I see what your saying in regards to the comment, about the BP ultimate eating more fuel- i agree that its not entirely true, yes it may cost more than the lower grade fuels, but thats just because like you said, the purity of the 98 octane, my understanding of organic chem is that yes a more pure form of a substance/ molecule means less impurities, so that the engine should be burning the fuel more efficiently and also providing more power in each reaction/ products produced, instead of different compounds (higher concentrations of them/more) making up the ultimate mix, meaning that because it's not as pure the reaction rate for each product in the fuelis different, which i assume leads more residue forming/ left (as seen as black smoke/ carbon by most)and not efficient energy production as the engine is trying to get through more compounds than it is built to by the manufacturers (so lower grade fuels may just mean burns off faster if its a low molecular wt), so yeah higher grade petrol=higher concentrations = costs more (what did you expect for more of something), the price you have to pay for better performing cars.
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