Hello Simon,
First off, I know nothing of the type of system you have there, but, I am familiar with the regular/normal configuration as fitted to my 4 SD1's since '93 and without exception expelling
ALL airlocks from an RV8 cooling system has never been straightforward.
From new, over time, as systems got crudded or even slightly modified, the task got harder rather than easier.
In significantly modified layouts with changed plumbing or component height, serious acknowledgement of the airlock issue has to be accepted.
Added to the mix, you have a powerful electric pump that seems (to me, at least) to be capable of pumping fluid much faster than the original water pump.
Maybe even faster than the natural flow capability of the system as a whole. If true, then the pumping function would become a triumph of power over resistance, leading to a significant propensity for the blades to cavitate.
Now add unwanted air to the equation along with sticky anti-freeze plus traces of oily contamination of the coolant and it becomes a 100%, guaran-damn-teed, bubble factory. Right from the get-go, a West Ham Fan's Delight.
Worse, in an effort to resolve the issue you substituted an even more powerful pump, creating even more cavitation, therefore encouraging even more bubbles.
Logically (in hindsight, of course) one might have thought the preferred solution would be to reduce cavitation by reducing the pump flow/power to match the natural flow capacity of the overall system..
In addition, we also know that pump speed is digitally controlled by a Gizmo which adds complexity to what is a fundamentally time proven simple system.
Does the Gizmo know (or even care) that the faster the pump goes, the worse the bubble problem gets, the less effective the cooling process becomes, the worse the temperature rises, so the faster the Gizmo wants to the pump to go!
A perfectly designed example of thermal runaway the nuclear industry would proud of.
Sadly then, you have reinvented the Wheel to come up with something square or oval which explains the bubbly discomfort.
Mind you, lets be clear, its not the pump speed/flow/cavitation that is the primary cause of the problem.
That, I venture to suggest, is unwanted air in the system.
So to eliminate the primary cause one will have to go back to basics.
Without any knowledge of your particular Cooling System layout all I can suggest is that you read my following essay on the subject of expelling air locks from a regular RV8 system and adapt it as best you can to your particular system.
http://www.vintagemodelairplane.com/pag ... ant01.html
Regarding your wish to redesign the plumbing to solve the problem, perhaps you just need to study a typical layout seen under any SD1 bonnet or other conventional RV8 application and add components such as expansion/overflow facility and pressure release, as far as is possible.
Even with an improved plumbing layout and airlocks eliminated, you will still have to match the water pump power to the flow capability of the cooling system to eliminate cavitation.
I dont suppose wishing you Good Luck and a Happy New Year helps much, but at least you get to smile a little.
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