DAN INA GLANDT wrote: >> He understands that to burn premium unleaded auto fuel and get good hp, >> a compression ratio of 9.5 : 1 works well at sea level. Most of his >> flying here will be in the 6000' msl range and up. Because the air is >> thinner up here, he believes that a smaller combustion chamber size would >> work and that with a pre ignition detector installed, he can determine >> when he would need to burn higher octane fuel if going to a lower >> elevation. The question is how much smaller would be the combustion >> chamber be to equal 9.5 to 1 at sea level. He is thinking around 51 cc, >> but we don't know for sure how to verify this. We realize it is >> complicated by a number of factors. >>
Joe Horton pointed out that I completely missed the point of this question. Other than the comments I made the other day about chamber volume being just one of the variables in compression ratio, I'd also mention that I don't think 9.5:1 is an acceptable compression ratio when burning auto fuel, even the 93 octane that I use. I've run 9.35:1 CR for the last 950 hours, and it's right on the ragged edge of what's acceptable. In fact, it leaves almost no margin for mistakes. I do experience detonation (which is not the same as "preignition") on occasion after a heat-soaking fuel stop in the heat of summer, or after idling for a long time in "traffic", such as waiting in line for takeoff at SNF or OSH. A summertime takeoff with carb heat on will yield detonation half way down the runway, which is not a good time for it. I have the instrumentation and experience to know when the engine is detonating, and have learned how to calm it down after a minute or so, but you're average guy wouldn't notice, and would be down in the trees in a minute or two. I'm talking about where I live, which is at 800'. Of course you could probably guesstimate how much higher the CR could be by using some kind of inverse of the "percent power" reduction of a 6000' baseline, but I think that's a really bad idea. The day will come when he'll fly somewhere lower, and the odds will be stacked against him. Of course you could say you'll just 100LL, but I used that kind of logic and almost ended up in the trees on several occasions. Contrary to what some folks think, compression ratio isn't a linear "free-power" lunch, and it's just not worth trying to eek out the last remaining drops of power at the expense of reliability and safety. I'll build my next engine with a max of 9:1 compression ratio, and that's with optimal quench volume. Sub-optimal quench volumes require an even lower compression ratio, in my opinion. There's more on quench at http://www.n56ml.com/corvair/95hp/ . Mark Langford ML at N56ML.com website www.n56ml.com
