Glenn, if it were only that simple.

Oftentimes, one or more of us will spec a cam or a converter, but realistically, the cam manufacturer should be contacted for a recommendation, then the converter manufacturer.

Please read this article from NHRA.

Racing Technology
Converter cues
by Wayne Scraba
Drag race torque converters: Love them or hate them, but if you select the wrong converter for your race car, you'll end up with a slug at best and an unraceable car at worst. What's the purpose of increasing the stall speed of the converter? For street-strip use or dedicated drag racing, increasing the stall speed allows the car to leave the starting line at a higher rpm, usually at a point where a modified engine is producing more torque. That translates into quicker elapsed times and increased speeds. Because of this, the most critical component in the performance mix of an automatic-transmission-equipped car is the torque converter. That should be no secret to most of you. The real secret is the way converters are rated and how the ratings affect the performance of your car.

When discussing torque-converter stall speed, two terms, and, consequently, two entirely different numbers are used: "rated stall," commonly called "footbrake stall," and "flash speed." What's the difference? Footbrake stall is just that. You load the converter by stomping one foot on the brake and the other on the gas. Watch the tach. The rpm at which the converter overpowers the brakes is footbrake stall.

Flash speed is quite different. Flash occurs the instant you release the footbrake and the rotating inertia, which is stored in the engine-flexplate-converter, is released. In many cases, this flash speed can be anywhere from 500 to 2,500 rpm higher than the footbrake stall speed. Often, a racer will stage at idle, hold the brakes, and "flash" the converter by flooring the gas pedal the instant the last yellow on the Christmas Tree comes on.

B&M points out that flash stall can be changed just by reworking the load of the converter. For instance, if a car has a 4.10 rear-axle ratio and a flash stall of 2,800 rpm on launch, a change to a 3.90 gear might increase the flash speed to 3,000 rpm. Why? B&M points out that the effective load on the converter went up. It also works the other way. If the same car used a 4.30 rear gear, the flash stall speed would go down. B&M explains this is because the converter has begun to couple up progressively as its load decreases. Thus, the combination with the 4.30 gear would make the car easier to move than the 4.10 and 3.90 combinations.

So far, so good, but herein lies the grief: Some converter manufacturers advertise the stall speed of their converters using the footbrake figure, and some use the flash-speed figure. As can be imagined, this causes no end to confusion — and it's compounded even more by the complex array of variables that contribute to converter stall speed.

Generally speaking, the larger the torque converter, the more torque and horsepower it absorbs. Because of this, a larger converter usually has less stall speed than a smaller-diameter converter. The variable is, of course, the fin angle inside the converter. Because of fin-angle changes, it is possible — in theory and in practice — to produce a converter in, for example, a nine-inch size that actually has more stall speed than an eight-inch converter. The stall-speed changes to be gained by reworking fin angles obviously are limited. The stall speed can only be increased so far before a smaller-diameter converter is necessary.

Many factors besides the converter have a direct impact on stall speed. ATI points out that engine type, engine power characteristics (and, more important, engine torque characteristics), bore, stroke, induction system, cylinder-head modifications, altitude, chassis weight, chassis modifications, tires, header tube diameter, and even body style affect stall speed. As an example, a converter that stalls at 2,500 rpm in a lightweight roadster with a small-displacement mouse engine might stall at close to 4,000 rpm in a '55 Chevy with a 454. Place one car at sea level and the other in high altitude, and a whole new range of stall speeds will result.

TCI concurs with ATI and stresses the importance of knowing complete car specifications before contacting the converter manufacturer. TCI added that nitrous-oxide, supercharged, or turbocharged applications also have a large influence on the size and type of torque converter required.

Basically, when selecting a converter, the converter stall speed should be coordinated to the engine-torque curve. It should come as no surprise that a high-revving, peaky, three-inch-stroke small-block will require a much looser converter than an oval-port, lumbering, four-inch-stroke rat engine.

Optimism, especially in the area of engine output, can be detrimental to converter selection. Turbo Action notes that inaccurate engine power and torque estimates provided by racers can seriously affect the performance of the torque converter. The same applies to optimistic dyno tests. To combat this, Turbo Action maintains that a converter manufacturer can never have enough information. It's up to you to supply it.

A-1 Automatic Transmissions agrees and notes that racers have a tendency to use torque converters with stall speeds that are too high. Said A-1's Marvin Ripes, "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 that appear in the torque-converter selection process that 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 Powerglide combination prefers a converter designed to work above peak engine torque." Ripes agrees with the other manufacturers with regard to information: Supply everything the manufacturer requests, and don't fudge on the specs.

As can be gathered, specifying a stall speed for any given converter is virtually impossible due to the wide array of probable engine and torque curve combinations. What about the stall-speed numbers published by converter manufacturers? Almost all manufacturers will tell you to consider these as guidelines only for stall-speed potential.

Before taking a closer look at the overall converter-selection process, one more item should be considered when dealing with high-stall-speed torque converters: efficiency. Typically, as stall speed increases, overall converter efficiency decreases — though, to some degree, this relationship has been tightened in recent years. For 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 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, as does the ATF, or automatic transmission fluid, temperature. That's why a large, high-capacity cooler is almost mandatory in an application that experiences regular hot-lapping.

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