Open diff -vs- Posi

[T4TWO]

Ok, A friendly but informative disscussion about "open diff's" vs one with a "posi" in your humble opinion.

Pro's and Con's of either types of rear ends?

Which would you install and why?

What size gearing would you install and why?






I'll start by saying that my build came with an open diff and I'm not sure what size the gears are in there now. Because I haven't got to that part of my build yet. But, I have always liked and had/have posi rear ends in my current/past Hot Rods. Anywhere from 4.10 to 5.36 gearing. My first hot rod had 30" tall slicks and I needed the 5.36 gears to turn them and get me down the 1/4 mile at 12 flat, + the car weigh a ton. One of my current street/strip cars doesn't weigh a ton (about 3400 lbs) and I have 28" tall slicks for her. So I opted for the 4.10 gears moving me through the traps about the low to mid 13 second range. Our other race car (about 2600 lbs) has 4.88 gears on a mini spool to move her down the quarter at 10 flat.

Recently I was able to pick up a used but reconditioned 3rd member with posi for my 56 pumpkin rear end. Gears in there are a 5.375 ratio. A bit large for my taste. But with 33" tall tires and a overdrive trans. I should be able to tame her down a bit and still cruise around all day on a tank of gas.

I know these cars are lite and do not require much. But curious as to what you peeps have in your ride and why you chose to go that route.

 

[RPM]

I use a 4:10 posi in my blown car, and a 3.73 open in the touring car.

A posi helps make the car leave quicker. But you don't want to get caught in the rain with big tires and a posi.

 

[Rick]

I hope you work in a tire store.Cause with a posi in these light cars your wheels will chirp like crazy.The reason?When turning a corner the inner tire turns faster than the outer one.A limited slip wont work either cause the wieght of the car is not enough to break the clutch's inside the pig.Ron (rpm) has a posi in his maybe he can shed more light on it.

 

[Keeper]

Well the rearend I currently have is 2.88 with posi. Once things are up and running I will see if they need to be changed out. I am looking more for cruising less for 1/4 mile times.

 

[EX JUNK]

I have a 4:10 Eaton posi unit in a '96 S-10 unit. I have experienced no "chirping" or excessive tire wear in almost 10K miles. I have good, positive launches with minimal tire slippage.

 

[Rick]

Then i guess i stand corrected.

 

[donsrods]

I'm running 3:00 gears in my T with 30 inch tires and a non posi. It is perfect for the way I drive. My wheelbase is only 100 and the car is light, so I don't want a posi as they can be touchy in the rain and can want to push the car through a turn.

On the other hand, my Son is running a set of 4:10 gears with a 31 inch tire and a posi rear end in his 30. His car is heavier and had a longer wheelbase, so he should be fine with that setup.

My 27 has a set of 3:70 gears, non posi, and it was absolutely perfect with the stick tranny and 30 inch tires. Cruised right along and had plenty of take off power.

Finally, my other Son's T has 2:73 non posi gears and he was just going to use them to get the car running then change to some deeper gears. He is running a hellacious cam that doesn't come in until 2500, so he figured 4:10's would be great. However, after driving the car for a year he has decided the 2:73's are actually pretty pleasant to drive around on and the car still launches harder than my T.

Don

 

[T4TWO]

Thanks for all the feedback. Keep them coming, this is very informative. And no I'm not running my T down the quarter, it was just to compare. I guess I will need to look for smaller gearing.

 

[bill4u6]

I run a powertrax in my s10 with 7:73 gears works real good I just dont nail it in a tight turn you have to learn how to drive with one.

 

[Telman2]

I'm running the same rear end I used when I built my car in 68. It came out of a 55 Chevy truck, it's an open differential and I believe it has 390 gears in it (I honestly don't remember). I've changed engines 3 times and transmissions twice but never felt a need to upgrade to a posi.

 

[ezrizer]

I spoke with Currie yesterday about a few things one of them being the different posi-traction options. From what I was told the clutch type (typical OEM) can chirp a bit in turns if the clutch pack is tight or the car is light or some combination of both. The other option is what they call the traction lock which uses a type of gear arrangement instead of clutches which is a more positive lock (no clutches to wear out and slip) in a straight forward direction and more forgiving in turns. Then there is the Detroit locker which I think is a heavy duty version of the traction lock and probably not necessary in a T-bucket.

 

[Todd]

Bill, is that a typo on your gear ratio?

 

[LumenAl]

I have a posi rear end... 335 gears, 29 inch tires, th350 tranny... wish I updated it to 700r4 to get the over drive... lost a little launch speed, but trust me... when I punch it passengers generally scream or yell... :dummy:

Down side to the posi is you have to be very careful in rain and not hit the corners as hard as you might if you had an open rear end...

 

[EX JUNK]

Since I do not have an OEM posi but an Eaton unit in my S-10 unit, that is probably why I do not have the "chirping" and difficulty in turns. It is also very stable in the rain. Living in Florida, where we get a lot of rain, I have had to drive in a few real "gully washers" and the car has remained very stable.

Another factor in my cars stability may be its weight and wheelbase along with rear tires. It weighs 1937 pounds and is 50 pounds heavier in the rear than the front. The wheel base is 108" and the tires are 285/70x15. A wider and more closed tread pattern, like many T's run, allows the tire to hydroplane and that can make for some pretty hairy handling in the rain.

 

[san-i-T]

I have a blown ride with 29 x 15.5 tires, 700r4 trany and 3.73 posi. Works for me!

 

[Ted Brown]

I run a powertrax in my s10 with 7:73 gears works real good I just dont nail it in a tight turn you have to learn how to drive with one.:biggrin:

7:73 gears ??? who would drive on the street with that low of a gear? and with the cost of fuel nowdays... hehe
All the old time pro race car builders and drivers, have told me years ago, to run as tall a gear as you can pull. Mine has had a gear of around 1:90 (33" tall tires) for a lot of years now.. If I am on the pedal hard (no spin at all out of the gate) it shifts out of low about 87MPH, and turns the tires about 1/2 a turn when it shifts (well that was with the Big Block) still the same rear gear now, and Chy. posi-lock, (NO it is a cone locker I believe, CRS hehe) not by choice, could not find an open leg when I wanted one... It used to scare people when it locked and un locked with a bang! For some reason it quit doing that lately. With this light of a car an open rear end works the smoothest, and if you really are into burning rubber, She will still burn both tires with an open rear end..

 

[Rick]

I have only one problem Ted.When i punch the car the front end comes up about 6-8 inches and make a left turn into the next lane.Not the best thing to do if you wanna keep the same underware on for a while.also it leaves just a 5 foot patch of rubber on the drivers side.Let it be known i have a 8 in ford with 2.73 ratio.and Mickey Thompson street slick compound.Look at my pics.

 

[bill4u6]

yes i made a typo i run a 3.73 gear:lol:

 

[HELLRAISER79]

I dont have my bucket done yet, but in my pickup an open diff was worthless. The posi was good until the carrier broke. Then i tried a mini-spool in an open carrier. I found out why they say dirt track use only. The mini spool broke and took the carrier with it. Im currently running a 3.73 gear on a moser spool and love it. I have 3.73 in 3 different cars and think its a perfect ratio. Btw the truck has 28" tall tires and goes 12.41@110 if you were wondering. And its not a drag car, its my street car. My bucket will most likely have 3.73 with a spool and some c-clip eliminators (very important).

 

[Ted Brown]

I have only one problem Ted.When I punch the car the front end comes up about 6-8 inches and make a left turn into the next lane.Not the best thing to do if you wanna keep the same underware on for a while.also it leaves just a 5 foot patch of rubber on the drivers side.Let it be known i have a 8 in ford with 2.73 ratio.and Mickey Thompson street slick compound.Look at my pics.

Hi Rick, I have set up AG/Super cars for years, no wheel stands out of the gate allowed, (can do them just for show) You can not steer with the wheels off the ground, besides that completely spoils all you setup... If you spend some time balancing out your ride, making sure you have the right length Radius rod/lift bars, you can end up with flat footing it out of the gate, without tire spin and at least one tire on the ground at all times.. almost forgot to mention the most important factor, unless Blown, exhaust system, the controller of Horsepower, where and when... Open leg rear ends are much safer on the street in light cars only...