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Thinking about this a bit more, it is difficult to compare results, as we all have varied conditions, add in the fact that different engines have various coolant and engine oil capacities. Plus the diesel models have the PTC electric heater assisting cabin heating. Petrol models don't, so we certainly don't have a level playing field, even to compare engine types or sizes.
From data, I see a 520d has a 5.2-litre sump capacity, the 535i has 6.5-litres, the 530d 7.2-litres, so there is quite a large difference in heat absorption, due to that alone.
Another factor is the mpg (or for time, litres/hr) we are achieving during warm up. That will have a major influence, as after all, it is waste heat that we are using to warm up.
HighlandPete
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