Originally Posted by
JohnL
“Use BP Ultimate 98 fuel only (1/2 a tank max at a time if your keen)”
Too sweeping a statement. Brands aside, and all else being equal, using a higher octane fuel than the engine actually requires doesn’t result in greater power output or better economy than using a lower octane fuel, so long as the engine can use the lower octane without detonation. If the engine runs fine on lower octane fuel then using a higher octane fuel will most probably result in a very slightly lower power output and worsened economy.
This is because the ****tail of chemicals used to increase the octane rating are added to the brew in substantial quantities, displacing a very significant % of the base ‘petrol’ content with substances that have a significantly lesser calorific value, i.e. there is less energy embodied in each CC of higher octane fuel than in each CC of lower octane fuel (i.e. lower octane petrol fuels have a higher calorific value per unit of volume than higher octane).
Note that an engine gains power output and improved economy as the CR is increased. As the CR is increased the need to use higher octane fuel increases, but this is only to avoid ‘detonation’ of the compressing air / fuel mixture. The power and economy increases comes from the higher CR, not from the higher octane fuel.