Fuel Question

The formula for calculating the octane rating of a mixture of fuels is:

( [ % Fuel A ] x [ Octane of Fuel A ] ) + ( [ % Fuel B ] x [ Octane of Fuel B ] ) = Octane of Mixture

so your 50/50 mix of 89 auto and 100LL would have an octane rating of 94.5.

If you don't want to do the math yourself, here is one of many online calculators:

http://www.wallaceracing.com/octane-mix-calc.php

Mike

Right Mike that is how I came up with about 20% for 91 octane 

Hi all,

It seems that most are in agreement that 91 with ethanol is better than 90 without. I have been using 90 without for a few years and a couple hundred hours with no issues. I'm in pa and fly 500ft to 7500 msl. 90 is the highest I can find without ethanol.

I'm not a fan of using ethanol in anything that doesn't run almost daily.

Project farm did a nice video between ethanol and ethanol free gas.

https://youtu.be/UvS_D4_lF5U

Does this help sway anyone's opinion or should I be switching to 91 ethonal?

Thanks

If you have been using 90 for years without problems then fine.

In reality the difference between 90 and 91 is nothing. I once talked to an organisation that tested Octane and they found huge variability on a batch to batch basis. so "95" varied between 92-97 and 91 between 88-93.

The Engineers at Rotax put a safety margin in too

Also Octane only really matter when the engine is at high power and has high compression. lead buildup on Pistons etc encourages detonation so I personally prefer to keep the engine lead free....except when I have no other option.

I even know a....how can I say this nicely...cost conscious individual whom had a small tank of high octane fuel for Take off and then switched to lower for cruise and when at Altitude !

 E10 (10% Ethanol) blend is the main ethanol fuel available in Australia. Its not very popular, as the alternative non ethanol fuels are readily available. We have ULP in 91, 95 & 98 RON (Research Octane Number).

Rotax advise that 95 RON is the lowest grade that should be used in their engines. Unfortunately 95 seems to be getting harder to find, so many Rotax's are fed 98 RON. 98 is significantly more expensive than 95.

E10 was introduced March 2005 - all new cars from then on will run happily on E10  but with slightly lower performance and higher fuel consumption, compared with the same car using one of the non Ethanol fuels. I would expect something similar from a Rotax sipping E10. I don't know anyone using E10 in their Rotax.

There is some E85 available but I have ever seen it and I cant imagine many drivers would use it.

Our small motors (lawn mowers/pumps/generators) recommend against using ethanol blends.

Australian Rotax owners regularly debate the perceived benefits of using 95 or 98 RON and the relative "keeping" qualities of the two fuels (general opinion is 98 doesn't keep well).

98 Ron delivers slightly more power than 95 and to a higher altitude, however the average pilot (like me) would be hard put to actually notice the difference.

Science tells us thet 95 RON maintains its qualities longer than 98 (more volatile fractions), when stored in a container open to atmosphere (fuel tank). It also tells us they both remain "fresh" in an airtight container, that is  75% + full, for at least 6 months. We are also told the addition of a significant amount of fresh fuel, to old, will restore almost all the lost fractions.

So most of use top up our tanks before the first flight of the day and do not put the girl to bed with full tanks (unless planning to fly within a few days).