Note: I think many programmers actually use PV=nRT to pre-calculate what the FCPW (full cylinder pulse width) would be at what's called STP. (Standard Temperature and Pressure, 100kPa & 25C) & then shrink from that based on run time readings. This is to save the math overhead done during realtime operation. I just do the math as I have plenty of processing cycles.
I'm not sure what Fred does, but MS I *think* does this as I see people talking about "base pulse" or something similar and that sounds like a pre-calculated pulse to me.
I just do the math, too, except for static components of it. I do have a base pulse width, but this is before things that intentionally alter lambda or are there to ensure lambda remains what is desired. IE, warm up and tip in and cylinder trim, etc. I find the "pre-calc everything" way very hacky... and yes, this is exactly what MegaSquirt does. They have no injector config, only a "ReqFuel" which is a dirty word meaning "base pulsewidth at STP" or similar.
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