How to Choose the Correct Torque Converter
Choosing a torque converter is like choosing a camshaft, but far more complicated. There's a lot of science behind torque converter selection, and there are far more ways to choose the wrong one than the right one. Your safest bet is to call your chosen converter manufacturer and give them your information; they have access to all sorts of esoteric data that they don't publish. However, you can get into the ball-park where ideal stall speed is concerned, using information obtained from an engine dyno and some data about your car's chassis and drivetrain.
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Instructions
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Take your car to a facility with a chassis dynamometer and have your car tested. The exact output isn't necessarily important; primarily you're after data that relates to the engine's torque and horsepower curve. If you have a turbocharged or supercharged engine, make sure to have the dyno operator record it and superimpose the boost curve over the dyno graph.
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For naturally-aspirated engines: Note your torque peak and use this as a baseline for your converter's flash stall. Under extreme load, a torque converter will briefly "flash" to a slightly higher rpm than its rated "true" stall speed. Since you're launching your car under flash-stall conditions, you need to start here; setting the true stall at peak torque will send the converter past its ideal launch point.
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For turbocharged engines: Note the turbo's maximum boost pressure, and find the rpm at which the boost pressure reaches 1/3 of its peak. Add this rpm to the peak torque rpm and divide by two to get an average; this is your target flash stall speed. Example: A Mitsubishi EVO peaks at 21 psi of boost pressure at 3,400 rpm; 7 psi (1/3 of 21) occurs at 2,800 rpm. Now, we add 2,800 to 3,600 (this engine's torque peak) and divide by two to get a flash stall of 3,200 rpm.
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Subtract 800 to 1,000 rpm if your car spends most of its time on the street, and leave the flash stall where it is for race-only cars. The average car weighs about 3,200 pounds; add 75 rpm of stall for every 100 pounds of vehicle/driver weight over 3,200, and subtract 75 rpm for every 100 pounds under 3,200. Add 500 rpm to the stall if the vehicle uses all wheel drive, and subtract 250 if it's front wheel drive.
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Add about 50 rpm of stall for every 0.10 your rear-end ratio falls under 3.25-to-1, and subtract 50 rpm for every 0.10 that the axle ratio goes over 3.25-to-1. Axle ratio affects your ideal stall speed because it effectively reduces torque output to the wheels. Our 3.25-to-1 axle ratio is common enough that you can use it for a baseline, but bear in mind that this is an estimate. Example: A Corvette's calculated flash speed given torque peak, boost curve, vehicle weight and intended use falls at 2,100 rpm. This Corvette uses a 2.95-to-1 rear axle, so we subtract that from 3.25 to end up with 0.30. Thus, we'll raise stall speed to 2,250 rpm (50 x 3 = 150, plus 2,100 = 2,250)
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Tips & Warnings
Turbo cars require a different approach, primarily since the turbo will continue to spool up as the torque converter engages. This effectively lowers the engine's torque peak while under load and thus the ideal flash stall rpm.
Example: The All-wheel-drive Mitsubishi Evo mentioned in Step 3 weighs 3,600 pounds (with driver and fuel), is mostly street driven and uses a 3.75-to-1 final drive ratio. We know from Step 3 that's it's engine-only flash speed falls at 3,200 rpm. Subtract 800 rpm for street use (now 2,400 rpm) and add 300 for vehicle weight (3,600 - 3,200 = 400, and 4 x 75 = 300) to end up at 2,700 rpm. Now subtract 3.25 from 3.75 (equals 0.40), and multiply 4 by 50 to end up at 200. Subtract 200 from 2,700 to yield 2,500 rpm. Now add 500 for the Evo's all wheel drive system and we end up with an estimated flash stall speed of about 3,000 rpm.
References
- "Race Car Engineering & Mechanics"; Paul Van Valkenburgh; 2004
- "Turbo: Real-World High-Performance Turbocharger Systems"; Jay K. Miller; 2008
- "Maximum Boost"; Corky Bell; 1997
- TCI Auto: Torque Converters Explained
- Car Craft Magazine: What's Up with Stall?
- Car Craft Magazine: How to Read a Dyno Sheet
- Photo Credit Stephen Dunn/Getty Images Sport/Getty Images
