Bazda wrote:E10 is not very consistant in the octane rating i've been told.
So if you tune on E10, another batch could out the tune out.
For what it's worth a car I tuned for flex fuel has had Mobil 98 poured into an empty tank and measured very near 10% ethanol - though one of the things that should really be obvious anyway, but becomes easy to appreciate is that having straight petrol in a tank then filling up with an ethanol blend doesn't suddenly bump what is in the tank to that ethanol percentage, even filling up on E85 into a tank which *seems* nearly empty of straight petrol usually tends to result in maybe slightly over E70 in the tank.
Say for sake of argument if you are getting ready for a dyno tune and decide to fill up on Force10, you could have say 15 litre of petrl still in the tank then you pour a further 30litres of E10 in there to get ready for your dyno tune... if you ponder the volumes involved:
Original 15litres petrol = 15litres petrol (funnily enough being 0% ethanol

)
New 30 litres Force10 = 27 litres petrol (90% petrol) + 3litres ethanol (10% ethanol)
So you end up with 45litres of fuel in the tank, 3 litres of which are ethanol - which turns out as "only" 6.6% ethanol, which can have a tangible effect on the tuning requirements.