Different usec / gallon values with no alternator?
 

I'm trying to verify some results I've been getting recently while going beltless. I've alternated over the past few tanks with my serpentine belt on/off.
After the first beltless tank, I went to refill my tank because, according to the MPGuino, I had run empty. When I filled up, I put in one gallon less than I should have, and the MPGuino had never been off before. I tried to add more gas, but overfilled the tank. I corrected the overage on my next belted tank or two, and then tried to repeat, this time on a road trip. I ran beltless for 2 hours, and was almost a gallon short versus my MPGuino again.

Since I'm running at ~12VDC instead of 14.4VDC, are my injectors not opening as far or as fast, so the MPGuino thinks I'm using more fuel than I am?

BTW, DCB - thanks for the MPGuino. Works awesome and is well on its way to paying itself in FE.

different system voltages can affect the injector opening and close times, yup. I might suggest changing injdelay to 559 from 500 to compensate for the slower injectors. This will subtract an additional 59 microseconds from each injector pulse.

http://ecomodder.com/forum/showthrea...html#post93630

Tuning Your MegaSquirt

According to that 0.2ms/V variation in opening time is what they put for a default number. So for a 2V drop the injector opens 0.4ms slower.

Your numbers might vary slightly but that should get you pretty close. Just change your opening time when taking the alternator off/on and you should be set.

Edit: I'm slow posting so use dcb's numbers :)

Thanks for the links, guys. I'm definitely headed in the same direction as the guy in DCB's link; I definitely detect the missing fuel when I look at my fuel logs and can say that it's probably due to the injectors opening differently and needing different delays in the MPGuino to work.
That being said, I'm not enjoying significantly improved FE although I have no alternator, power steering, or a/c spinning when I remove the belt. I know there is energy being saved there, so I probably need to tackle the voltage issue. Thanks!