math to calculate mpg from injector pulse width

[GasSavers_RoadWarrior]

Whoops, come to think of it, the vacuum bias on the FPR really only compensates for the manifold vacuum the injector is spraying into.

[EH3]

correct. there's nothing electronic about it. it's just a spring loaded diaphram.

obviously the problem is that at high vacuum the fuel pressure is about 8psi lower than at ambient/low vacuum. that number even varies between honda engines with DOHC VTEC typically having fuel pressure at 50psi ambient versus ~42 for SOHC and non-VTEC engines

[GasSavers_RoadWarrior]

Yeah there's 8psi less push, but 8psi more suck on the other side of the injector.

[Dalez0r]

Originally Posted by EH3

obviously the problem is that at high vacuum the fuel pressure is about 8psi lower than at ambient/low vacuum.

Correct. But, since the manifold is 8psi lower (due to the vacuum) the amount of fuel flowing per % of on time is the SAME as if you were at WOT and there was no vacuum (and higher fuel pressure to match!)

Should be able to calculate fuel consumption soley based on the injector's duty cycle and rated flow at your operating nominal fuel pressure. The catch is, some of the 'on' time is taken up by the amount of time it takes to open (and close) the injector, during which time it's not flowing at 100%. There should be some math to correct for that effect, which is primarily noticed at very low duty cycles...

[monroe74]

dalezor: "Should be able to calculate fuel consumption soley based on the injector's duty cycle and rated flow at your operating nominal fuel pressure."

That idea makes sense to me, and it seems like a very good way to calculate fuel consumption.

There seem to be a bunch of products/systems that do it in a way that seems to be inferior: they calculate fuel use indirectly, based on airflow. I think they do it this way because they're relying on OBD2, and OBD2 delivers airflow data, but not injector data (I'm talking about generic OBD2; the statement I made doesn't apply to certain specific manufacturers). SG is an important example of a product that seems to have this problem (although it has many other strengths).

Anyway, does anyone know of any existing product/system that does it the 'right' way? I'm a little surprised that I'm having a hard time finding such a thing.

Of course one example of a project doing it the 'right' way (in this regard) is this one:

http://forum.ecomodder.com/forumdisp...mputer-26.html

Any other examples?

[monroe74]

The SuperMID, of course. I forgot to mention that one (as an example of a product that watches the injectors).

Any others?