There's more. Many factors outside of the converter have a direct impact upon the stall speed. ATI notes that the engine type, engine power characteristics (and more importantly, the engine torque characteristics), bore, stroke, induction system, cylinder head modifications, altitude, chassis weight, chassis modifications, tires, header tube diameter, and even body style have an effect upon the stall speed of the torque converter.
As an example, a converter that stalls at 2,500 rpm in a lightweight early Nova with a small engine might stall at close to 4,000 rpm in a '57 Chevy with a big-block. Place one car at sea level and the other in Denver and you'll have a whole new range of stall speeds (the quality and quantity of the air has a direct influence upon engine torque).
TCI agrees with ATI's recommendations, and they stress the importance of having complete specifications of your car available before contacting the converter manufacturer. TCI adds that nitrous oxide, supercharged, or turbocharged applications also have a large influence on the size and type of torque converter required. Remember, with some of these artificially aspirated engine combinations, the increases in torque can prove considerable.
Engine Torque Curves And ConvertersBasically, the idea behind selecting a converter is to coordinate the converter stall speed to the engine torque curve. Unfortunately, optimism can have a detrimental effect upon converter selection, especially when it comes to engine output. Turbo Action notes that inaccurate engine power and torque estimates provided by the customer can have a serious effect upon the performance of the torque converter. The same can apply to optimistic dyno tests. To combat this, Turbo Action maintains a converter manufacturer can never have enough information.
A-1 Automatic Transmissions agrees, but they also note that there is a tendency for enthusiasts and racers alike to use torque converters with stall speeds that are too high. A-1's Marvin Ripes explains: "When recommending torque converter stall speeds, we tend to be conservative. In order to come up with a stall speed for a given combination, we rely upon years of experience with countless different automatic transmission combinations. There are some trends, which appear in the torque converter selection process, that tend to influence the selection of stall speed.For example, a three-speed "prefers" a converter designed to work at peak engine torque, while a Powerglide likes a converter designed to work above peak engine torque.
Obviously, specifying one particular stall speed for any given converter is virtually impossible with varying engine and torque curve combinations. What about the stall speed numbers published by the various converter manufacturers? Almost everyone will tell you to consider these numbers guidelines for stall speed potential.
Before taking a closer look at the overall converter selection process, there's one more item to consider when dealing with high stall speed torque converters, and that's efficiency. Typically, as the stall speed increases, the converter efficiency decreases (although to some degree, this relationship has been tightened up in recent years). As an example, a small-diameter performance converter will almost always slip more than a large, tight OEM converter.
In most cases, a typical performance converter will slip anywhere from 3 percent to 5 percent at coupling (part throttle conditions). When a high stall speed converter is installed, the engine rpm increases under certain conditions and so does the fluid temperature. That's why a large, high-capacity cooler is almost mandatory in street applications. Remember, too, it's almost impossible to over-cool an automatic transmission.