dusta said...
def makes a dif .
MY11 forester . Tank of unleaded around town will give me 400-450ish .
Premium will give 500-550ish
The reason you get a better mileage on premium in a modern car is because of the electronics which control the engine. They have a knock sensor which detects when the engine is pre-igniting and if detected, retards the ignition timing until it stops pinging.
So you can pretty much run them on the lowest grade of petrol but the timing will then be significantly retarded and the power and mileage reduced.
You should use a grade of petrol which will allow the engine to maintain the ignition timing at the designed point, rather than have the electronics continually retarding it to account for the low octane rating of the cheper fuel. A retarded ignition means less power and less mileage.
However, it also means longer engine life because the fire isn't lit until the piston is closer to top dead centre, which greatly reduces the peak loads on the crankshaft bearings, pistons etc.
Getting back to the fuel then, all mineral based petrols have almost identical heat content and thus in identical engines with identical settings, they will give the same mileage.
This is a rude shock to everyone who thinks high octane fuels automatically mean more horsepower. That is, put 120 octane av gas in the old FJ holden and it will be much more powerful. It makes no difference because the heat content is the same as 87 octane standard petrol.
The difference is that with the higher octane fuel you can modify the engine to run a higher compression ratio and more advanced ignition timing and other tweakings which will all result in more power. You can't do that on standard fuel because the engine would self destruct due to continuous pre-ignition.
As already mentioned, ethanol has a lower heat content per litre and so can actually reduce mileage.
To run cars on straight methanol, (which is actually ethanol but it's de-natured with a few percent methanol to stop people drinking the stuff) you have to bore the jets out to nearly double size to allow for the extra fuel burn requirement.
close to the fuel bung recommending to use premium fuel. Should I change to premium? What will happen if I keep filling up with non premium?
its for sale by the way 
