Mr Demon, you are entitled to your views, however I would question the validity of some of your statements.
"If you want less BHP buy normal petrol" The calorific value determines the amount of power that can be extracted from fuel, of course a retarded ignition timing does produce less BHP than an advanced timing, but the knock sensors whose function is to identify pre-ignition only have a very small adjustment potential, the difference between fully retarded and fully advanced is 3 to 5 degrees so on a naturally aspirated car the potential in percentage terms is about 2% more bhp on a fully advanced engine, The higher the octane fuel prevents pre-ignition and in the absence of knock the engine will run in its maximum advanced state, If there is some knock detection then the ecu will retard the timing until knock is absent, of course load, temperature and atmospheric pressure all have an affect on knock generation.
So technically you are right that more power can be derived from a higher RON but that difference is so small it ceases to be "real" power a driver would notice, and of course we all drive around at full throttle all the time to take advantage of that extra power.
On a Turbo engine, yes there will be a bigger difference, the forced air does lead to create pre-ignition so more complicated engine mapping is called for and a greater degree of adjustment in the potential for knock sensor adjustment, usually lower compression ratios are the norm from the naturally aspirated engine - but a difference of 40 BHP just from ignition timing adjustment between a 95 ron fuel and a 99 ron fuel is unbelievable, unless of course the car develops 900 plus BHP in production form.
You also say, "As for cleaning, well it's is a well known fact that these new direct fuel engines coke up over time" OK, there is perhaps some validity in this comment, however DI engines with over 75K have very similar carbon build ups as non DI engines, There is a school of thought that the intake of fuel from a non DI engine washes deposits from the back of the valves and therefore keep them clean, but, in a DI engine there is no advantage in using high detergent fuel or other cleaning additives in fuel as the DI engine injects its fuel after the valves therefore no amount of cleaning agents within the fuel will have any effect on carbon build up above the valves or in the intake manifold.
There was a fantastic amount of R&D that went into the development of DI before it was launched and hence the development of EGR systems and a different type of catalyst construction, however, all manufactures - and I mean all, are looking to introduce DI as the next step forward in engine design, and this is just a stepping stone to all electric or fuel cell technology for the future.
DI engines also run a very lean mixture, NA engines typically burn fuel/air at 14.7:1, DI engines 40:1 - therefore MPG is better, DI places fuel in the most advantaged spot within the combustion chamber to get the cleanest burn and extract the highest calorific value from the fuel.
I wholeheartedly agree with your comment regarding the "Feel good factor" from using these fuels, If spending a few quid more per year gives that reassurance then it works for you, it works for the fuel supplier, it works for the advertising campaign and everyone is happy.
Im not trying to say everyone that swears by a fuel supplier, type of fuel etc etc is stupid, I just question the real benefits and the claims made by companies in their marketing blurb.
Then again, I don't buy bottled water I get it out the tap, I do buy economy or "value" range eggs, I don't buy anti wrinkle creams or whitening toothpaste because all you are buying into is the advertising gimmick.
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