At the risk of getting flamed for doing so, I'll try posting something a bit more considered than "brand A sux beacoz da guyz at da servo sed so".
smythie wrote:A higher RON/MON fuel gives a tuner more scope to advance timing thereby improving burn efficiency and potentially getting better bang per stroke.
(edited slightly smythie, hope you don't mind)
I reckon that sums up what's most important. Energy density has no effect on power (all other things being equal) unless the capacity of your fuel delivery system (or the computer that controls it) is at its limit. Economy yes, power no.
In a theoretical world it doesn't matter if your fuel has less energy density if the RON is 10 points higher and you can use it. Your AFRs will read lower at Lamda, but you'll make more power over a more volatile fuel that doesn't let the engine reach its full potential.
Sure, if they cost the same per litre it will cost you more to make that power. But if they had the same energy density, you'd expect to pay more for the higher RON fuel anyway. If it costs less, then it's win - win (or maybe win - breakeven). Given what people on this forum spend on mods, I wouldn't think an extra $200 a year on fuel is going to break the bank.
Of course in the real world, you have to know that your fuel system actually can supply more fuel and the engine can actually benefit from the extra advance. Most can supply at least 20% more fuel and all (turbos in particular) can benefit from more ignition advance (production engines don't yet run efficiently).
Practically speaking, 98RON dino fuel and 98RON E10 should make about the same power. The dino fuel will physically require less fuel, but will produce more heat and pollutants. So power wise, all 98s (or 91, 95, etc) should be roughly the same.
Take 90 litres of 98RON dino fuel and add 10 litres of pure ethanol and you'll have the ability to make more power, cleaner and with less heat. That's probably about where United 100RON sits (at a pure guess). If it (or VPR) were available in Brisbane, I'd be using it.
Take 15 litres of 98RON dino fuel and add 85 litres of pure ethanol and you'll "potentially" make truckloads more power, slash emissions (would likely pass with no cat) and run cool as can be. That's why I said all serious high performance cars will be on this in a few years (once again, all other things being equal).
"Potentially" because whereas most ecus will adjust a certain amount based on what the O2 sensor tells them when in closed loop, it's a big step to expect them to firstly work out that 20% more fuel is required and another for the fuel system to actually be able to deliver it.
Other factors to consider are how much heat is in the combustion chamber as this has a great bearing on detonation .. and also what level of contamination is present. Hint: A cool burning fuel that is a natural solvent will assist on both these fronts.
So, on virtually every issue that actually matters, ethanol is a superior fuel to the dino fuel we've been using for the past century BUT it will work your fuel delivery system harder to due a lower energy density. That's not opinion, just fact.
Of course this is all performance based and ignores issues of energy security, sustainability, peak oil, storage, production and environmental concerns. I'm a fan of ethanol primarlily because of its performance potential and will leave the discussion about food crops, cellulitic waste and subsidies aside.
