trav_19,
I spent years restoring old 60s and 70s muscle cars for a hobby. I’ll try to take a shot at explaining this.
The engines in those cars were really quite unsophisticated compared to today’s engines that use variable valve timing and computer controlled ignition. As such, it was very important to match the torque converter stall speed (in an auto tranny car) with the operating range of the cam shaft installed in the engine.
The “bigger” the cam shaft (meaning more overlap and more lift) the more top end horsepower you could generate with the engine. However, the bottom end suffered as a result (meaning poor idle quality and less low end torque). (Since there was no variable valve timing back in those years, you just had to pick either top end power or low end torque when you designed the engine.)
A mild streetable cam shaft in those years had an operating range of about 1,500 RPM to 5,000 RPM. A “big” cam shaft in a typical 60s muscle car might have an operating range of say 3,000 RPM to 6,5000 RPM. That means that the engine won’t really start producing good power until you pass 3,000 RPM. I a big heavy muscle car like a Chevelle, you need the engine to get up to that speed before the torque converter starts to act on the turbine to rotate the output shaft and put power to the wheels. (You might get away with a lower stall speed if the car was relatively light like a Plymouth Barracuda, for example.) A torque converter is really nothing more than a fluid pump! Understanding the internals of a torque converter would really help you get your mind around this whole issue.
Here’s a great article with pictures that explains how a torque converter works.
http://auto.howstuffworks.com/torque-converter.htm
All of this said, I think that the problem with your Dad’s car creeping at idle (unless the brake is applied) is not related to the stall speed of the torque converter. This was a typical problem in cars with “big” cam shafts in them in those years.
As I mentioned a big cam shaft causes poor idle quality. That caused many carburetor tuners in those days (me included) to bump up the idle speed screw on the carb to keep the engine from stalling at idle. Raising the idle speed to perhaps as much as 1,000 RPM or even higher, was not uncommon. I’d say your Dad’s got the idle speed set a bit too high and that’s why the car creeps. Since it’s really not a big problem, many choose to just let it like that. Others carb tuners back then resorted to drilling bypass holes in the carb butterfly valves to improve airflow a bit at idle. This was a typical tuner trick to deal with big cams.
The main question I have for your Dad is what are the specs of the cam he has installed in the engine, and does the stall speed rating of his torque converter match the operating range of the cam.
Hope some of that helps.
