Howard wrote:

Paul:
    Cavitation is a phenomenon resulting from restricted suction..... a pump
with a restricted outlet will not cavitate.... it will stall..... a
different phenomenon, and not  harmful to a pump as cavitation is.   Actual
cavitation results in greatly increased wear and will literally eat away at
the metal in a pump......   I believe there is a confusion of terms
here.......In cavitation water goes to vapor under suction....  basically
the same thing as happens when a vapor lock occurs in a car.

                                                                H.W.

That is interesting Howard. I did not know there was a distinction.
In that light I don't know whether the Mazda pump is stalling or
cavitating. Since it is a closed loop, pressurized system how can
you tell?

Paul Lamar

Scott Gettings wrote:

This explanation makes the most sense to me, as well (IMHO).  Restricting the
outlet should create a huge pressure rise, which is commonly seen in many pump
applications.   However, drop the inlet flow, which drops the pressure creating a
relative vacuum and many pumps will cavitate like crazy.

Just try it on your pool pump.  Shut off the input side and the pump goes nuts, yet
obstructing the output just gives you a huge pressure head.  When a boat prop
cavitates, it is from low input flow -- a low-speed phenomenon.  Dump a bunch of
high-speed water into the prop and it won't cavitate.

Scott Gettings
-- 

I agree with most of this but neither one of these examples is a closed
loop system. Inlet flow and outlet flow are always the same in a closed loop
system. The only difference in the system is how much restriction you have which is
the sum of all the restrictions. Block, rad, thermostat and a restriction
in the pump of the return flow of about .66 square inches. This last one 
is by far the least amount of area the coolant must flow through. The second 
worst restriction with normal size hoses is the thermostat itself. 

If you add up all the areas of the passages in the block you will find they 
are far larger than 0.66 square inches. The same goes for all the tubes in 
the rad. It makes sense that if you reduce the resistance to flow in a 
closed loop centrifugal pump system you will increase the flow rate up 
to a point limited by the RPM when the pump stalls. If you have less 
restriction you then can increase the RPM and flow without stalling 
the pump.

Paul Lamar
 
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