i have mild knowledge about engines and transes,but don't have a lot of knowledge about rears. hoping you guys could help me out a bit. i am goona need a new rear in the future and i'm trying to decide what the best for me would be. i have 310hp and 340 tq give or take. this will be mainly a street vehicle. im looking for something posi but if limited slip does me better than that works. What type of gears would get me good get up and go, will do alright on the highway, and get me decent gas mileage. don't think those three go together well but im just looking for a little bit of help. thanks.

"Posi" is short for "Positraction" which is just an old term Chevrolet used for limited slip. Choosing an axle ratio without knowing anything about your engine and trans is like trying to nail Jell-O to a tree. On here, you don't benefit if you think your cam specs are secret. Which transmission are you using? If automatic, what rpm does the torque converter "stall" at? And which axle do you have? Look at it from behind the car, you'll hafta crawl under. See that cover? Are there 9 bolts around it, or are there 10? 10 bolt gives you alot more options, but some claim the 9-bolt is stronger. 10 bolt will survive the power you're claiming.

First, Posi is limited slip. There is no difference between them. Posi is Eaton's trade mark for their limited slip differential.

The 3.42 is a very popular gear. It works well with the 700R4 transmission. The 7.5 rear end that you probably have will hold up to an engine like yours. I recommend a new 28 spline Eaton, 3.42 gears and 28 spline axles. You didn't give any more information on your car, so I don't know if you already have the 28 spline axles or not.

In several third gens I've run engines making more torque than that with 26-spline axles without incident, even with repeated blasts down the quarter mile. No point buying stuff you don't need.

yea its a 10 bolt. i have a 700r4 with a stock trans. all the stuff about is stock. 350 with just mild stuff done to it. bored 30 over and some bolt on stuff. what is the difference between a 26 and 28 spline? and is a 10 bolt the best rear to work with.

yea its a 10 bolt. i have a 700r4 with a stock trans. all the stuff about is stock. 350 with just mild stuff done to it. bored 30 over and some bolt on stuff. what is the difference between a 26 and 28 spline? and is a 10 bolt the best rear to work with. also whats the benefits of having a higher/lower rpm with a torque converter?

28 spline is stronger. 10 bolt gives you more choices of gear ratios and more choices of different types of posi. Think of the stall speed as the rpm at which you engage the clutch with a stick shift.

okay posi is the same as LSD?? i thought LSD was a type of open differential.. basically when i jacked my car up i went to the passenger side and rotated the tires clockwise and the driver side rear was going the opposite way. if i was standing on the passenger side and spun the tire clockwise, then ran over to the driver side it would also be spinning clockwise. do i have a posi, LSD or open diff?

ok now im all confused. i thought posi was when both wheels moved the same direction and limited slip was when only one wheel turns. i thought a burnout with a posi both wheels spin but burnout with limited slip only one wheel spins? can we get this cleared up please.

Posi and limited slip, as mentioned above, are the same thing. Two stripes on a burn-out (providing the limited slip is functioning as it was designed). An open rear end is a one-wheel peel. Check the following link for 10-bolt info:

https://www.thirdgen.org/forums/tran...-10-bolts.html

JamesC

ok yea that whole link helped me out a lot. thanks a lot.

A limited slip differential has clutches or some device that limits the differential's ability to spin freely. It will apply power to both rear wheels as best it can so that traction is improved. Some types of limited slip differentials (LSD) work better than others. An Eaton uses clutch discs and preload springs to create friction and lock the axle gears to the ccase so tht they can not freely spin. An Auburn uses cones to do the same thing. A Detroit Locker uses drivers and couplers with teeth that engauge to lock both axles together with no slip at all when under power. The Governor Lock uses a governor and a cam along with clutch plates to lock the differential. A Posi is an Eaton LSD. Posi is a trade mark name for Eaton, along with Positive Traction. Ford called their LSD a Traction Lock, Mopar had the Auburn Sure Grip and Spicer Power Lock, Pontiac had Safety Trac, which was a Borg Warner LSD. There are many others. A differential that only spins one wheel is a standard or open differential, NOT a limited slip.

okay so if both my wheels move but both move opposite ways (one moves like its in drive, the other like its in reverse) then i have a posi unit or an open unit?? that link did help other things a lot but i know i can see what i have but turning my wheels while on jackstands. just not sure which is which..

I think you have lsd/posi when the tires do that. If you were to turn your driveshaft they would both spin the sameway. Only one real way to find out, Get out there and PAINT THE ROAD!

painted already lol. when i first stomp the gas and do a wheel stand for the first 2 or 3 seconds only the passenger wheel moves then driver side starts spinning to. thatd be LSD right? they dont both spin immediately but like i said after about 2 seconds both start tearing the street up.

Pretty sure.

well that pretty much puts a smile on my face then! gotta get some 3:42 gears now

If you have the tires off of the ground and the transmission in neutral then you should turn one wheel and the other turns the same direction. This is limited slip, or "posi". If one turns one way and the other turns the other way then it is a standard differential or a worn out limtied slip. The whole idea of a limited slip is to lock both axles together so that they both go in the same direction. If they are not locked then one can spin without the other.

If you start a burnout and one wheel spins, then the other one kicks in then I'd guess that you have a standard differential that has done too many one wheel burnouts and is trying to weld itself solid. There is a good chance that a differential failure is in the near future. One wheel burnouts are very hard on a differential and can do serious damage to it.

just bought the car 3 months ago, engine blew within weeks, only done 2 wheel stands, everything else was just a quick wheel chirp when leaving a stop light. i wouldnt be surprised at ALL if the rear end went out of it. bought the car for 350 bucks and it was def worth no more than 350 bucks. problem after problem after problem. me and this car got off to a bad start. think im gonna sell it before something else goes wrong. If i get a rear end out a 4th gen (its 1 1/2 to 2 inches longer i already know) will i be able to keep these wheels i got or will they rub? they're pretty close as it is..

big gear head is correct.

what size gears give you the following

good get up and go
65 mph under 2500
decent gas mileage
durability

all with a 10 bolt

with a 26" tire, 3.42:1 with 700R-4 is 2000 rpm at 65 mph with the torque converter locked up. You should be well over 20 mpg, even with a 383.3.73:1 puts you just under 2200 @ 65.

With stock-headed LO3, and/or extended interstate use, go 3.42:1. Otherwise, 3.73:1 is about perfect with 700R-4

yea i do a lot of every type of driving. lots of back roads, lots of 55 mph roads, and a few freeways so a 3.73 will work best? the mpg was the incuded with a 350 and 700r4?

Now that post just confused me. I've never once yet read or heard what anyone's miles per gallon was running any V8 with 700R-4 and a 3.73:1 axle. Never checked it when my '88 Camaro had that combo. I have heard, read, and tested for myself, miles per gallon of camaros with V8/700R-4/3.42:1, and most were 22-23 mpg, with LO3s, LT1s, carbureted ZZ4s, every kind of SBC. if you use a tight, low-stall converter, you can run without converter lockup, but that's another 200 rpm. And 2400 at 65 mph is extremely annoying. The common consensus is that a 4.10:1 gear is just plain too much with a 700R-4. If you need a 4.10:1, then you need to convert to a built 200-4R. Going the other direction, I have experience with a 350 Camaro exceeding 25 mpg at 65 mph with 700R-4 and 3.08:1. Switching to 2.73:1 did NOT help. Ever since then, every time I bought a camaro or firebird with 2.73:1, I've always replaced it with a 3.08:1 gear, (usually followed by some other gear, but that's irrelevant) and every single time, 3.08:1 has helped both mileage and acceleration.

so what is the 2.73 ideal for and what is the 3.73 ideal for

2.73:1 is ideal fot a boat anchor, literally. 3.73:1 is the most fun you can have with a 700R-4!

is there anything else i should know about when it comes time to getting my rear replaced? i know i should use 3.73 and from what you guys said 28 spline is the way to go, is there anything else i should know? and if i had the parts already to take a 10 bolt and get it rebuilt and the new stuff done how much is that goona run?

what are 3.42 gears like

You need a mega install kit with new wheel bearings, you need new lube, and you need new axle seals. Then you'll need to tell your tire size so we can get you the correct part numbers for the speedometer gears. Installation runs near $500 if you pay a shop to do it for you. Otherwise, you need some special tools.

so really a 3.42 is the same as 3.73 just with mroe money involved? what kind of customizing would be needed to convert my stock 10-bolt to a 3.73? and how much we talking about all this? including parts and labor

I think I've now confused you. I apologize. For a 3.73:1 setup: new 3.73:1 gears, around $200 new limited slip, around $500 new mega kit, around $150 new lube, around $20 new seals. around $5 used 28 spline axles $? new 28 spline axles, around $250 that's your parts. Add $500 for labor of pro installation. To go 3.42:1 can be done much cheaper. Start by getting the axle assembly (around $100 in my area) from any '90-'92 V6 Camaro or Firebird. It already has 28 spline axles and 3.42:1 gearing. Then figure $400 for a conversion-style locker, and $20 for lube, and $10 for a rear cover gasket. To compare a 3.42 gear to a 3.73, the 3.42 gives up a little bit of acceleration. The 3.73 gear gives up a little bit of gas mileage.

ok yea sounds like if its only 2-3 mpg difference then i'd rather go with 3.73. thanks for your help with the whole rear issue. now one more question. is a stall converter the same thing as a torque converter

I would use the Eaton Posi. Anything else is probably not going t o work the way you want. Some are cheaper, but the Eaton is worth the extr money.

so i need to buy a whole new differential and everything?

so i need to buy a whole new differential and everything? on jegs i see these 3.73 gears that are second series and third series. whats the deal with those

If you have 2.73 gears and a standard differential then yes you need a new one. If you have 2.73 gears and a posi then you can get the thick 3 series gears to fit the 2 series differential that you have now. The 2 series differential is for 3.08 to 2.41 gears. The 3 series is for 3.23 to 4.10 gears. The difference between a 2 series and 3 series differential is in the location of the ring gear flange.

no i was talking about 2 series and 3 series ring gear. does 2 series gears go with 2 series differential and likewise for 3 series?

oo so the standard differential is an open dif and a 1 series. the 2 series is a stock lsd, and 3rd series is like eaton posi or something correct?

if you have 2.73 gears and a standard differential then yes you need a new one. If you have 2.73 gears and a posi then you can get the thick 3 series gears to fit the 2 series differential that you have now. The 2 series differential is for 3.08 to 2.41 gears. The 3 series is for 3.23 to 4.10 gears. The difference between a 2 series and 3 series differential is in the location of the ring gear flange.

Click on this picture to make it larger. These are all Eaton cases for the Chevy 12 bolt, but the difference betweenn "series" is the same as any other rear end. Notice the difference in the location of the ring gear flange on each case. The one in the front left is a 2 series, the one in the center is a 3 series and the one in the rear right is a 4 series. The ONLY difference in these cases is the location of the flange. This is because of the diameter of the epinion gear head. A 2.56 gear set has a much larger pinion head than a 4.11, so the ring gear needs to be moved farther away from the pinion gear center line for the 2.56 and closer for the 4.11. The series has nothing to do with what type of differential it is. Your 2.73 uses a 2 series and you need a 3 series for gears in the 3.23 to 4.10 range. The 7.5 is the only rear end that has thick gears available for the 2 series idfferential, so 3.42, 373 and 4.10 gears can be used on the 2 series idfferential.

ok it makes more sense now. so what is an eaton posi considered

The Eaton posi is available in different series. The old Chevy 12 bolt had a 2,3 and 4 series Eaton. The 8.2 10 bolt only had a 2 and 3 series Eaton. The 8.5 and 7.5 never had an Eaton installed from GM but after market Eatons are available in 3 series for both of them. There were no 4 series differentials of any kind for the 10 bolt rear ends.

ok i looked at the 28 splines and they are just under 2 inches longer. are the 26 stock factory axles? and what will have to be done in order to put 28s in

The 26 spline axles were used up to '90 and then they went to 28 splines. The length is the same, only the diameter and number of splines changed. In '93 the rear end was made wider for the new body. 28 spline axles from '90 to '92 will fit your rear end with no problems, as long as you also have a 28 spline differential.

so any 28 splines besides 93+ will be the exact same fit as the 26?

Should be.

one more question is a stall converter the same thing as a torque converter

A torque converter has a stall speed. Some have a higher stall speed than others. A stock converter has a low stall speed, usually below 1000 RPMs. An engine with a large cam and other high performance parts will have it's torque peak at a higher RPM, so to get the most performance from the engine the stall speed of the converter is raised so that it is closer to the torque peak of the engine. This is done by changing the over all diameter of the converter, changing the stator and other internal parts. By increasing the stall speed of the converter you will allow a high performance engine to get closer to it's torque peak before the converter begins to lock up and start moving the car. Torque converters are used to tune a drag car for better launch, 60 foot times and over all ET. There are endless combinations.

ok so if you take a stock 210 hp chevy and make it 350 hp its best to get a new torque converter and stuff

Wipe that smile off your face. You don't have a posi (or LSD). As others have said, "Posi" is just a brand name for LSD. Same thing. No difference.

If you launch with one wheel spinning, and the other catches up, no matter how quickly, that's not posi.

Very simply, a posi (or LSD) axle has a set of clutches or cones inside the differential unit that lock the two axles together, so they should always turn together under however much power you can generate with your own two hands.

Here are the tests to identify a posi.--
1. ONE rear wheel off the ground, transmission in neutral. You should not be able to turn the wheel.
2. BOTH rear wheels off the ground, transmission in neutral. Turn one of the wheels. The other wheel should turn the same direction. This is true whether you turn right or left side, forward or backward.
3. BOTH rear wheels off the ground, transmission in gear or in Park. You should not be able to turn either wheel, any direction.

Once you identify it as a posi, then you can test the health of the clutches by repeating test #1. Make a jig of some sort so you can turn the axle with a torque wrench. It should take 110 to 140 ft-lbs. of torque to turn the axle.