Hi Guy,
the coolers do NOT have low efficiency, in fact they are very good. When we contracted a Professor of Chemical engineering process control to set up a cooling and radiator rig he explained that the results were normal, you see a very small drop in temp. There is a multi-factorial equation that I cannot remember which involves airflow, temp rise in the airflow through the heat exchanger, fluid flow and temp drop in the fluid. The "take home answer" is that with high fluid flow you only see 1-2 degrees drop.
IF you see 5 degrees drop then your fluid flow is too low and you need a far larger heat exchanger, therefore the right decision is to Increase the fluid flow. The actual "heat dumped" equation pushes you to a small temp drop but a very high fluid flow rate.
I do not know what results you will get with a Rotax, but they are clever engineers and I am sure worked this stuff out 20+ years ago. Will be interested in your results.
