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Octane requirements for HP500?

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Old 02-24-2012 | 07:42 AM
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SR-71
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Default Octane requirements for HP500?
I recently purchased a Fountain 38EC with twin HP500's. Reading through the Mercury engine literature I was surprised to see Merc recommends 87 octane for these engines. Is this a misprint?
Last edited by SR-71; 02-24-2012 at 05:35 PM.
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Old 02-24-2012 | 07:44 AM
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nope,,,,,87,,,its a low compression motor,,,thats all it needs
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Old 02-24-2012 | 07:47 AM
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A.O. Razor
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No misprint. They run fine on that. Personally I find 89 is the best for them. 91 and up is too high Imo. Better combustion with the lower octane stuff. They are only 500 hp 502s.
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Old 02-24-2012 | 02:34 PM
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If I run better then 87 I have a transom full of black soot at end of day.
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Old 02-24-2012 | 04:09 PM
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Originally Posted by GoFastSonic
If I run better then 87 I have a transom full of black soot at end of day.
Me too . I tried 93 and all it did was create a make work project .
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Old 02-24-2012 | 04:50 PM
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I run 87 in my 525efi, no problems.
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Old 02-24-2012 | 05:04 PM
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What is the compression on a hp500 by the way???

Mi engine is 9.3 to 1. I been runnin 89........seems to do ok.
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Old 02-24-2012 | 05:38 PM
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According to the Merc manual the compression ratio on the HP500 is 8.75:1.
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Old 02-24-2012 | 05:42 PM
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87 octane.
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Old 02-24-2012 | 06:16 PM
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agreed 87 is all you need.
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