Just a thought ... if there wasn't any rpm change, and you heard or felt surging, it could be a torque increase that causes a pitch increase, and increased thrust.  And if MP was constant, then the only thing *that* leaves is mixture.  If you're running lean of peak, power depends significantly on mixture.

The thing is, this sounds backwards.  If it's worse when you turn the boost pump ON, then it sorta seems that you were running so far rich of peak that additional fuel pressure degraded power?  I'm not aware that the engine can even produce a mixture that overrich, except at very low power settings.
Charley

On Sep 30, 2013, at 7:58 AM, Art Jensen wrote:

Looking for thoughts from the collective minds of the LML.  Last flight in my IO-550 normally aspirated legacy, I noticed surging in the engine with the low boost pump on. I did not notice any surging when I turned the boost pump off. Also did not notice any fluctuation in RPM. Nor did I see any fluctuation in RPM when I downloaded the engine data. The obvious question would be what are the possible causes and does it make the airplane unsafe for flight as there is no surge when the boost pump is off. I don't understand how I could feel and hear surging in the engine yet see no fluctuation in RPM, so I'm stumped. Any ideas would be appreciated, thank you. Art

Sent from my iPhone; Art Jensen
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