Hello,
I'll be building a Franklin 6A-350 (220hp) soon, and have a question about the compression ratio I will use. The stock CR is 10.5, and I have the option to do 8.5, 10.5, or 10.9 CR on the new build. I live in New Hampshire, and we get cold weather. Would an engine built with 8.5 CR be easier to start and maintain in the cold with today's fuel, or would using 10.5 CR not be an issue? With the 8.5 CR pistons, horsepower would be down to about 205, but have heard that the trade-off in less detonation (in the cold) might be worth it. Thanks.
John

I wouldnt worry about it. First, you should (must) always preheat your engine prior to starting in cold weather (I define as around +20 F). If you do so, compression is irrelevant. Once the engine starts, those cylinders and heads are pretty warm, so again, shouldn’t make any difference.

And, at least under “normal” circumstances, you shouldn’t be able to get a properly set up small aircraft engine into detonation.

I’d go with the middle range, probably.

MTV

MTV,
Thanks. I will be looking into engine heaters, and you are right, once it's warm will be fine. And you are in Montana, you know a bit about the cold. Best regards. John

Johntoo wrote:MTV,
Thanks. I will be looking into engine heaters, and you are right, once it's warm will be fine. And you are in Montana, you know a bit about the cold. Best regards. John

John,

Actually, I learned about cold ops during my 29 years in Alaska, the last 19 or so in Fairbanks. Never parked a personal plane in a hangar, and work airplanes lived outside a lot as well.

MTV