[TR] 4 vs 6 vane water pump impellor

[Randall]

> All speculation and theory so........ does anyone know the 
> advantages/disadvantages of 4 vs 6 vane? 

Water pump design is an extremely complicated topic, even for a pump running at constant rpm, let alone with the widely varying rpm,
head and flow parameters that are inherent in automotive cooling systems.  And the number of vanes is a relatively minor variable in
the performance of the pump.  Things like vane shape, clearances, housing shape and so on have a much larger effect on results.  So,
my opinion is that it is impossible to state categorically that a 4 vane pump is better or worse than a 6 vane.  

In other words, this discussion is futile, until and unless someone gets a sample of each specific pump and does some specific
performance tests on it.  Even then, it is going to be hard to say which is "better", since I've never seen any evidence that lack
of water flow is a problem with our engines.

One thing, though, there is no such thing as "moving the water too fast so the radiator can't cool it".  A long-persistent myth,
perpetuated by those who slept through high school physics (or took auto shop instead).  What can happen on cars with the pressure
cap on the high pressure side of the radiator (like the TR2-6) is that the pressure differential across the radiator rises so high
that the cap opens and lets coolant blow out the overflow.  THEN the engine overheats, due to lack of coolant flow (and coolant).
I've also seen non-TR engines where the hose on the low pressure side would literally suck closed, blocking off the flow.

My own experiments with 4 vs 6 (on a TR3 motor) showed no measurable difference in cooling performance.  But I didn't bother taking
any flow or pressure measurements.  With a stock 4 vane pump, and adequate airflow through the radiator, the engine stays cool even
in 115F weather (which is more than the driver can stand).

Randall