Tom,
So nice to hear your clear and concise description of these issues. I would
like to add a little to your discussion of pressure and flow in the
radiator. When the water pump is not turning, the pressure would be the
radiator cap pressure everywhere in the system. When the pump is turning it
creates a pressure differential plus-and-minus the nominal cap pressure. If
the radiator was the only restriction in the system, then the inlet pressure
would be increased and the oulet pressusre decreased by equal amounts.
However, if the cap is already at its maximum pressure it would then relieve
the excess pressure until it is again at its nominal pressure and now the
pressure on the outlet side is the full pressure difference less than the
cap relief pressure (e.g. 15 psig). This may make the outlet side acually
below atmospheric pressure which is why they put a coil spring in the outlet
hose to avoid having it sucked closed. If we add a restriction at the
thermostat; i.e., ahead of the pressure cap, then this pressure drop
subtracts from the pressure at the cap and consequently increases the
pressure on the outlet side of the radiator. This has the effect of
increasing the average pressure of the system, which is good. You can
easily extend this discussion to the case where the cap is on the outlet
side of the radiator
Bob
>At 01:27 PM 8/30/97 -0700, you wrote:
>>Tom,
>>
>>Thanks for your input on this subject. A quick questions however: Several
>>months ago you suggested that I move the radiator cap on my Tiger.
>>Frankly, I did not understand this suggestion as my Mk-1 has a copper
>>"header" tank that has the cap on it rather than on the radiator itself.
>>Through trial and error, I have discovered that it is best to keep the
>>header tank no more than half-full. In addition, It should be noted that
>>the header tank is mounted on the fender well, well above the top of the
>>radiator. Am I missing something or is everything after the Mk-1
>>different. Sorry to ask the question but I just do not have enough
>>experience with Tigers yet. Plenty with small block Fords and other LBC
>>though!!
>
>The cooling system is dynamic and has different pressures in various
>locations. The system was designed with various fixed orifice holes to
>control distribution, and one variable (the thermostat) to control
>temperature. The water pump impeller is the lowest pressure area in the
>system, and if it is turned too fast or poorly designed impellers designs
>are used, cavitation (the formation of steam bubbles) occurs. The inlet
>side of the pump and the tank it's connected to are the next lowest
>pressure areas. The pressure cap only "sees" the local pressure, so the
>pump inlet tank is the proper location for this device. By relocating the
>cap to the top of the inlet tank, the cap relief pressure is effectively
>increased by the pressure drop through the radiator core.
>
>The original header tank should never be over 1/2 full or expansion will
>quickly reduce it to that level. You could simply re plumb the header tank
>to the right hand side of the radiator, but the plumbing would be awkward.
>Most of the "relocations" I have done or seen, use a non pressurized
>expansion tank, just like most late model cars do. This will keep your
>system fuller and make it more effective.
>
>The restrictions of air flow are a major problem for Tigers. At low
>speeds, air recirculation around the radiator reduces its effectiveness.
>At high speeds, inadequate air flow paths restrict air throughput. The two
>holes on either side of the radiator are major culprits. All air entering
>the engine compartment from the front should be forced to pass through the
>radiator. Full shrouds and electric fans can help at low speeds. At high
>speeds, you have to get the air "out" once its through the radiator.
>That's why you see so many cars with supplemental vents such as LAT hoods.
>A front end air dam is also effective to reduce the pressure under the car
>to aid hot air flow as well as adding high speed stability.
>
>Tom Hall
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