Mercury Racing blue motors: 500EFI, 525 EFI, 575 SCI. Pros/cons/maintenance/etc.
#62
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#63
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#64
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No sir. The exhaust valve is well open there. No part of the combustion process is happening then.
Your typical gasoline combustion event happens between 10°-15° ATDC . We have to start the spark and get some flame going well before the main combustion event. Thus our early spark lead of 32°-36° BTDC. Your exhaust valve is opening BBDC. If you where having the main combustion event when the exhaust valve was open, it would be extremely noisey and have poop for power..
BTDC = piston before top dead center
ATDC = piston \after top dead center
BBDC = piston before bottom dead center
ABDC = piston after bottom dead center
#65
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It would have to be an extreme case, but what about carbon buildup from the slow burn of the higher octane? More of a theory than I think it's actually happening here, but I'd like some opinions of some of you guys