Compression Ratio advice
#1
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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
Thanks
Rob
#2
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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.
#4
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.
Last edited by razor1115; 08-21-2002 at 07:42 AM.
#6
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Join Date: Apr 2001
Location: LOTO / Overland Park, KS
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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?