Rubberchicken said:
nigelphoto said:
but in the normal course of events you will be doing your engine a great favour to stick to 91RON
Why?
I get the cost aspect, but my engine doesn't care about cost. (Oh indeed...
) So why would you be doing your engine a favour?
Err, because as likely as not its whats specified in the handbook. The manufacturer spends hundreds of thousands on R&D and there are an unquantifiable amount of variables throughout the rev range and under a miriad of different conditions which effect combustion, all of which are controlled by the ECU and various sensors around the engine. The characteristics of petrol are that 95RON ignites at a higher temp (which comes as a result of higher compression in a tuned engine) but burn time, brake mean effective pressure, economy, mechanical longevity and everything else will be the same as 91RON, whatever else the fuel company marketing spiel might say. So if your engine is designed for 95RON you will damage it by trying to cut costs and run on 91RON unless you retard the ignition or compensate by accelerating very slowly especially going up hill. However, the same is not true the other way round; if your engine is designed to run 91RON then spending the extra on 95RON achieves nothing except for the satisfaction of spending more money. Although it will not cause any deleterious effects, there is no gain either and it certainly won't magically increase the longevity of any internal components. Contrary to popular misconception unleaded petrol requires special grade steel valves, but not hardened valve seats unless its an old iron head. Running 95RON in an engine requiring 91RON isn't going to do anything positive or negative to the valves/guides/seats: running 91RON in a high CR engine will cause pinking which will burn the tips of the spark plugs, very quickly damage the crank bearings etc. Before ECU's/cats/fuel injection (ah, the good ole days) one could do a plug chop and see exactly how an engine was burning the fuel, but regrettably thats all in the past (except for us silly old booggers who still ride BMWs with carbs!) so it is best to stick closely to the manufacturer's specifications for oil/fuel/plugs etc as that will give the ideal working conditions and compromise between emissions, fuel economy, performance, mechanical longevity etc.
PS Sorry this is so long and boring, yawn, yawn