Tony, just curious — did your neighbor with the Beech Sundowner have an STC for auto fuel?

it cant be done legally as there is no STC available. The engine has an STC and the AA5 and 5A have STC’s but no AA5B and O360A4K combination. Also most all auto gas has ethanol in it so no STC will allow that anyway.

BUT….. My “friend” uses 90 octane ethanol free gas in a tiger with no issues. Petersons tested the O360 with 89.7 octane with no detonation. The FAA required 91 octane to allow a margin of safety. So I would never do such a thing because it would not be FAA approved but I wouldnt consider it unsafe as long as you can get E free gas where you live which is pretty hard to do. I would never use gas with ethanol in it because it could damage plastic parts in the fuel system like in the fuel selector valve. I would be afraid of that thing swelling up and jamming in the wrong position at the wrong time. There are other issues too with ethanol that will make it unlikely we will ever see any auto gas STC’s in the future so we are stuck with 100LL

I have the STC to use 91 octane Mogas in my AA5A Cheetah. Here in Canada most 91 octane is ethanol free. In any event, I test every jerry can before filling the plane. Yes, you can mix 91 Mogas with 100 LL and in fact the STC calls for using 100 LL at regular intervals.

The 150 and 160 hp AA5 has an STC for 91 octane but the AA5B does not. The 180hp engine has an STC but the AA5B airframe does not. To be legal the airframe and engine have to be certified. airframe plumbing which has hard bends or is close to heat could induce vapor lock. the tiger flows 2 gallons per hour more than the cheetah so thats why it would need to be tested.