I have a field switch, so when I get my MPGuino, I will be able to tell you a % gain. I won't be able to say MPG improvement, because I won't have it calibrated by then.
Sometimes, however, when I have the alternator off, the engine feels funny. Like the throttle is closing partway. It goes away when I turn the alternator back on. I suspect that it is the ignition getting less powerful with the low battery volts.
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