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.

  >>

Your urban legend reporter here, just back from the SCCA National
Championship races at Mid Ohio Raceway near Mansfield Ohio. I noticed that
those pesky owners of retired Trans Am cars had kept the cooling tricks above
as built into their cars. On most of the Chevy powered cars there is a ball
of snakes run of small hoses bleeding  coolant back to the suction side of
the systems. The pressure caps are 22 pounds.
I didn't see any systems open, so I can't say what size, if any, restrictor
they may be using, but I doubt that it would be as small as 5/8. The engines
are turned up to 8,800 RPMs. Since there is much extra effort put into the
cooling systems on these engines, and the cost is around $40,000.00 a pop,
whatever system the builders install is probably what you ought to run. If
nothing else, the double pass radiator puts both inlet and outlet on the same
end of the radiator. That could help on a lot of installations. I will pass
along to Smokey Yanuk that he has not instrumented the inside of a chevy
head, and has no idea what is going on.

Lynn E Hanover

Is he still alive?  We had one of his 7/8 sized Chevelles down in Midland
back in the 60's. I don't believe everything he said came from his admittedly 
fertile mind. A lot of what he knew came directly from Chevy R&D :-).

Paul Lamar
 
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