With all the talk of the new disposable Continental being installed in the SR20, some mention was made about overleaning the engine being a culprit in the recent rash of compressionless engines. Apparantly, exhaust valves are being blamed or suspected.
I think there is a a bunch of misinformation running around regarding leaning in general. An engine running lean of peak can actually have lower exhaust gas temperature than when running rich of peak. (Particularly if you lean it until it quits) In our Turbo Cessna 210 with a long running continental (knock on wood), I am very aggresive in leaning and typically burn about 14.9 to 15.2 GPH.
HOWEVER, we run Gammi Injectors and have an engine anylizer tracking each cylinder. This is the only way to properly lean an engine for maximum life and efficiency.
A problem with a stock Continental (or Lycoming for that matter), is that the source of fuel (injector or carb) is not perfectly designed to provide an even distribution of fuel vapor to each cylinder. This inconsistant delivery can cause one cylinder to be leaned perfectly while another is over-leaned (bad) or underleaned (very bad).
If you watch the engine anylizer in the 210 as you are leaning, you get a Peak Temp of around 1620 degrees F. If you enrichen the mixture or lean the mixture, this temp drops of quickly. With the Gammi’s, since they are tuned to each cylinder, you can safely go lean of peak, get the temperature down to 1545 or so and only lose a little RPM and of course speed. Every cylinder is monitored and there is less than 10 or 15 degrees difference between each. The Gammi’s do a great job.
Incidently, this is very similar to leaning and old Piper 140 or Cessna 152. Remember, you were lucky the thing cranked and did not worry about the broken EGT. What did you instructor do? He said lean it til it runs rough then enrichen it just a little. Guess what? You are running lean of peak. However, with just 4 cylinders and no Turbo, this was pretty safe for the engine. Shock cooling is what kills most trainer engines. (3000 feet above the airport five miles out, no problem)
Now, relavence to this discussion -
The TCM problems seem to be exhaust valve problems that are not related to poor leaning. Poor leaning techniques may kill one or two exhaust valves, but probably not all, especially in such a few hours.
Cirrus does need to get the engine anylizer system worked out with ARNAV as soon as possible so that the engine can be properly leaned and monitored.
As a final thought, I wonder if shock cooling has something to do with the problems. My understanding is the plane just loves to fly, does not want to come down without lots of planning, and has little drag. With the throttle pulled back and the mixture pushed to rich on a descent, that’s a lot of cooling real fast. (Gas is a coolant in the engine). The 210 I fly does not like to come down without a lot of planning or airspeed build up. However, drop the gear and it drops like a rock. I know most controllers give you all the time in the world to come down but other than popping the chute, how are you current owners getting down? (Open the door and stick your foot out til you get to 120 kias?) Maybe a nice slip will do the trick.
Hopefully the SR22 will have drag brakes. I can’t imagine flying it without them.