 |
     |
 |
|
|
Sorting out Torque Converter Confusion – Part two
|
 |
|
How Important Are Converter Dimensions?
|
By Wayne Scraba Photos courtesy of J.W. Performance Transmissions |
Last month, we began our look into torque converter technology by examining the need for increased stall speeds in racing applications. Obviously, changing the diameter of the converter changes the stall speed. But does size really make that much of a difference? It depends.
Generally speaking, the larger the torque converter's diameter, the more torque (and horsepower) it absorbs. Because of this, a larger converter will usually have less stall speed than a smaller-diameter converter. There is a caveat, though, and that's the fin angle within the converter. Because of fin angle changes, it is possible (both in theory and in practice) to produce a converter in something like a 10" size that actually has more stall speed than an 8" converter. There are some limitations to stall-speed change attainable from reworking fin angles. The manufacturer can only increase the stall speed so far before the builder is forced to go with a smaller-diameter converter. Many factors outside of the converter have a direct impact upon the stall speed. The engine type, the engine power characteristics (and, more important, the engine torque characteristics), bore, stroke, induction system, cylinder head modifications, altitude, chassis weight, chassis modifications, tires, header-tube diameter and even the body style have an effect upon the stall speed of the torque converter. For example, a converter that stalls at 2,500 RPM in a rear-engine dragster with a “little” small-block might stall at close to 4,000 RPM in a '69 Camaro with a big-inch rat motor. Place one car at sea level and the other a mile up in Denver , Colorado , and you'll have a whole new range of stall speeds (the quality and quantity of the air have direct influences upon engine torque). Companies such as J.W. Performance Transmissions stress the importance of having complete specifications of your car available before contacting the converter manufacturer. They add that nitrous-oxide-assisted, 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.
How Does The Engine Combination Affect The Torque Converter?
The technicians from J.W. Performance Transmissions point out that the main idea behind selecting a converter for your racecar is to coordinate the converter stall speed to the engine torque curve. It should come as no surprise that a high-revving, peaky, 3-inch-stroke small-block will require a much-looser converter than a small-port, "lumbering", 4-1/2-inch-stroke big-block. Unfortunately, racer optimism can have a detrimental effect upon converter selection, especially when it comes to engine power. Inaccurate horsepower and torque estimates provided by the consumer can have a serious effect upon the performance of the torque converter. The same can apply to optimistic dyno tests. To combat this, a converter manufacturer can never have enough information. It's up to you to supply it.
There is a tendency for racers to use torque converters with stall speeds that are too high. When it comes to recommendations, most good converter manufacturers tend to be conservative. In order to come up with a stall speed for a given combination, they most often rely upon years of experience with countless different automatic transmission combinations. There are some trends that appear in the torque-converter-selection process which tend to influence the selection of stall speed. For example, a three-speed-automatic combination "prefers" a converter designed to work at peak engine torque. On the other hand, a two-speed-Powerglide combination "prefers" a converter designed to work above peak engine torque. As you've gathered by now, specifying one particular stall speed for any given converter is virtually impossible because of the wide array of probable engine and torque-curve combinations. What about the stall-speed numbers published by the various converter manufacturers? Almost all of the manufacturers will tell you to consider these guidelines for stall-speed potential only.
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 overall 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% to 5% at coupling (part-throttle conditions). When a high stall speed converter is installed, the engine RPM increases under certain conditions, and so does the ATF temperature. That's why a large, high-capacity cooler is almost mandatory in an application that sees street driving (drag cars can benefit from a cooler, as well, but that's another topic). Remember, too, that it's almost impossible to over-cool an automatic transmission.
Special thanks to J.W. Performance Transmissions for supplying critical information provided in this article: J.W. Performance Transmissions 1826 Baldwin Street Rockledge , FL 32955 PH: 321-632-6205
|
