Ok, so after some members help and a ton of measuring I have now figured out that I have a 515ci, 9.6 comp ratio engine (502 efi). I ran with regular all last year with base timing set @ 8BTDC. I'm assuming I should be running 91 or 93 but not sure how to figure out timing with a dyno? Any suggestions?

Are you trying to find out what the stock timing tables are set at or are you trying to figure out how much timing it will take?

The stock Merc EFIs were built around 87, but your CR is a little higher. Assuming the timing tables in the MEFI unit haven't been altered I'd stick with 91/93.

There are some software interface options that allow you to log the data coming off the MEFI unit so you could see what the motor is doing in addition to any knock or knock retard taking place.