For all my motoring life (50 years) I have believed that higher octane rating had no effect on fuel consumption, and have used the minimum recommended fuel. Higher octane was only to avoid detonation (pinking), and avoiding it by a bigger margin conferred no benefit.
However I saw an argument somewhere recently that modern engines were able to detect pinking and automatically adjust ignition timing, and therefore a higher octane could enable more favourable timing.
While this argument has some technical plausibility it doesn't mean there is a benefit in actual practice.
Does anyone have any hard data?
Thanks
I did the same 70 mile round trip commute 5 days a week for 5 years in both Jazz and Civic and could check like for like during both warm and cold weather, it is a given that mpg drops during cold and improves during warm weather whatever fuel you use. What I found was with 97 and 99 RON versus 95 RON you could have more pep or more mpg, trouble is when you got more pep you accelerated and drove that much quicker and got same mpg as 95, if you laid of a bit on the loud pedal you could use less throttle to get same performance and get up to 10% better mpg.
Engines have had one or more 'knock sensors' on the engine for many years now, and ECU's have got cleverer. The knock sensor is like a microphone listening out for certain frequencies of sound that indicate that pre-ignition is happening (due to octane of fuel and combustion conditions) the fuel air mixture burn actually peaks too soon and is trying to push piston back down the way it came (reverse crank direction) without passing through top dead centre position, this is very damaging to engine and what the knock sensor / ECU will do is retard the ignition (fire the spark later in the stroke until the knocking stops) - this robs the engine of power but is there to protect it from the damage caused by pre-ignition.
High octane fuel contains additives to resist pre-ignition and these additives actually reduce the amount of hydro-carbon (the stuff that burns) in the fuel, actually lowering the energy content of the fuel, but allowing the spark timing to be advance more, which allows the energy in the fuel to be utilised much more efficiently in the engine.
The engines answer when it detects knocking is to lower the engine output to protect it, if you put a fuel in that resists knocking over a broader range of engine conditions (ie higher octane) the ECU can run with advanced ignition spark timing which makes engine more peppy or allows a lower throttle opening for a given power output = more mpg.
Last year I did fill up at local Tesco with 99 RON (don't normally use their fuel) before taking 4 adults and luggage to North Wales, I was impressed by the pulling power up the Welsh hills and how cleanly the engine ran.