How to Choose the Right Stall Torque Converter
Torque converters are more than just simple fluid clutches that allow your engine to idle; these devices are actually a case study in complex function resting upon an elegantly simple design. Torque converters multiply torque transmission at low speeds and seamlessly transition to near direct-drive with vehicle RPMs, making them more closely related to continuously variable transmissions, or CVTs, than to clutches. Picking the right stall speed can knock as much a half second off your quarter mile time while leaving your car driveable and as free of compromise as possible.
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Instructions
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Examine your car's engine specifications or take it to a shop with a chassis dynamometer to have it measured. Note the engine's peak torque and the rpm that it occurs at.
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Add about 15 percent to your engine's peak torque figure if you used a chassis dyno instead of factory horsepower ratings. The engine, transmission and wheel require a certain amount of horsepower to turn, meaning that such "wheel horsepower" will register lower than the flywheel horsepower you need.
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Order a torque converter that matches your engine's peak torque output and peak torque rpm as closely as possible. If you can't find one that matches it precisely, err on the side of caution by choosing one with a slightly lower stall or higher torque rating. You'll lose a little performance with a stall that's too low, but you'll lose performance, fuel economy and driveability with one that's too high.
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Tips & Warnings
You'll need to take an additional step if your engine is turbocharged in order to compensate for turbo lag. Examine your engine's turbo boost curve to find the rpm where the turbo first hit half of its maximum boost, also known as the half-boost rpm, or HBR. Find the halfway point between the HBR and peak torque and use that as your target stall speed and torque output at that rpm as your target torque rating.
For Example: if you car's torque peaks at 5,000 rpm and to turbo hits its half-boost point at 3,000 rpm, then your target stall speed should be about 4,000 rpm. If your HBR falls at or above the peak torque, either set the converter stall at peak torque as you would a naturally-aspirated (non-turbo) engine, install a larger camshaft to move the torque peak up, or install a quicker-spooling (possibly smaller) turbo to move the HBR down.
