Jeff L wrote:

Based on pilot experience, what is your critical altitude for LIVP
turbos?  >FL220 has been my observation, with 34" MAP not being
maintained at higher altitudes.

Do you start manually leaning your mixture as the MAP decreases past
critical altitude?


Jeff,
As a matter of convenience and safety of my passengers (namely my Judy) I normally operate my IV-P at or below FL 220, but we've had it to FL 240 several times to top weather, and a few times to FL 250. That's the highest I tested and certified it to in 2008--for several reasons. I normally climb at 32" MP and 2500 RPM, full rich to these altitudes, and have no trouble maintaining this MP to FL 250, but don't know how much "reserve" is left at FL 250--if any. I cruise at FL 220 at 29" MP and 2300RPM, LOP. This generally gives me about 170-175 KIAS and 260 KTAS or so, depending on the temperature, and a fuel burn of about 14 GPH; usually just a bit less. The airplane will go faster, but the fuel burn goes way up and the time savings is truly miniscule.

It's normal that as you go up, you'll get to an altitude where the air is so thin the turbos can't compress it to 34" any more. You can improve this by making sure the waste gate closes completely with minimal bypass. The compressed air bled off to pressurize your cabin also affects this performance. You can also improve the MP if you're at the max capacity of the system by increasing engine RPM, but this, like flying up higher, has some rather serious down-sides.

Glad to talk to you off-line if desired?

Bob