Thank you rickair.
I have just a few specific follow up questions (since you know this stuff).
1. When will the hobbs start counting and when will it stop? (i.e, when the master is turned on/off, when the ignition is turned on/off, or when the engine is running (oil pressure in the green)).

2. You said an a/c tach can be calibrated to a normal operating rpm, say 2300. So if you go less than 2300, the tach time slows down. But, if your RPM's are above 2300, does the tach time count faster than actual time??

Thank you