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912 fuel

Guest hatrack

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Ol' mate the other day said he only runs his 912 UL on ULP as opposed to premium ULP. I would have thought that premium fuel would have to be better. Am I wrong?



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98-octane mogas is a more dense fuel than 95-octane, that is to say it contains slightly more megaJoules per kilogram...remembering that a kilogram is a fixed weight (we are assuming a gravitational attraction of 1G throughout, and weight is mass multiplied by gravitational force) whereas a litre is a varying weight dependent on the temperature of the fuel (which affects its density). Technically we should buy our fuel by weight, not volume. We should also calculate fuel-flow in kilos per hour (or pounds per hour) instead of volumetric units per hour.


That said, theoretically the denser fuel should yield slightly lower fuel-flow for horsepower developed. However, the difference in practice may be marginal, and users would have to do a cost-benefit analysis of the two fuels to determine which produced the best fuel-flow versus cost. Given the diurnal variations in fuel-pricing, such a test would need to be conducted on a daily basis in order to determine on any given day which of the two fuels gave the better value-for-money.


98 octane mogas should not contain alcohol, but will likely contain alkyl phosphate compounds as octane-enhancers. 100 octane almost certainly will contain alcohol (because alcohol raises the octane-rating slightly), and anything below 95 octane will most likely also contain alcohol. The simple test (known as a "phase-change" test, where water is used to extract alcohol from fuel, thereby increasing the apparent water-level in the sample) is also the best field-test for alcohol-adulterated fuels.


Two other octane-boosting compounds may be encountered at times, namely MTBE and ETBE (methyl tertiary butyl ether and ethyl tertiary butyl ether). These compounds are not known to have adverse effects on fuel-system components, but alcohol (ethanol or methanol) are known to affect some seals and tank-linings. There is no easy test for distinguishing MTBE and ETBE from methanol or ethanol. Since MTBE and ETBE are usually more expensive than alcohol, "no expense is spared to keep the cost down."


FWIW, I am running an 80HP 912 on 98-octane from a supplier just across the road from my local airport and encountering no signs of filter-clogging or waxy substances accumulating in main-jets, idle-jets etc.


You pays your money and you takes your choice. 95 or 98 - both work, it's your call.


One handy tip, if I may: If you are not going to fly for a while, drain your fuel tank and use the mogas in your car! The octane-enhancing compounds boil-off rapidly. Old fuel is well below nominal octane-rating and may cause detonation and engine-damage in a very short time - if you can even start the engine on it in the first place.


"There is no fuel like an old fuel, and old fuel is like no fuel".



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