charles harbert wrote:

Thx for the info and you obviously have the experience. I would think that the
5/8" hole is acceptable for a 12A producing around 150 hp, but would be
unacceptable for a 180 hp 13B, or a 300hp 20B. The reason I say that is that hp
and heat are directly related.

The cooling system was designed to be able to flow a certain volume (and
pressure) w/o cavitation. They originally sized it thruout to balance the flows.
You could probably idealize the system as equivalent to about a 1" pipe.

I assume the 5/8" hole takes the place of the thermostat in warm weather (how do
you block the bypass?). I too have drilled holes in my thermostat (4@1/4") to
increase the flow slightly and as a safety factor, if it should stick. From my
work in hydraulics, we always applied a nozzle coefficient. A butt type opening
(washer) is 15-20% less efficient than a well designed orifice (thermostat).  I
would suspect that if you began to overheat due to higher hp that you might
increase the size of the hole because you have artificially restricted the flow
to less than the original design.

The real key IMHO, is to have adequate radiator capacity and good airflow thru
them. It seems we spend too much time trying to optimize the other parts of the
sytem, when it's really just having adequately sized radiators.

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