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Old 03-15-2009, 04:18 PM   #6 (permalink)
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Originally Posted by dcb View Post
I don't know, I just measure it with the gas pump

There's lots of variables as I said. I would just top off the tank and fudge usPerGallon so that mpg looks about right, then true it up at the next fillup...
Exactly! Don't get wrapped up in the name, i.e. uS/gal. Think of this as some arbitrary unit of time that your injector must stay open on average to spray one gallon of gasoline.

But your injector is an on/off device. It doesn't stay open all the time. Rather, it is pulsed on for a few units and then off again for a bunch of units. Your engine control computer determines how many times per second and how many units it stays open. Sensors for RPM, temperature, load, and many other values are fed to the ECU. The ECU then adjusts the number of times per second the injector is opened and for how long. Keep in mind that a TBI injector isn't necessarily opening and closing in synch with the engine RPM. It's just that the average amount of time it's open has to be the number of units needed to spray gasoline at the proper fuel/air ratio.

I also tried to calculate my injector fuel flow but was waaay off. After wrestling with the calculations for a week, here's what I did instead.

I zeroed in on my uS/gal number by driving at a steady speed on a level highway and, based on my previous fuel log, guessed that I'd be getting 21 MPG. I pulled over several times, adjusted the uS/gal setting up or down, drove again at the same steady speed until I saw 21 MPG on my MPGuino. That got me in the ballpark. As it turned out later, I was within 1% of my best calibrated uS/gal.

Then I started the fillup-fillup-adjust cycle. (1) Fill the tank, reset the car odometer, reset the MPGuino tank values. (2) At the next fillup, note the gallons needed to fill the tank, note the gallons the MPGuino says you used, do not reset the tank values yet. (3) Adjust the uS/gal setting up or down and then check the MPGuino tank gallons used again. It will recalculate up or down depending on your setting change. Keep adjusting the uS/gal number until the tank gallons used is the same as the amount of gas you just put in the tank. Now you're calibrated very close. Do the same over the next couple of tankfulls to get the calibration even closer.

As dcb says, you need to identify the type of injector. In its basic form, the MPGuino is not designed to handle peak and hold injectors. If yours is that type, you've got some work ahead of you in both hardware and software.
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