RS246.com Forum :: The World's #1 Audi R, S and RS Enthusiast Community

Right, I know this has been done in the past, but my main reason for resurrecting it is the recent appearance of Tesco 99RON fuel local to me, which I have used for my last 3 tankfulls.

My own experience with this fuel is that she does seem to pull a bit stronger (that may be in my head), I'm definitely getting slightly better MPG on the same runs (assuming my DIS is accurate, but I'll shortly be calibrating that), and it's as cheap as some 95 unleaded (I'm currently paying 96.9).

I know that RON isnt everything and that the MON value is also important, and that in the US they take an average of the RON & MON values.

The other thing that's been said in the past is to avoid supermarket fuels because they don't contain all the right additives - some people have gone as far as to say that they're "crap" and can have a negative effect on high performance and highly tuned cars. However, if these fuels are produced to certain standards (EN whatever), then surely they have to comply with a set of given parameters, which in theory should make comparable fuels consistent? (I've a feeling I might get shot down for my logic on that one, but it makes sense to me...)

So please :-

• * is there a definitive answer to the quality and performance of the so-called "high octane fuels"?
  * what are your own experiences?

I think there is more to a fuels performance than just the octane number. A fuel also has
a calorific value which tells you how much energy (what you are really interested in at the
end of the day) is released on combustion. You also have to consider how quickly and how
completely the fuel burns.

There is good information on the 99 RON fuel available from Greenergy's web site:
http://www.greenergy.com/products/99_octane.html

I believe they add 5% bio-ethanol which increases the octane value but reduces the calorific
value and is volatile.

use Vag-Com do a CF run on your normal fuel in 6th gear from 2k upto at least 5500rpm.

Clear Fault codes

Fill with 99 or what ever run around a while the try again. That will tell you if it is more resistant to det.

But as Jonny rightly says if they are using additives that reduces calorific value you have lost any gain you found reducing det.

Based on my limited experience of Tesco's 99 RON fuel it gives noticeably better performance than Optimax. If it were easier fo me to get then I'd use it all the time. When I went from 97 RON to Optimax, I thought that the performance was better but it was never 'obvious'. But when I went from Optimax to Tesco's 99 RON, it definitely was.

Not sure about the technical aspects of reducing det vs loss of calorific value. However if the positive effects of the former are greater than the negative effects of the latter then it's an overall win.

Regarding RON and MON, they are simply two different scales of a fuel's "resistance to knocking", measured at different RPM on the test engine. MON is done at a higher RPM so better reflects high speed and/or high load performance. You could argue that the US method is better because it uses both, but ideally we should have both quoted. That is what Greenenergy have done in their spec sheets. Their 95 RON fuel has 85 MON, and the 99 RON fuel has 87 MON. It would be reasonable to assume that most 97 RON fuels are therefore 86 MON.

MON test also has much higher air charge temperature.

According to Miller's technical information their octane boosters (CVL, octance plus) give a pretty good
boost to MON as well. I think MON gives the most accurate representation of a fuels performance.
It is possible to have high RON, low MON and vice versa and vary in summer, winter, batch to batch
etc. - basically, a minefield

Has anyone tried that new 102 octane stuff from BP?

I saw some in the Newport Pagnell service station just down the road from Aston Martin, but they wanted £2.42 per litre!!