Dr G W Owen wrote:
>The direction of flow of the water is more important than some would
>have you believe. The water pump works by pushing the water off the
>impellors, this creates a pressure drop and water flows into the pump to
>replace the stuff that has just been "pumped". The important part of
>this is the "pressure drop". If your car is running close to boiling
>point (be that 100C with no pressure cap or 110C with a pressure cap)
>then the drop in pressure will cause thewater to instantaneously boil
>and turn to steam within the pump. Remember Water boils at something
>like 50C on top of Everest. The water pump does not pump steam very
>well and the flow around the engine stops.
Boiling within the pump is usually referred to as cavitation.
Perhaps its not so much the direction of the flow as it as the place the
water pump draws its water from. My vintage race car has possibly the
worst water pump design since the aforementioned Model T. The Fiat 1100
motor in my Formula Junior has the water pump on the front of the head!
It actually works by sucking hot water out of the front of the head and
sending it off to the radiator at the other end of the car. Needless to
say, I did have some problems with this setup. I managed to make it
work without changing to a different pump setup, but I'm still
considering changing it. Here's what worked for me in spite of this
awful design:
The builder of the race car had thankfully designed in a surge tank that
sits well above the level of the water pump. This keeps a small "head"
of water acting against boil-off in the pump. I changed the filler neck
on this surge tank so I could increase the cooling system pressure from
the antiquated value of 7 psi to 14. It is also necessary with this
setup to frequently crack open a petcock on the head to release any
trapped air. I normally do this once before each event, and check it a
few times after having drained the water for any reason. I also added a
flow restrictor just downstream of the water pump to prevent cavitation.
In most cars, a working thermostat performs this function. Mine had
none.
The race setup on this motor also changes the way water goes back into
the engine. The Fiat setup has the water inlet on the front of the
block just BELOW the water pump in the head. Duuuh. What Fiat did was
to make the water holes in the head gasket smaller at the front. Double
Duuuhh. What was done to my motor was to pop out the 2 freeze plugs on
the side of the block, grind the surface reasonably flat, put threaded
holes on either side and bolt on a flanged pipe that sends the cool
water in at both ends of the block. The original inlet was blocked off.
The other way I've seen it done on these motors is to use an external
water pump mounted in place of the generator. In that case you can draw
water from the bottom of the rad and send it into the block (or head)
first. I think with enough pressure in the cooling system, and enough
back pressure against the pump, you can make almost anything work.
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