Thread: Special Thanks & Weird -25% LTFT

but in the case of tuning the airflow is the variable not the fuel

The injector constant is just the +y in the very famous algebra equation 1x+y=z. While you may not see it as a variable of airflow, the reality of the operating system of the PCM does not really see it as anything but part of a bigger "how much fuel" equation. In this case "air" (there is no "air" in a fuel injection equation in reality, its more just a fictitious number for calibration reasons) is calculated on the left, which in the PCM happens to be defined as fuel injector constant x airflow x commanded AFR. The output as "fuel" on the right as injector pulse width.

The real equation is largely just a quadradic one, but you get the idea.

No, the fueling is not a variable. You know the fueling it's the IFR. Everything is based off airflow; the fueling, and the spark. The ends to me doesn't justify the means. There isn't an injector constant, it isn't OBD1. There is an IFR table that is measured and flows a certain amount.

i can appreciate thinking a different way though. It's like how everyone used to scale the tables on lsx stuff but now you can just double the stoich and half the IFR to do the same thing.

A clearer explanation may be offered like this. Ill try to draw a crude example of roughly how I believe the stock "master fuel equation" to work inside of the PCM.. Please do not nitpick or use something like this without caution. This is roughly what the PCM will do when it is putting out about 50 horsepower with 33lb injectors.

calculate 'airflow variable':

Maf freq 5000hrz. PCM references MAF configuration table for mass air calculation resolution... 42.5g/s worth of air is determined to be entering the motor at this time.

Calculate desired lambda:

PCM references PE/Openloop lambda/closed loop stoich for commanded AFR value.. example will use a lambda of 1.0 (14.7 afr)

Calculate "expected fuel"

PCM references injector constant table, and determines that mass injector flow is will output (230mg x 6) 13g of fuel per second

Calculate "required injector duty"

PCM then takes the "required air" and the "available fuel" and cuts the difference based on commanded afr. End result is 30% required injector duty cycle. (there is alot that goes into obtaining the IPW numbers from this, but this is the laymens version)

The point I am getting at, is the fact that the reality is the adjustment of the equation that sits on top of the injector pulsewidths does not have to line up with anything if it works. You can use the system to your advantage, and make ACCURATE FLAT adjustments to the maf curve (no, not just saying x6% to the whole non linear table) much easier in the injector constant side of the equation.

There isn't an injector constant, it isn't OBD1.

There isnt REALLY anything in a PCM.. to say that "injector constants disappeared in obd2" is crazy.. its the same math, the same tables, the same IPWs, just displayed differently.

Originally Posted by darkhorizon

A clearer explanation may be offered like this. Ill try to draw a crude example of roughly how I believe the stock "master fuel equation" to work inside of the PCM.. Please do not nitpick or use something like this without caution. This is roughly what the PCM will do when it is putting out about 50 horsepower with 33lb injectors.

calculate 'airflow variable':

Maf freq 5000hrz. PCM references MAF configuration table for mass air calculation resolution... 42.5g/s worth of air is determined to be entering the motor at this time.

Calculate desired lambda:

PCM references PE/Openloop lambda/closed loop stoich for commanded AFR value.. example will use a lambda of 1.0 (14.7 afr)

Calculate "expected fuel"

PCM references injector constant table, and determines that mass injector flow is will output (230mg x 6) 13g of fuel per second

Calculate "required injector duty"

PCM then takes the "required air" and the "available fuel" and cuts the difference based on commanded afr. End result is 30% required injector duty cycle. (there is alot that goes into obtaining the IPW numbers from this, but this is the laymens version)

The point I am getting at, is the fact that the reality is the adjustment of the equation that sits on top of the injector pulsewidths does not have to line up with anything if it works. You can use the system to your advantage, and make ACCURATE FLAT adjustments to the maf curve (no, not just saying x6% to the whole non linear table) much easier in the injector constant side of the equation.

Thas a pretty decent map of what it's doing. But there is still other tables that is using the grams of airflow that it calculates and doing it that way means that all of them are off. A serious one is the spark. While you may be getting the correct lambda I still like the computer to know what is going on.

Originally Posted by darkhorizon

There isnt REALLY anything in a PCM.. to say that "injector constants disappeared in obd2" is crazy.. its the same math, the same tables, the same IPWs, just displayed differently.

When obd2 hit it because way more complicated than just an injector constant.

Originally Posted by Kyne

Thas a pretty decent map of what it's doing. But there is still other tables that is using the grams of airflow that it calculates and doing it that way means that all of them are off. A serious one is the spark. While you may be getting the correct lambda I still like the computer to know what is going on.

If you do that while making more serious power, you won't have any ability to adjust for dynair past 0.92 g/cyl.

In other words, zero adjustment of timing after a medium/low boost setting.

The 3800 PCM is so simple, it likely won't matter either way.

If there were other algorithms running along side that main one, which could directly influence the main algorithm, then it'd be a problem.

Example being that algorithm that estimates the temperature of the air as it reaches the cylinder found in the later LSx PCMs. That would not be one you'd want to fudge.

Here, if you get it to behave, then you win.

I found very quickly that the rate of evaporation is crucial for dialing in the AFR during warm up with very different injectors from stock. The spray pattern of say disc style injectors are more like a fuel hose than a mist. Larger injectors are more stream like as well, even in the pintle style. That correction is non adjustable on your PCM. I simply ran open loop for longer. Also found, the spray pattern should be the factory straight down. Anything else is pretty difficult to tune around. No point in making it more difficult than it has to be.

Goodluck finding accurate data for injectors though. There are many companies out there just making up data or "borrowing" it from other companies.

If you really want the PCM to know what's going on, you're also going to have to know what the stoich of your fuel is. Unless you know how much ethanol is in there, you won't know how to adjust. And if you adjust the stoich value, you have to adjust every value in the PCM that references an AFR.