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I haven't the slightest clue about how reliable this method is, that's what I hope to find out.
Unfortunately, my 98 Civic does not have a MAF sensor! However, hopefuly there can be another way. Right now I'm looking at PID $04, Calculated Load Value (CLV), defined as:
CLV = [current airflow] / [(peak airflow at WOT@STP as a function of rpm) * (BARO/29.92) * SQRT(298/(AAT+273))]
There's airflow! As for the other variables:
peak airflow: dunno what this value is
WOT = wide open throttle
STP = standard temperature and pressure (25C, 29.92inHg)
BARO = barometric pressure (maybe same as manifold air pressure MAP ?)
AAT = ambient air temperature
As you can see, the last two terms correct for temperature and pressure. If BARO happens to be from the MAP sensor, then we've got that. If AAT happens to be intake air temperature (IAT), then we've got that. Even if we don't have these values, the result won't be changed by a great deal.
Peak airflow is trickier. The easy and completely inaccurate route to get peak airflow would be to just multiply the engine displacement by RPM (divided by 2, since it takes two revolutions in a 4-stroke engine). However this assumes 100% volumetric efficiency (VE), which is generally not the case. We'd need the map of VE to RPMs, which of course the ECU happily holds within its silicon.
So we're missing a few crucial things. Perhaps we could at least get some value that's proportional to MPG. I'll try some things out.
Anyone else here have any ideas?
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1998 Honda Civic LX
In VERY early planning stages now
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Awesome avatar from DannyWork@Flickr
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