Compression Ratio advice
 

This is my question should I run 9.0 to1 or 9.5 ? I was building a 496 but I came across a gen6 502 block so a 540 is in order, anyhow here is my combo.GM 088 Casting SQ. port Iron heads ported on the exhaust side, 2.25 intake &1.900 inconal ex. Crane hyd Roller (part number139741) int. 234 duration @.050 & .610 lift ex. 242 duration @.050 & .632 lift Single plane Victor jr.(dont have a carb anymore) what do you think I should get 850 cfm mech. sec or 750? I like the Demon that my frend has. Callies 4.25 crank & Eagle 6.385 rods ,stellings headers, the boat is an old 24' Python that I am in the middle of rerigging.I know that a lot of you guy are running similar combinations.I usually run Sonoco 94 in my little cigarette,but sometime I am forced to run 91 octane marina gas.

Thanks
Rob

I would try for 9:1. The 1/2 point of compression is only worth 5 HP and could put you into detonation if bad fuel is incountered. I think 9.5 to 10:1 are ok with aluminum heads. Just my opinion.

yup what vette said 9.5 woud be pushing it on 91 octane without aluminum heads. High compression is not as important as high cubes. Stuck in Kalamazoo.

I built nearly identical engine to yours...first time around went with a bigger cam because these cubes need some air (bigger cube engines make big cams seem smaller). However, vacuum at idle was so low that the carb was having trouble, I believe, getting a good signal. Went with the same cam you have only in a Gen VI. Great idle, no more loading up. I would run the 9:1 CR. Currently, I run 9.3:1 with the same heads and have zero issues. I also had the pistons end milled to bring the CR down from 10.3:1 to 9.3:1. I set full advance @ 36 deg but will bump it a little to see what the engine likes. Just got done jetting primaries - couldn't get secondaries to richen up. Low fuel pressure when sec. opened up. Replaced fuel pump. I will regulate press @ 7 psi. Go back this weekend and re-jet carb, as it will be way rich now. On the carb, I have a 950HP, mech secondaries. Even at 100% VE turning 5500 rpm you should only need around an 860 cfm. I went a little larger for future upgrades.

rws;

yep go 9:1 with Iron. I would go with an 850 Race demon for your carb. I have 2X775 RS Race Demons on mine and love them.

Follow up question on the topic. How would you lower compression, say from the stock 8.7:1 to around 8:1 for bolting on a blower? If you already have flat top pistons, then just use heads with larger combustion chambers?

larger comb area in heads, reliefs cut/sharp edge deburred in pistons...