So I put an MSD PN 85551 Distributor on my firebird. I was thinking about taking it back off and advancing the timing to get more HP out of the engine. What I need to know is how do you know how much to advance it, whether to lock out the advance, and whether to lockout the vacuum advance. Any suggestions on how to do these, which ones to do, and how much? Thanks Zyd

MSD 85551 doesn't have vac advance
As it comes out of the box it has the slowest advance ramp rate.
Only way to determine what advance you can run ,is either on a dyno or drive at low speed in top gear and accel.If it knocks you have too much / too fast.
Trial and error ; will be affected by gas you use , CR , head design and other factors
Normally only racers lock out their timing

Thank you!