The Stovebolt Site Forums The Shop Area General Truck Talk Hydra-matic and rear end gear ratio?

Running original hydra-matic in my 54 3100, and closed driveshaft. Wondering if anyone knows what the rear end gear ratio would be? Was there more than one rear end gear ratio offered with the hydra-matics?

3.90 same as other 54 half tons

Apparently the 3.90 was the one and only ratio available for '54 3100's regardless of the transmission used.

And, the ring/pinion can be changed to a 3.55 set, as long as you use a 47-53 ring-gear carrier.

Thank you all so much. Found out what I needed to know.

tc, see that you also have a truck with the hydra-matic. Have you changed your ring-gear carrier to the 3:55?

Be careful about swapping ratios behind the HydraMatic. It has a fluid coupling, not a torque converter, so it's not as forgiving about breakaway torque from a stop and a smooth transition of power between gears. It's entirely possible you could end up at the wrong torque point in the curve when shifting, with no way to adjust the throttle pressure to get the engine back into the RPM range where it belongs. Converters virtually double the torque from the engine just before the input shaft starts to turn as the vehicle pulls away from a stop. This allows manufacturers to use higher gear ratios without sacrificing low-speed performance. A fluid coupling acts like a manual clutch, with little or no slippage when locked up, but no torque multiplication on breakaway. Converters have a maximum RPM efficiency of 90%, with 10% slippage even at its best lockup point. That's the reason for having a lockup clutch inside the latest generation of converters with computer control of the lock-in point.
Jerry

Ted,

Yes I have changed the rear-end behind my '54 Chevrolet 3104 Hydra-Matic pickup to 3.55. This truck is in the middle of a complete restoration, so it might be a year or more before I drive it.

The 3.55 in my 4-speed Suburban convinced my that the conversion is worth it (even though the increase in speed, decrease in RPM, is only about 11%).

A friend who has a 53 GMC 1/2 ton with a Hydra-Matic changed his rear end to a 3.55 several years ago. He drove that truck on a 5,000+ cross-country trip a few years ago. He'll be driving it to the ATHS show in South Bend this spring (from Boston).

I'll take my chances with the 3.55 behind the Hydra-Matic. And, if I am wrong, I have a spare Hydra-Matic rear-end that I can swap back in (I'd be willing to bet that I will not need to do so).

If you're building a cruiser, not a working truck, the higher ratio will probably be OK. The shift points on the hydro were optimized for load-pulling or hauling applications at or near maximum gross weight, and full-throttle operation. Running a Cowboy Cadillac instead of working the truck will give you more options.
Jerry

I work my 1/2 ton trucks as 1/2 ton trucks.

Maybe you have driven a Cowboy Cadillac, Jerry, but I have never considered doing so.

Great information guys. Thanks again for all your help.

Hey Hotrod,whats the difference between a fluid coupling and a torque converter?

The torque converter has a third item called a stator that redirects the fluid as it leaves the driven torus and re-enters the drive side. Before the driven vanes start to move, the fluid velocity actually helps the engine turn and increases torque. Fluid couplings only have two parts, a drive torus attached to the crankshaft, and a driven torus splined to the transmission input shaft. The vanes of the torus are straight on a fluid coupling, curved on a torque converter. The stator has a one-way clutch built in so it cannot turn as it's redirecting fluid. Once the driven side of the converter starts to turn, the stator rotates with the assembly and the torque multiplication goes away. Brcause of the convoluted fluid flow, the torque converter cannot achieve a good lockup. There's always about a 10% slip between input and output speed.
Jerry