Throttle a pump harmful?

am (Gary V. Deutschmann, Sr.) writes:

[...snip...]

But there is a better way than clamping down the output feed line.
Install a T-Fitting in the output line and a line connected to the
T-Fitting as a return line to your sump, you can install a valve or
clamp this line to increase output from the feed line.
This method works well on all pumps, keeps heat buildup lower and
places less stress on the pump.

Thanks for the answer. This does respond to the core of the
question that I was asking. So what you're saying is that operating a
pump at a higher head does a few things:

1) increases wear/stress on the impeller/motor
2) increases heat production/reduces efficiency

Both sound reasonable and plausible, but I have heard that lower flow
rates can make some pumps work less (that they can work more
efficiently at heads greater than 0ft than they do at 0ft). This has
been asserted for the Rainbow Lifegard Quiet One pump on this very
newsgroup. This assertion is also plausible if the efficiency of
a pump is nonlinear (goes up at lower pressures, then drops again
at higher pressures, going back to zero at the pump's max head).

Now, what I really wonder is: does anyone have any actual numbers to
support either set of assertions. These numbers might only apply to a
particular make/model of pump, but any empirically gathered numbers
would help to satisfy my curiousity.

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller