> > Correct, there is no lockup clutch. However, the nature of a fluid
> > coupling
> > (as opposed to a torque converter) permits it to effectively lock up
> > once it
> > reaches a certain rpm. At that point, the entire fluid mass is
> > rotating
> > around and 'locking' the driven disk to the driver. Fluid couplings
> > have
> > straight vanes, and just a driver and driven member. Torque converters
> > have
> > curved vanes and three elements. This is an oversimplification that
> > perhaps
> > someone can expound upon, but the fluid coupling does act as if it is
> > locked
> > up.
> >
>
> John. Even the fluid coupling will have some slippage. Usually, fluid
> couplings have a bit less slippage than torque converters (especially at
low
> rpm) as you said, but in order to transmit the torque through the fluid,
some
> slippage is necessary.
As everything begins to spin together, it is not too bad. Much less than a
torque converter. And, since you are not multiplying torque, you are not
experiencing those losses, either.
> you get the multiplication. Of course, there is a price to pay, and that
is
> the higher slippage.
Exactly. That's why lockup clutches on trannys with torque converters are
very important. Depending on the stall speed of the converter and the
weight of the car, you can see a 10% drop in engine rpm once the lockup
clutch is engaged, at highway speeds. That's a measurable improvement in
mpg. Fluid couplings would show a much smaller improvement. Hard to know
what it is, since we have no cars with fluid couplings and lockup clutches
that I am aware of.
There's a thriving aftermarket for replacing Borg-Warner 65 and 66 boxes in
Jaguars (poor ratios and no lockup feature and a very fragile tranny in this
application because of the weight of the car) with modern GM automatics with
lockup converters and overdrive. Better ratios, stronger, better mileage
and less expensive to maintain or repair.
John
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