Al Gietzen wrote:

Add a hose from the heater outlet back to the input to
the rad. Since the restriction in the return side of the water pump was
probably put there to enhance the water flow through the heater,

re-routing

the heater flow back to the input to the  rad should enhance the flow

through

the engine slightly without modifying the pump or adding another cooling
core.

I measured the heater outlet pipe today and it  is about .53 inches in

diameter.

The area would then be 0.22 square inches. About one third the restriction
area (.66 square inches) in the outlet of the pump. So using this heater

outlet

should help the flow greatly with an unmodified pump. The total area (.88)

is still

not up to the area of a 1.25 inch ID hose or 1.23 square inches.
Modifying the pump is still a good idea IMHO.

My opinion is that it is doubtful that the restriction at the pump outlet
has anything to do with flow to the heater; there is plenty of pressure drop
through half the engine for that.  More likely it was just a convenient way
to handle placement of the thermostat and exit flow.

Yes and I wonder why the Mazda pump designer screwed it up so badly.
Probably assigned this job to a engineer right out of college :-)

More importantly; I would question whether extracting flow at the heater
outlet and routing back to the radiator is a good idea.  My understanding is
that the flow goes from the pump through the intake/exhaust port side of the
engine; and then back through the spark plug side.  If that is correct, then
you may be increasing the flow through the radiator and one side of the
engine, but likely decreasing the flow on spark plug (most critical) side of
the engine.  That would definitely not be good.

Al

Just the opposite Al. Here is a view of the cooling system. The water goes
in on the spark plug side and comes out on the exhaust and intake side.

Not ideal for a heavy duty engine IMHO. The rear rotor runs hotter. Most heavy 
duty engines and water cooled aircraft engines use external water
manifolds to distribute the cool water evenly. Early NSU engines had it
right. Unfortunately they did not stick to it and went with the Mazda method 
on later engines. Max Bentele took credit for this axial flow design while 
at Curtiss Wright.
 
Paul Lamar
 
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