95 or 98 RON?

ph001 said:
Generally speaking ALL fuels come from the same UK refineries and ALL meet the same quality standards (EN228 for petrol & EN590 for diesel). The only difference is the additives which may boost octane and help prevent gumming up of fuel lines / injectors etc. There have been rumours in the past that some supermarkets have on occasion imported 'excess production' from outside Europe but I suspect the administrative complexities of doing so are a pretty big deterrent as they would still be legally obliged to meet EN2228 and EN590 if they wanted to sell it in the UK.

That said, Ethanol content does have an impact. Ethanol is added by almost all manufacturers as it's a cheap easy way to boost octane rating and because it's an oxygenator it results in less harmful emissions (the primary reason for government legislation for it's use). The vast majority of fuels in this country are UPTO 5% ethanol but 10% has been rumoured for a long time and is undoubtedly on it's way. It's supposed to be clearly identified on the pump in terms of an E no. e.g. E5, E10 etc.

Ethanol has a lower calorie value than petrol so inevitably mpg suffers (about 3-5% for E10) BUT it has good anti-knock properties so may allow more ignition advance and hence power output. It's very hygroscopic unfortunately meaning it absorbs moisture. This can be a problem if you leave fuel sat in a tank for prolonged periods as you can get quite a lot of water forming at the bottom of the fuel tank and cause starting problems / rough running. Finally it can react quite badly with certain plastics, rubbers and seals, but any car manufactured after 2002 is likely to be fine and EVERY car manufactured after 2011 HAS to be compatible by law.
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Interesting thing happened yesterday/today. Filled £20..00 in Tesco yesterday and used it all by late today......weather has been fab....so called in to my regular Sainsbury's and put in another £20.00the difference is amazing. So I'm going to check these ethanol levels.
 
Ambient air temperature (cold is better), damp/humidity (more water in the air is better) air pressure (high is better) and altitude (lower the better) makes more difference than the fuel grade TBH, like a natural intercooler. But V. Power for me anyway :driving:
 
I tend to use Shell Vpower (or whatever they call it this week) and Tesco 99. I do so few miles in the Z4 each year that cost is not really a factor. if i can't get these i just tend to put a splash and dash of 95 in. To be honest there is no noticeable difference to me when using the different fuels.

I had an e91 330i in the past and on anything other than super unleaded it ran like a dog it was hesitant, thirsty and the engine just felt flat. As soon as the super was back in it was a different animal, i asked BMW to check it out a few times and they could newer find a problem so guess it just liked the finer things in life.

I also had a 4.8i V8 X5 and again there was no noticeable difference in performance between fuels and given the size of the fuel tank and the speed it gulped fuel it ran on regular unleaded most of the time
 
I always run 98 RON, I found that in my last car (MX-5 NC), if I bought £10 95 RON and £10 98 RON, the car would go just as far.
 
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