Not sure EFI alone is the problem. I run a stock Merc GEN VI BBC with EFI and no super charger at elevations between 1,535 (Castaic) and 6,225 (Tahoe) with no problems at all. As might be expected at higher elevations I lose 10-15 mph, but the boat runs great on a Bravo 1 24P 4-blade prop.
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