How many of you have an automatic transmission in your hot rod? Whether you love them or hate them, automatic-equipped machines are the most common, and one area that's sure to create confusion is the torque converter selection process. Choose the wrong converter and you'll end up with a dud at best and an undriveable car at worst. Following is a look at selecting the right converter for your car.
Stall SpeedWhy increase stall speed? For high-performance applications, a higher stall speed allows the car to leave at a higher rpm (usually at a point where the engine is producing more torque). This translates into better performance, quicker elapsed times, and increased speeds. Because of this, the most critical component is the torque converter. That's no secret; the real mystery is how converters are rated, and how those ratings have an effect on your car's performance.
When discussing torque converter stall speed, there are two different terms: "Rated Stall" (it's common for enthusiasts to call this "foot brake stall") and "Flash Speed." What's the difference? Foot brake stall is just that. You load the converter by stomping one foot on the brakes and the other on the gas. Watch the tach. The rpm at which the converter overpowers the brakes is simply the "foot brake stall."
"Flash speed," on the other hand, is quite different. Flash occurs the instant you release the foot brake and the rotating inertia (which is "stored" in the engine flexplate) is released. In many cases, this flash speed can be anywhere from 500-2,500 rpm higher than the foot brake stall speed. In the case of a race car, a drag racer will stage at idle, hold the brakes, and "flash" the converter (flooring the gas pedal) the instant the last yellow on the Tree comes on. The same technique can be used with a street car to increase the engine rpm level when the car leaves a stop light.
According to B&M, flash stall can change just by reworking the load that the converter sees. For instance, if the car in question has a 4.10:1 rear axle ratio and the flash stall achieved is 2,800 rpm on a launch, a change to a 3.90:1 gear might see the flash speed increase to 3,000 rpm. Why? B&M claims the effective load on the converter went up. It also works the other way.
If the same car had the rear gear swapped for a 4.33:1 ratio, this would cause the flash stall speed to go down. B&M explains the reason for this is because the converter has begun to couple up progressively as the load it sees gets lower. Thus, our theoretical combination with a 4.30:1 gear would make the car easier to move when compared to the 4.10:1 or 3.90:1 combinations.
So far so good, but herein lies the grief: Some converter manufacturers advertise the stall speed of their converters by the "foot brake" method, while others prefer to use the "flash speed" figure. As you can imagine, this causes no end to confusion-and it's compounded even more by the complex array of variables, which contribute to converter stall speed. More on these variables later.
Converter SizeGenerally speaking, the larger the torque converter, the more torque (and horsepower) it absorbs. Because of this, a larger converter will usually have less stall speed than a smaller-diameter converter. The Catch-22 is the fin angle inside the converter. Because of fin angle changes, it is possible (both in theory and in practice) to produce a converter in something like an 11-inch size that actually has more stall speed than a 10-inch converter. The stall speed changes gained by reworking fin angles obviously have some limits. You can only increase the stall speed so far before you're forced to go with a smaller-diameter converter.
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.
Picking The Right ConverterHow do you know which converter is right for your application? It's almost impossible to buy a converter "off-the-shelf" and expect it to be right for your car. The only way to tailor the converter to your combination is to check with the experts (the converter manufacturers). ATI builds hundreds of different 8-inch converter combinations and has computerized over 7,000 different vehicle combinations.
With the criteria so variable, most aftermarket manufacturers require the following information:
ChassisVehicle weight: This tells the converter manufacturer how much work is needed to reach a certain elapsed time. It affects gear ratio and helps indicate what converter flash rpm can be used and still remain effective.
Body style: An initial calculation of the horsepower/gear ratio will be done. Then it might have to be adjusted if the car in question has a large frontal area. This gives the manufacturer an idea of how much air the car is "pushing" as it enters speed traps.
PerformanceStreet car, street/strip car, or bracket racer? The manufacturer needs to qualify your needs as accurately as possible, because they might already have significant information about your type of combination on file. Further to this, there are major differences between converters for class racing or bracket racing or street/strip duty.
Average e.t., mph, 60-foot times, and rpm at the e.t. light: The manufacturer wants to see what the present performance capability is with your combination. How efficient is your converter? How efficient is your entire combination? If you're building a new car, they can estimate how fast the car will run and help to choose the right combination of components before mistakes are made.
EngineWhen it comes to the engine, the following information is required: *Bore and Stroke*Type of Cylinder Heads*Cam Lift, Lobe Centerline, and Duration at .050 inch*Carburetor and Manifold*Compression Ratio*Primary Header Tube Diameter
Gear RatioWhat type of tranny (and its gear ratios), rear gear ratio, and tire size: This is required to calculate the actual output gear ratios (overall ratios in the respective gears), engine rpm, and vehicle mph. It is possible to have a good elapsed time and high trap speed but still have a torque converter that is all wrong for your car. Trap speed rpm will tell you if the converter is operating efficiently when going through the traps. Typically, 100 rpm of torque converter slippage equates to a loss of 1 1/2 mph through the traps.
Suspension & TiresType of rear suspension: This is helpful in diagnosing problems with your car (or combination). In some cases, a loss of performance has nothing to do with the transmission or torque converter-it's in the chassis.
Tire size and code: Through extensive testing and customer feedback, performance converter manufacturers have identified the characteristics of many tires. Tire growth, sidewall characteristics, and compounds are taken into account when selecting a torque converter. This testing has also determined which tires work and which don't with automatic transmission combinations. Much of this applies to the newest high-traction street tires now available.
MiscellaneousIn addition, information on your fuel type and if you use nitrous oxide or if the engine is supercharged or turbocharged: The tach type is considered, as is information on motor mount mid-plates, flexplate bolt circle, past performance, past equipment tested, and so on. Expect a barrage of questions before the right converter is selected.
As you can see, one size certainly doesn't fit all when it comes to torque converters. All of the major companies employ skilled technicians with a wealth of information available, but it's paramount that you provide accurate information about your combination. If the selection is done right, the performance of your converter will almost seem transparent. You won't be sorry and the cost of the telephone call is minimal when you consider how poorly your car will run with the wrong torque converter.