Michael :
See if you agree with this reasoning : There are two major flow
resistances, the radiator, and the block/head. The radiator is between the
radiator cap and the pump inlet, the block/head is between the pump outlet
and the radiator cap. So, if the cap opens at 4 psi (gauge), the
theoretical limit for pressure at the pump inlet is about 18 psi lower (4
psi + 1 atm), and the pump outlet pressure is limited only by the flow
capacity of the block/head. (I don't know what the absolute pressure for
cavitation is, but I assume it's pretty low, less than .6 psi. I'm also
assuming the problem is "suction" cavitation.) Assuming the block/head has
the same flow resistance as the radiator, the pump outlet pressure could be
36 psi or about 83 feet of water above the inlet pressure. Dividing by the
square of the ratio from 7000 rpm (where I've been told cavitation occurs
with the stock pump) to 500 rpm, gives a maximum dynamic head of about 5
inches at idle, or about 2.6 feet at 1250 rpm.
On Thursday, March 18, 1999 8:50 PM, Michael Marr [SMTP:mmarr@idcnet.com]
wrote:
>
> Yes, that's correct. The head seen by the pump is a combination of
static
> head (the difference in height between the level of fluid on the suction
> side and the level of the fluid on the delivery side) and dynamic head,
> which is causied by the friction experienced by the fluid as it flows
> through the block, hoses, radiator etc. In this case, static head is
> negligible. Dynamic Head is proportional to the square of the flow rate.
> The relationship between head and flow through a system is known as the
> system curve. In theory, your calculated heads are correct if your
> assumption for the initial head is correct. Actually, the dynamic head
at
> idle will be minimal - certainly measured in inches, I would think,
rather
> than feet. Remember, the system pressure can never rise above the
pressure
> rating of the radiator cap.
>
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