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Everyone says back off the 10.5 compression ratio, but thats the STOCK compression for this engine.
My 1969 merc monterey 390, it came from the factory with 10.5:1. Why mess with it? Whats a good camshaft that delivers high upper end torque without giving me valve clearence issues? Thanks.
Also, in a higher compression engine has anyone tried that water/methanol injection?
Just my two cents on this,
In 1969 leaded premium was available, now it's not.
I'd be suprised if the compression ratio actually worked out to be 10.5:1 once you get done doing all you can to make the engine move air efficiently in the intended rpm range. Equalizing the combustion chamber sizes, flycutting the pistons if needed for valve clearance, setting up the proper quench area... all these things usually drop the compression ratio a bit unless you invest in a set of aftermarket pistons and have someone massage them into the desired dome (or dish)shape and compression ratio wanted.
Don't forget if the engine still has the original heads they did not have hardened valve seats yet as leaded gas was still around,so be sure and have that upgrade done before any damage is done running unleaded gas.
Or if you can get your hands on AV gas it's equal to 100 octane leaded. Going rate is around $4.10 a gal.