[Jay S]

If the Lunati HR cam mentioned has a 112LSA with 4* of advance the maximum target compression for a fairly stock 389 should be at around 8.7:1 for 87 octane. With the cam mentioned, stock induction and exhaust in a heavy car the engine ideally would pump around 150 psi, with a good tune 160 psi max of compression at sea level for 87. Higher SCR than that and it will probably bite you at some point.

The more mods you do to the engine, bigger cam, better induction and exhaust, the higher the octane threshold is, there isn’t really a straight up answer for maximum compression for 87 octane. I have ran up to 10:1 on 87, but it wasn’t needing to climb any mountains either. Since the engine needs to climb a lot of elevation your probably going to need to have the AFR a bit on the lean side at sea leval, or it will be really rich at 5000 ft. I doubt you will regret being a little conservative on the compression.