Saturday, November 3, 2007

Extreme Leaning

I'm not a big fan of "extreme leaning," by which I mean continuous operation on the rough side of peak EGT. Leaning an engine until it runs rough due to lean misfire (then leaving it like that) is not something I condone. Call me old-fashioned, but I don't believe in flying behind a rough engine.

But there are times when you need to coax maximum endurance and/or maximum range out of your engine (such as when you are low on fuel), and there's a safe way to do that. It's a technique Max Conrad used on long over-water flights. I forgot to mention it in Fly the Engine and should probably go back and include a mention of it. (Fortunately, kind of quick revision is easy to do when you use Lulu.com as your publisher.) The technique is this:

1. Lean the engine to peak EGT.

2. Switch to one mag.

3. Continue leaning just until you feel the rumble associated with lean misfire.

4. Switch back to both and see if the rumbling stops, which means that the other magneto is providing enough "ignition assist" to cause reliable ignition of the thin air-fuel charge. If the engine is smooth, leave it there and you're done. You've got the engine leaned to where the leanest cylinder will not sustain combustion reliably unless it has both mags contributing to the ignition process.

If switching back to both mags doesn't cure the rumble, start over again and repeat the procedure using the other mag.

This is not something I recommend as an everyday general operating procedure, but it's a good trick to know if you are trying to make the 6966 miles nonstop from Casablanca to El Paso, TX, in an O-360-powered Comanche 180 (as Max Conrad did on November 24, 1959, a record that still stands today).

2 comments:

Matt Barrow said...

Hmmm...with a GAMI injected TNIO-550 and now a TSIO-550, I typically run 90-150 or more LOP.

Are you referring to carburated engines?

Have you read John Deakins' articles on AvWeb regarding engine management? Your opions if so...thanks,

Kas Thomas said...

I have read a great many of John's (well-written) articles, including the engine-management ones, although I can't claim to have read them all.