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Automatic Transmissions:

A Torque Converter Primer

Explanation by Jack Tertadian


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Q.>>>: Is anybody out there familiar with torque converter operation and specs? I am looking into an aftermarket unit that "has a stall speed between 300-800 RPM above stock". What does that mean? I'm looking to upgrade, and want to know if the increased power is worth the cost of the unit. Any input is appreciated. I'll be posting real-life trial results for all of the other owners with Automatic Transmissions, so we all can benefit. ---Matt Booker


A.>>>: The Torque Converter is a round, doughnut-shaped device that has a driven-by-crankshaft side, and a driving/attached-to-transmission side. The DRIVING and the DRIVEN sides consist of VANED surfaces, closely abutting each other, with hydraulic fluid between/ inside them. When the DRIVING side rotates, the DRIVEN side is propelled into rotation by the hydraulic fluid rushing around, forced by the driving vanes into the receiving vanes.

Between the two halves is a STATOR, that can redirect the driven fluid in such a way that the SLIPPAGE (driving side moving faster than driven side) forces fluid hydraulically HARDER into the driven vanes, so MULTIPLYING TORQUE output...until the STALL SPEED is reached.

STALL SPEED (put most simply) is rpm where the engine can't go any faster with throttle floored, because the driven side is at 0 rpm. Picture being at stop sign, HARD on brakes stopped, and you FLOOR the throttle but car doesn't move at all--what does the tachometer show? That is stall speed of your torque converter.

[Note to tech heads: This is not exactly the stall speed; it is a bit above because of the technique not allowing any motion, but it is very close and gives the essential idea.]

If you stall at 1200rpm, but the car makes more torque at 2500rpm, then you are delaying max acceleration until car moves fast enough to get rpm to 2500. A higher stall speed would get you into the engine's "powerband" faster, and would be less "doggy/slow" on takeoff launch.

A "looser" (higher stall speed) converter thus can accelerate the car faster if matched well to powerband of engine. A looser converter also is a little less efficient--it allows more slippage, so at high speed you lose a bit of horsepower to that slippage (converts to heat). If you select a proper converter, you can gain both ET and MPH...but sometimes you gain ET but lose mph because of less efficient/higher slippage.

HOWEVER, if you have a "LOCKUP" torque converter where at some speed/gear the converter has a lockup clutch that engages and so NO LONGER uses the hydraulic/slippage action, that topend efficiency can be regained. I suspect strongly that 3S automatics lock up at some point, but at what rpm/load/speed..., I don't know.

If your car can't spin the wheels on takeoff, and is a little slow off the line, but picks up when your rpm's climb higher, a looser converter could very well help acceleration some amount. Nitrous oxide, which supplies pure torque, works well with "tighter"/stock converters. Without NOS, a looser converter may be a good idea.  ---Jack Tertadian

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Article 2000-2004 Jack Tertadian, All Rights Reserved.
All Images 1995-2005 Bob Forrest, All Rights Reserved.