VCCA Home Forums TECHNICAL - 1945 AND NEWER 1955-1957 STOCK 56 DIFFERENTIAL HELP

Looking for some advice on a complete overhaul kit to rebuild the differential on my 5tock 56 bel-air.
Can Anyone suggest a kit that they used or a kit that someone else has used and been happy with ?
Much appreciated.
Paul.

It would on what parts you require and which parts come in a kit

What differential gear would be the best all around ratio to run
with a 200 4r overdrive transmission ?
Any recommendations ?

...4.11..

After contacting several transmission places that sell the 200 4r transmissions they
recommended I stay with the 3.55 R & P. Think I"ll go with their recommendation.
. Gene as always....

Selecting the appropriate rear axle ratio is always tricky with converted drivelines.

The key question is whether you are expecting acceleration or smooth driving at highway speeds.

I learned a long time ago that the key to good highway performance is to have the engine running at rpm’s just beyond the torque peak of the engine at your cruising speed. That way the engine goes “back up” the torque curve if there is a slight grade or anything to try and slow the vehicle. Otherwise the transmission will frequently be downshifting when there is any additional load.

The axle ratio is one part of the equation that includes transmission ratio (this is an overdrive transmission), tire rolling radius, and engine torque peak.

A great example of this is that many cars will not achieve their top speed in top gear. The engine is operating at too low a speed to produce the power needed to overcome the rolling resistance and aerodynamic drag. Down shifting let’s the engine run faster so it produces more power so the car goes faster. I have had numerous cars that fit this scenario.

If the 1956 has a stock 265 engine the RPMs may be too slow with the .3.55 and cause it to labor and overheat.

The engine is a 350 , although the speed limit here in Arizona is 75 m.p.h. I cruise
at 70 m.p.h. on our roads, Rpm's at 1900 /2000.

I can understand why they recommended you stay with the 3.55 rear gear ratio. When the transmission is in overdrive your drivetrain is operating like it has a 2.38 rear axle. That is pretty low.

A lot depends upon how the engine is tuned with respect to valve timing, spark, and fuel delivery. Gene is correct that you do not want to overload the engine. A term we often used was “lugging” then engine. That described when you stepped on the accelerator and the engine speed would not increase due to load. That is really hard on bearings.

How responsive is the throttle when you try to accelerate slightly when driving at 70? Does it downshift immediately or will the car accelerate?

I expect it drives ok on I40 in your area. How does it drive when you are 2 lane roads in the mountain areas around you?

No problems with downshifting when accelerating at 70, no lugging. On a long stretch of hilly road state hwy 89 about 35 miles into chino valley,AZ it sometimes
feels like it's slightly lugging , That's why I was asking if the 3.70 might be a better chose ? Timing is advanced to the max without starter kick back or pinging
The carburetor is a Edelbrock # 600 c.f.m. Rusty I appreciate your help and your knowledge .
Your thoughts on 3.55 or 3.70 ? Would a 3.73 fit into the 56 housing ? I'm still trying to get the ratio that will work best for me..............

Hi p.k.,

Thanks for continuing this discussion. While I am no expert in drivetrain conversions I have spent a lot of time working with matching engine performance curves to vehicle performance. As you can imagine that was a lot of math with gear ratios and tire size.

One rule that I have learned the hard way is that in most drivetrains small changes do not make much difference for general driving. Obviously race cars are a different situation. In those scenarios a 2 or 3% difference could be the winning advantage.

From my perspective the 5% gain from 3.55 to 3.73 would probably not be very noticeable.. That change would hardly be a 100 engine rpm increase at 70 mph in overdrive. For a carbureted engine with distributor style ignition for timing that change is not worth a lot.

I know this sounds crazy. It might tell you a lot if you spent a few bucks and some time on a chassis dyno. At a minimum that would give you an idea of the engine torque and power curves. Then you have at least some basic data to help decide what changes make sense.

Thanks Rusty, If I spent that kind of money I'd not be living in this house, but the dog house...

I can relate to that scenario!

Here is a neat little formula that I found when considering changing to OD transmission in my '51


[ (Axle Ratio x Vehicle Speed x Transmission Ratio x 336.13) / Tire Diameter ] = [ (3.73 x 65 x 1.00 x 336.13) / 31 ] = [ 2628 ]
Note: 336.13 is used to convert the result to RPM = [63360 inches per mile / (60 minutes per hour x Pi.)]