Tuesday, Sep 17, 2019 at 10:34
Torque converter slippage is governed by the design of the torque converter vanes, and the speed that the torque converter operates at.
In the old days, torque converter specifications were supplied with a "stall ratio" number.
This is the maximum amount of torque a torque converter can produce when stalled - because torque converters multiply torque.
You rarely see "stall ratio" mentioned today, it seems it's no longer an important number, as it once was.
You can have "sloppy" torque converters with a high stall ratio, where there's a substantial amount of slippage, with a vastly-increased amount of torque output - as compared to "stiff" torque converters with low stall ratios, where slippage is minimised, and torque multiplication is reduced.
"Sloppy" torque converters will produce more heat because of their increased amount of slippage.
It's all to do with turbine and stator vane angles and shape, and the amount of oil recirculation within the torque converter.
Most vehicles today have relatively "stiff" torque converters to try and reduce slippage and heat buildup.
But - the operating speed of the torque converter is also important, as regards slippage.
As the RPM of a torque converter increases, slippage is substantially reduced - until, at maximum RPM, slippage is relatively minimal.
As the RPM of a torque converter is reduced, slippage increases, torque output increases - and heat buildup increases.
As most people tend to try to keep engine RPM's lower to improve fuel economy, they are generally operating their torque converters at a higher rate of slippage, than they would be, if the engine RPM's were much higher.
This is where lock-up torque converters come into their own. When towing at highway speeds, lock-up TC's eliminate slippage completely, and thus reduce heat buildup.
But accordingly, a locked up TC when towing at highway speed,
places more strain on the motor - so this is the reason that the TC unlocks, once the towing load on the motor becomes much higher, as in climbing long steady grades.
Steep grades make it fairly obvious, that there's a need to unlock the TC or change back - but long steady grades, where the slope doesn't seem all that great, impose considerable load on engines and drivetrains, because the load increase is subtle, but very steady, and leads to rapidly-increasing heat load on the engine and transmission.
Cheers, Ron.
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