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Still pretty sure you need to adjust fueling to benefit from higher octane unless the car is designed for it.
So let’s not brush aside a valid point you made here. Our L6s don’t have timing dynamically adjusted based on EGT, O2, Knock sensor etc data. So it can’t detect fuel quality and give you extra performance.
But with higher octane what you get is a cooler burn (which you can argue can get some minor ponies) reducing potential for knock. Don’t forget that 4* and 5* fuels burned much cooler with their lead content.
Then there is the matter of better detergent packs that go into more expensive fuels. Case in point: my remapped diesels always smoked like a chimney when at full throttle on supermarket fuels, but never smoked on Esso, BP or Shell fuels. Even my S2000 would idle better in hot standing traffic when using super unleaded v normal stuff.
Better detergents = cleanser burn and less plug fouling etc.
As to the question of cost: if you take my average of 1000 miles a year at say a lousy 17mpg due to the lead in my right boot, it equates to 267 litres. At the current local cost of £1.48 for super and £1.37 for regular unleaded it would cost:
Regular: £366
Super: £395
The £30 difference is worth it to me as an insurance policy against knock let alone the benefits mentioned above. How much do we spend on pointless rubbish on our cars that isn’t vital to their operation and yet we can fret about a small difference at the pump?
Finally, super unleaded fuel is “up to” 5% ethanol and if you live in the south east the Esso super is currently 0% and RON 100 (even if labelled 99).