GoMoG, forgive me if I might suggest that I think we may be discussing the same thing...differently... ? (-:

I am with you on the slightly different operating temperatures of EFI engines compared to that of my old carb fed lump.

EFI seemed a logical step to reduce emissions and I think the changes in design managed to do so by a number of means, perhaps primarily by weakening the mixture more accurately than a carb could, in terms of adjustment relative to expectations of variations in engine loading..? bringing the term lean burn into the engine description dictionary...?

The first time I drove in convoy with Colin (AKA Spanner Juggler), in his then new EFI +8, I was aware at every traffic stop that his rad fan was constantly running, whereas mine was not, which made me aware of the lean burn aspect of evolution in engine development...?

Expansion tank location.... I can well understand there could be reasons I am not aware of, whereby the expansion tank might benefit from a different location, however given my understanding of the possible operation of the system, gravity would seem perhaps not to be the primary force involved in returning coolant which has been forced into the "expansion" tank, to then return to the rad as things cool back down..?

If I might explain my thinking.... On my +8, the expansion tank has a clearly defined water level, indicated by text and a couple of arrows.

The indicated water level, if adhered to, would equate to the tank never being more than under half full..?

With the tank never more than half full, would seem to suggest that the system designers required that there be an air space maintained within the tank..?

The requirement for an air space...? Apparently it may be the case that water can not be compressed, whereas air can, thus as the coolant is forced into the tank as the engine temperature rises, it compresses the air space within the tank, and dependant upon variables, that pressure could reach up to a max of just under 15lbs/sq inch without issue. 15lbs/sq inch being the release pressure of the pressure cap...?

With however many lbs/sq inch of pressure built up in the tank by the expansion in the system, my guess is the air pressure rise in the expansion tank would be enough to push the water back into the rad as the engine cools, even if the tank was lower than the filler neck of the rad....?

As for how low a tank might be mounted, I guess one might get involved in all sorts of calculations perhaps even contemplating the possible effects of vacuum etc.etc.etc., but then this is a Morgan and I suspect that although quite a few scientists and very smart folk might own a Morgan, I doubt they would have bought into the idea of Morgan ownership in search of engineering perfection...? (-:

As ever more than happy to be corrected if my logic is flawed.... I have been wrong before..(-: