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Also, don't forget what BSFC stands for: Brake-Specific Fuel Consumption. That means (pretty much) fuel consumption per horsepower, or per foot-pound of torque, or per unit of load. (Those measurements are somewhat interchangeable, at least in this context.) So you have to calculate or guess at the amount of HP, the amount of TQ, or the actual load on your engine to get an idea of BSFC.
My gut feeling is that you would make so many assumptions to make the numbers calculable by a reasonable person that the numbers would be slightly better than guesses. You'd have approximations of approximations of approximations...
In general, you can use injector opening time as a proxy for fuel usage. But it is very much a relative measure, it is an approximation. As noted, other factors (pressure differential across the injector valve, flow while the valve is opening and closing, etc.) can have a pretty significant effect. The more conditions differ between two measurements, the less reliable the approximation is.
Note that the MPGuino does just this, uses injector-open time to work out fuel consumption. But it has a lot of fudge factors built in, and most of its users (hopefully) realize it is an approximation.
But once you start multiplying approximations by estimates and dividing by guesses, the reliability of your results get more and more questionable.
-soD
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