Kevin,

 

I fly an ES with an IO-550 and our Yahoo group just had a very detailed discussion on this topic.  High boost is for priming.  Low boost is your backup in case of an engine driven fuel pump failure.  The low boost is used for takeoff and above 10,000 (or lower under high ambient temp conditions) for vapor suppression, and switching tanks.

 

The Continental and Lycoming fuel systems are as different as any two fuel systems could be.  But any low wing plane I've ever flown (Mooneys and Pipers) all had boost pumps used for takeoff and landing with Lycomings.  My Mooney 252 (TCM TSIO-360) didn't use a boost pump for takeoff and landing, took some getting used to.

 

Mike Easley

Colorado Springs

Super ES flying