Hi Chuck, I have a Megasquirt EFI system running well on my 51 passenger car, using an assortment of junkyard sourced parts. It uses feedback from a wideband O2 sensor to trim the air-fuel ratio to whatever you specify, so I'm guessing it could correct for any change needed with ethanol+ gas? I always run non-ethanol mostly because it's supposed to stay fresh longer. The Holley Sniper systems looks like it would adapt well to a Mercury Manifold and would be less expensive than the Rube Goldberg contraption I cobbled together. I have not used one though...
That said, I'm seeing only ~1-2 mpg better these days compared to the Holley 94 and loadamatic ignition, usually getting 18 or so with mixed in town/country driving and not stomping on it too much (I have OD). There's more power but I attribute that to the dual carb intake. Have you checked your air-fuel ratio and timing? Ken
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