Can anybody tell me what exactly the "Single Fire FI Size @40 psig" and "Double Fire FI Size @40 psig" constants are, and what the values assigned to them represent??
I had my chip reprogrammed to correct a lean at WOT condition but the new chip now has a lower value for both of these constants then the original.
the fuel injector size constants are a base line.
they are "a" basic value for the ECM to use for all calcuations (there are other basic things too).
a change to the FI size constant makes a global change to the amount of fuel delivered (very simular to adjusting fuel pressure) inputting a smaller size injector constant makes the car run richer and vise versa.
the single VS double fire refers basically to cruising VS accelerating, in a batch fire system the injectors are divided into two banks and they alternate firing on each stroke (single fire) but when you accelerate they fire together (double fire).
normally you change both of these values up or down at the same time, keeping them equal.
RJ does that mean the ECM is assuming 40 psi fuel pressure? 40 is the uppr end of the stock pressure envelope. OR since the O2 sensor is going to adjust anyway the 40 number isn't critical?
the ECM has no way of knowing what the fuel pressure is set at or what size injector your actually running.
the 40psi is just for injector rating, that way if you put a larger injector in a "stock" motor (say replace a 25lb injector with a 30lb'er both rated @ 40psi) you should be able to put the new injector value in and have it run the same (in theory).
since most of us are re-tuning because we have made modifications (more cubic inches/cam/heads ect ect) we have to adjust the programming to make it run right.
sometimes you can't raise fuel pressure enough to get rid of a global lean condition, so you lower the injector constant, that way the ECM holds the injectors open longer (since it thinks the injectors are smaller than normal).
You want to get the tuning as close as you can even though the O2 sensor does try to correct things. the O2 can only react so fast to changes and there are limits to how far the ECM can go (rich or lean) so its best to have it as close to optimum in the tuning as possible (it doesn't have to be perfect but close is good).
your in luck I ran into the answer this summer....
the answer is: the O2 is not used at WOT...BUT thats not the whole story!
IF the MAF car is running close to 128blm's or alittle on the rich side of that, during normal driving. then the ECM will go with the prom's programing (over-riding the O2).
But if the car is running lean (BLM's constantly way above 128 during normal driving) for a long time then the ECM will richen up everything, including WOT (even if it doesn't need it). the ECM learns over time (as long as the battery isn't disconnected, that erase's the memory) and when it see's that the car has been running lean most of the time, it makes a stored memory of it. so instead of starting out trying to maintain a 128blm (normal) it will go with a 138blm (or whatever its been having to adjust to in the past). it will also apply that BLM to WOT (yep I've seen it, WOT but showing 138 blm and adjusting injector pulse width to richer regardless of O2 readings).
so basically its important to get normal driving tuned in close to 128blm's (cruising, part throttle) before doing the WOT part. that way the ECM thinks everything is fine and won't make an "across the board" adjustment based on the O2 sensor's history.
of course anytime you disconnect the battery (long enough) that memory is wiped out and the ECM goes back to what's in the prom. so if your car seems just a little off at the dragstrip (and you know your WOT tune was good) try disconnecting the battery for 30seconds.
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Constants - Questions
