FastKat

iTrader: (10)

It seems like automatic transmissions and torque converters are rated at either a max horsepower, or max torque, or sometimes but not always both. So which one actually causes them to fail?

My first guess is torque because that is how hard something is turning... and by turning too hard against resistance, eventually something's going to break. But where does that leave horsepower? And why is it used in rating transmissions and converters?

FirstYrLS1Z

iTrader: (4)

I'll chime in as torque(rotational force) applied to too much resistance(traction).

Excessive fluid heat destroys clutches !

LS-ONE_DAY

HEAT is the #1 cause of transmission failure. Horsepower and Torque would be secondary. Some companies like FLT doesn't even advertise a "Max" hp/tq number because it depends on the application.

FastKat

iTrader: (10)

Yea, I know heat is really bad for transmissions... I'm thinking more about catastrophic failures, not the failures that result from continuously high trans temps.

So if it's all about torque, why do some companies use max horsepower specs on converters and transmissions?

mrvedit

HP is basically Torque multiplied by RPM.
There certainly is a maximum torque for any mechanical device.
But RPM is also an issue as excessive RPM will cause centrifugal force to cause bulging, bending and therefore bearing failure. Excessive RPM in an automatic transmission will cause the fluid to pool/move in unintended ways reducing clutch pressure.
I would "guess" that unless you are exceeding 7000 rpm, then max torque is what you have to worry about. And as mentioned, overheating will kill things long before the max torque is reached.