Rear Axle: Avanti rear end swap and positraction

[keithwahl]

Has anybody changed their rear end from 3.31 to 3.73 and what do you notice in drivability?
Has anybody added Positraction- if so where do you get parts and what does hat do for your performance?

Thanks,

Keith

[RDWEAVER]

Fuel mileage will probably decrease somewhat and your engine will be turning faster at cruising speed. Twin traction will aid traction and keep only one rear wheel from spinning. Do you have a 4 speed transmission?

[Jeffry Cassel]

It makes car a little quicker but at hiway speeds the RPMs are right up there. I'm happy with a 3.31.

[Mike Van Veghten]

The 3.73 will add greatly to the driveability of the car. The engine will not "lug" while driving slowly and then when you hit the throttle hard, the engine will rev much more freely and accelerate the car much quicker.

If tuned well, the mileage "may" not change that much. Also, this depends on how...you drive. If you are hard on the gas pedal, yes, the mileage will drop. If you drive without always slamming the throttle, the mileage "may" even pick up a little. This is because the engine doesn't have to work so hard to get the car moving.
If you do more city driving than highway, then yes, you may see better mileage. If you do more freeway/highway, you may see a slight drop. When you want to pass someone on the freeway, the engine, again, will not struggle to accelerate, it will pick right up and run well.

Many people have the idea that the engine is better with higher (smaller numbers) gears. For cars/engines before about 1970 or so, the smaller engines were not built to run at lower rpms. That's just not so. They NEED the help of the transmission first gear and the rear axle gears to keep them in a "happy" rpm area where the torque output is the greatest. If the engine is running below the "meaty" part of the torque curve, the engine struggles to accelerate, it's just not in a happy place. If the engine is running along at a higher rpm/meaty part of the torque curve, the acceleration is much snappier.

While nothing is free, there is no guarantee that the mileage will change all that much from the 3.31 gear...in either direction. Much of that will depend on your driving.

As for the positraction, yes, a very worthwhile addition addition, especially with the 3.73 gears. Without, the likelihood of constantly spinning one tire is high. With the Posi unit, that likelihood goes way down.
Also...all the additional power that the rear tire will see with the new gear set, the positraction (two tire working) will accelerate the car that much faster than if only one tire is driving the car.

NOTE - This all supposes that the housing you have is a Dana 44.
Many if not all off-road companies have Dana 44 parts. Gears, 4 or 5 different posi units to choose from, bearings, gaskets, etc.
I had my Dana 44 gone thru completely, ALL new parts, from axle to axle by Currie Ent., in SoCal. The gear in this axle...3.73..! In one of my 54 Conestoga's.

Mike

P.s. - Many will site that newer cars have a higher ratio, the engine runs slower at a given speed. Why is that..? Because the entire PACKAGE was designed to do this. From the cam shaft, to the compression ratio, to the transmission design, to the axle ratio...AND don't forget, these cars are computer controlled. The fuel and ignition timing are setup much different and have the ability to constantly change with the slightest load and rpm change.
Can't do all that with any car of yesteryear..!

[studegary]

From the AOAI Forum, I get the idea that you are converting to an overdrive automatic transmission. In that case, I would advise going with a lower ratio (higher number) differential. Studebaker's name for limited slip is Twin Traction (what you refer to as Positraction, a GM name).

EDIT: Three ways to get the different ratio and Twin Traction; 1) buy a unit that someone has removed from a Studebaker, 2) buy from a Dana supply house, 3) buy the entire assembly NOS from SI.

[JoeHall]

Without overdrive, a 3.73 will feel like a 5-speed, being driven in 3rd/4th. You will sense a need to upshift, but it's not possible. Without overdrive, I have found 3.07 to be best rear gear, but a 3.31 will suffice. Neither are as good as 3.73 on the low end for jack rabbit starts, but I spend much more time on the upper end, cruising at road seeds. An OD provides the best on both ends, and 3.73 was standard issue in 62-64 Hawks that came with OD. My favorite rear gear for OD is 3.54. They are harder to come by, but that's what I have in both GTs with OD. (One has 700R and the other T85.)

To give some idea of rear gears without an OD transmission: with 205/75-15 tires, in high gear, a 3.73 moves the car 22 MPH per 1000 RPM; a 3.54 goes 23; a 3.31 goes 24, and a 3.07 goes 26. For example: a 3.73, at 3000 RPM, goes 66 MPH (not factoring TC slip for automatics without TC lockup); with a 3.07, at 3000 RPM, the car goes 78 MPH. If you have driven your Stude enough to know the RPM range you prefer, you can choose from the numbers above. I have found Stude V8s to be happiest at 2200-2700 RPM. Happiest meaning less noise, heat, blow by, oil consumption, and wear on accessories (WP, alternator, PS pump, etc.), while getting more MPG.

With a 3.73, in OD, the final drive ratio becomes 2.61, which moves the car about 29-30 MPH per 1000 RPM, and about 75 at 2500. When you want a faster spinning motor, just use the selector to lock out OD. That works with modern AODs and with older Studebaker T86/85s. I prefer direct drive around town but, at 55 or above, prefer OD.

[Dick LeRoux]

Switched to 3.73 on my '63 R2 and cruising is much more enjoyable. Don't ask about the speedometer because it just fluctuates and bounces. I just drive by RPM and keep myself up with traffic. I figure I'm good at about 65 - 68 mph and keep to the far right lane as much as I can when on the freeway. I do my best to avoid I-5 & 405 and will search out two-lanes for any road trips which suits me fine.

[sbca96]

I went the other way around .. 3.73 to 3:31 .. I also went from a T-86 to a TKO 5 speed which balanced everything out nicely.

Tom

[1953champcoupe]

I run 3.70 gears in my 38 Chevy with tall 15 inch tires and a four speed. It is fine on two lane roads running at 60 MPH but on the interstate the engine is spinning about twice as fast as my daily driver at 75 MPH. The 53 Stude (58 Hawk chassis) I am building will use the original 3.31 gears with a 700R4 so I can cruise at slower RPM. If it's geared a little two high I have a3.7XXX TT rear end in a parts car that I can replace it with. Unless your car has an over drive or you only drive in the city I'd stick with the current gears.

[Dick LeRoux]

Tom,
I went the other way around .. 3.73 to 3:31 .. I also went from a T-86 to a TKO 5 speed which balanced everything out nicely.

My mistake in writing this & not checking before posting. I did as you did and went to the 3.31.

[JoeHall]

Tom,
I went the other way around .. 3.73 to 3:31 .. I also went from a T-86 to a TKO 5 speed which balanced everything out nicely.

My mistake in writing this & not checking before posting. I did as you did and went to the 3.31.

Whew, you just renewed my faith in common sense. LOL. If you think the 3.31 is nice, you oughta try a 3.07. That would increase speed about 5-6 MPH at any given road speed above 60. For example, a 3.31 with 27" height tires rolls 60 MPH at 2500 RPM, and the 3.07 rolls 65 MPH at 2500. For calculation purposes, with the same height tires, the 3.72 rolls 22 MPH per 1000 RPM; the 3.54 rolls 23 MPH; the 3.31 rolls 24 MPH, and the 3.07 rolls 26 MPH.