2 and 3 are the external interrupt pins, they are special because they can differentiate between a rising signal and a falling signal. So essentially one pin fires a tiny bit of code when the injector opens and one the other one fires a tiny bit of code when it closes.
This eliminates any guesswork in the interrupt.
The LCD wires were chosen because they were convenient for standard arduino layout and left some multi purpose pins open. Will probably change in the prebuilt code someday.
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WINDMILLS DO NOT WORK THAT WAY!!!
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