Re: Conversion of a ‘56 Ultramatic to a ‘55 400 & A Few Engine Rebuild Questions

Posted by HH56 On 2021/1/31 10:59:42

The 56 trans will not need any different linkages or mounts and will just bolt in place of the 55. The only visible difference is the 56 unit is a bit smoother appearing and several pounds lighter because the case is aluminum instead of cast iron.

It sounds as if you are going for a bit more performance with your engine rebuild. As Riki says, the trans was NOT designed for high horsepower and to go farther, the trans turned out to be for all practical purposes the weak spot in the 55-6 Packard lines. Basically, the Ultramatic was a very early automatic designed entirely in house around a straight eight engine that averaged around 165 hp and less than 300 ft/lbs of torque. Unlike competitive units that came into use for other mfgs 55-6 models, it did not have the benefit of hindsight, collaborative production or high volume experience when for competitive reasons it was hurriedly modified and then bolted onto a V8 engine whose output averaged considerably more at around 250hp and 350 ft/lbs.

With the mechanical components being almost identical but engine output increasing a substantial amount, that along with some teething issues with the many hydraulic changes needed to make essentially a single speed non shifting transmission with torque multiplication into an actual two speed gear shifting unit with torque multiplication things started to give and transmission issues were numerous. For 56 they revised and refined what they could in the valve body to minimize the harsh and runaway shifts but the mechanical limitations were still essentially unchanged even though engine power was increased even more.

To sum it up, the Ultra will perform adequately and for the most part reliably if driven sedately or with some decorum but if you plan on fast getaways from the stop light to take full advantage of a high horsepower engine or if you find yourself in driving conditions that require a lot of up or down shifts you may soon be reduced to limping home or worst case suffer the indignity of calling a car hauler to be carried home.