- 1). 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.
- 2). 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.
- 3). 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.
- 4). 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.
- 5). 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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