I found a long article about diesel engines and automatic transmissions on the International web site. Here are a few paragraphs which I find interesting. The bottom line points out that diesel engines tied to auto trannys usually don't use all the published torque available. I'd like the REAL numbers as described in this article for the GM, Ford, and Dodge products. That would be interesting!
From: http://www.internationaldelivers.com...l_bus/178.html
"It's easy to claim "I've got more torque!". It's more difficult to understand the process of engine-converter matching and why that claim is meaningless. Bear with me, please.
You do know that the common method of rating transmissions is with the maximum engine torque level that can be absorbed. In the case of manual transmissions the peak torque produced by the engine is balanced against the capability of the transmission. For instance, the Spicer 6+ (code 13MAG) is rated at 530 Lb.Ft. and we couple it to our engines that have that peak torque value. In actual operation the transmission is subjected to that torque level whenever the engine passes through the peak torque speed range while operating at full throttle, either on acceleration or lug down when climbing hills.
Allison also rates their automatic transmissions and approves their application based on engine torque output. But, in reality the AT-500 series transmissions are never subjected to the engine peak torque. The operating characteristic of the torque converter (which is a fluid, not a solid, coupling) is such that it reaches the maximum torque transmission level at zero output speed. This condition is referred to as "stall". At the stall point the gearbox of the transmission is subjected to the maximum torque level which is determined by the engine input torque and the torque multiplication ratio of the converter. It is this condition that Allison uses to establish the "real" rating of the engine-transmission combination.
You can experience the stall torque situation by operating a vehicle as follows: with the engine running and warmed to normal operating temperature, transmission in Drive, set the service brakes tightly with the foot pedal and press the accelerator pedal to the floor. The engine speed will rise from low idle to a speed above peak torque and below maximum horsepower and stay there. This is stall speed. Do not hold this condition for more than 10 seconds because all of the engine power is converted to heat in the transmission fluid and you can overheat the system.
For purposes of best acceleration performance the engine and converter combination (or "match") are selected to cause the stall point to occur at an engine speed higher than peak torque where the resulting torque level is less than the peak. Since this match point speed and torque level can vary from one combination to another the advertised rating of the transmission would be hard to define and describe in simple consistent terms. Every matchup is evaluated on an individual basis with care taken to not exceed the gearbox capability at the converter match point."
