Colin wrote: >Correct me if I am wrong about aviation carbs, but the range of adjustment has to be set initially, and then is pilot adjustable from there. Also base jetting would have to be correct for total possible richness, true?<
I should probably just let this go, which was my first instinct when I read Colin's comment "Expect any time you change fuel grade to completely re-tune." But I'm afraid this is going to give somebody the wrong idea about the possibility of running their aircraft on more than one type of fuel. I disagree with "Expect any time you change fuel grade to completely re-tune." because it is such a definitive statement. There are times that you can change fuel grade and never know the difference. Take my brother's plane, for example. A 1956 Cessna 172 with an 0-300. It has an STC to run auto fuel. My understanding of this STC is that it's nothing more than a piece of paper that allows you to pour auto fuel in it. No adjustments were made to the carb, yet it'll run 100LL or 93 octane autofuel, and in the 60 hours that I flew it, I never noticed a difference between the two. I never had to "completely retune" when I filled it up with autofuel after running it on 100 LL. The fact that it will run with no problem on 93 octane autofuel probably means it was designed to be run on 80/87 octane avgas. There are people who probably do the same thing with other planes, and could tell some difference (especially in the vapor lock area), but my point is that some planes can swap fuel interchangeably and not even notice, and that is probably due to the low compression ratio. And most cars that are designed to run on 87 octane can have 93 poured in it and you'll never know the difference except you're wallet will be lighter. I'm not disagreeing that it CAN make a difference in basic adjustments, especially on something like a racing engine that is finely tuned to run perfectly on a certain mixture and composition of fuel. But we all know that aircraft engines are a far cry from racing engines running on the edge of efficiency. Most of the guys on this list are running VW engines, and many of them are homebuilt engines. To tell these guys with Posas that if they change fuel grade they're going to upset their already crappy running carb is a disservice, I think. The number one factor that determines what kind of fuel they can burn is compression ratio. My point is that IT ALL DEPENDS. You can't just make a blanket definitive statement like "Expect any time you change fuel grade to completely re-tune." and expect it to always be true. That was my point, and I'm tired of arguing it. By the way, there are two very informative articles on autogas vs avgas at http://www.eaa.org/education/fuel/autogas_vs_avgas.pdf and http://www.eaa.org/education/fuel/autogaspart2.pdf Mark Langford, Huntsville, AL N56ML "at" hiwaay.net see KR2S project at http://home.hiwaay.net/~langford
