too much of a curve so the speedo bounces up and down...


Thanks,
Dave

 

i actually had it routed up through the car on top of the tranny through the same path that the shifting cable went and then under the dash from there but i have since then moved it to nearly the original path. i cut the other speedo cable off right under the dash and then pulled the plug piece in the firewall all the way through so i had a good sized hole to fit the other end of the cable through and then routed it right under the brake booster and intertwined it in the brake line to hold it in place properly so it wouldn't fall and cause and problems. i prevented any major bends in the line if at all possible but mainly kept to the old cables routing system. i haven't exactly tried it yet though but i hope it is ok... i finally got my entire motor in and together on tuesday night and when i started it the head gasket never sealed properly and then completely blew out and filled the motor with water! so i skipped school yesterday to tear the entire thing apart AGAIN for ANOTHER rebuild with some $60 felpro head gaskets this time and hopefully i can get it all together tonight and try this one more time... i still haven't got to try out the new 383 stroker and the new Turbo400... so i'm very anxious as well as nervous that it'll even work