I finally found out the reasoning behind the magic 5/8 restrictor on the
output of the Chev V8 block water passages. Its an urban legend.

The idea is to increase the pressure in the block to reduce
steam pockets. Nobody is actually instrumenting the heads with temp probes
to see if there is indeed hot spots and steam pockets. This is all racing lore 
passed from one racer to another. Here is how it goes. 

Putting a restrictor in there results
in a lower cavitation speed for the pump so it is run 30% to 50% slower
than the crank speed. Consequently the coolant flow is reduced through 
the block so you now need a higher capacity racing pump.

Why? Picture what happens if you try to pump water
up to the top of a hill. You start the pump and take the hose in hand while
walking up the hill. Somewhere along the way as the hose gets higher
and higher the water stops coming out of the hose and the pressure 
on the outlet side of the pump reaches a maximum. Is this input cavitation 
or output cavitation? I don't know. The point is the pump stops pumping 
but it is still rotating. A higher capacity pump will pump to a higher 
pressure so that is what is used. This is the bigger hammer approach to cooling.

More racing lore. A two pass rad works better despite four to eight times 
the rad restriction. Apparently the rad restriction is way less than 
the 5/8 inch block outlet restrictor.

If you blow a head gasket the rad is over pressured and the thin oval tubes 
try to go to a round shape crushing the fins and blocking future air flow.
This ruins the rad so you need a new rad if you blow a head gasket.
Why the pressure cap does not pop off relieving the pressure beats me.

The picture shows the breathers from the back of the heads to the coolant
outlet side of the block. This is supposed to vent the steam pockets. 

Paul Lamar
 
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