To be a bit more accurate, it's not horsepower dependent, really,
it's the slip ratio of the torque converter (which is a bit more
complicated than a fluid coupling). High slip results in torque
amplification.
Effectively it's a lower gear ratio.
Your comments of efficiency and fuel consumption are correct -- as
well a some reduction in top speed as the torque converter will slip
more at high speed if it's a high rpm stall one than if it's a low
rpm stall speed one.
Fluid couplings (in the automotive sense, that is) do not have a
variable stator and therefore do not have a dual stall speed like a
torque converter does. The variable stator (of any of about three
main types) allows one to have a high stall speed fluid coupling
under power at low speed and a low stall speed fluid coupling at high
speed. Some are simply one-way clutches, the most common current
type, but in the past there have been a couple different active
systems. The one most people over 50 would know is in the GM
Powerglide 2-speed automatic. Buick used them in the Twin Turbine
(along with two separate torque converters if I remember correctly)
and at least one other GM division had an active one.
Lockup torque converters are great, by the way....
Peter
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