BPW and Injector math

[gregs78cam]

this would be my first guess for estimating where going static would occure:

find RPM with highest injector duty cycle, let's say it's 75%.

now, you know you can run up to 25% more fuel before you run into 100%DC, though you may go static before then, but for theoretical purposes, 100% is the target. take your BPC and multiply it by 1.25 (to make it 125% of it's original value),

Actually in this 75% DC scenario you would want to multiply your BPC by 1.33333. Basically the inverse of number. As an example lets say you were at BPC of 75, and were hitting 75% DC, you would divide 100 by the percent you are at and multiply your BPC by that.

[RobertISaar]

Actually in this 75% DC scenario you would want to multiply your BPC by 1.33333. Basically the inverse of number. As an example lets say you were at BPC of 75, and were hitting 75% DC, you would divide 100 by the percent you are at and multiply your BPC by that.

possibly, i don't remember off-hand, i'd have to fire up my bench and switch between two BPCs to see what the effect on BPW(and therefore IDC) is.

[gregs78cam]

possibly, i don't remember off-hand, i'd have to fire up my bench and switch between two BPCs to see what the effect on BPW(and therefore IDC) is.

Well yea, I was just saying that the math is done this way, to get from 75% to 100%. I have no idea what the real correlation is or how the ECM will change the Injector PW.

[EagleMark]

Greg and Robert. Your theories are all good, but I was looking for something different. Not a test on when they go static.

A mathmatical equation for the BPW/BPC. Since all the Sync/Async settings are in Usec and Msec there has to be something?

BPW/BPC = 1461.5 * (cyl size / gm/s of fuel) so what is 1461.5 ?
In a 5.73L engine comes out to 135 BPW/BPC.

So... BPW/BPC = 1461.5 * (cyl size / gm/s of fuel) = 135. Where is 100%? Although they may go static at 85%... because the off time is more important then on time at the high end.

[gregs78cam]

So you want to see the equation that the ECM uses to come up with Injector PW?

The only way to know what 100% Injector DC is, would be to use the same equation that the ECM uses to calculate Base Pulsewidth, plus all of the adders; i.e., Temp, Load, RPM, TPS, too many variables that are always changing.

[1project2many]

Yes! There has to be a point where it is static... right? RPM...

Yes. It's when the airflow is equivalent to the point at which the desired AFR requires the injectors to be on full time...
Translation? Run a dyno simulation. Where the predicted HP is equal to injector flow, you're done.

Think backward... injector is static at 100% DC. What's the fuel flow at that DC? What HP can it support? If the engine flows more air than that then it will starve for fuel.

Put a pair of 20#/hr injectors on a big block and you'll be out of fuel somewhere just off idle, right? But... what if your cam has only .100" lift??? The injectors will provide plenty of fuel. You can't know what the engine needs for air while you're trying to tune the engine for fuel.

If the VE table is already tuned you can work the calculation backward to see if the injector will go static. But, how could you tune the VE table without the injector?? All I can think of, at least for us mortals, is simulation software.

I suppose if you're in the process of tuning and you're seeing the VE tables climb toward 100% then you could make an estimate to see if changing the BPC will be enough. But if you calculated the BPC correctly then the fact that you're nearing 100% VE already tells you the injector's not really large enough.

It would be good to have the fuel equation broken out and we might be able to make some neat tools if it were. I think your observation that BPC shouldn't be moved more than a small percentage is perfect. If you're tuning and VE is climbing above 100%, you really need to change the fuel pressure or the injectors then recalculate BPC for the new injector flow.

BPW/BPC = 1461.5 * (cyl size / gm/s of fuel) so what is 1461.5 ?
In a 5.73L engine comes out to 135 BPW/BPC.

I haven't had time to look at this but it's not the same for PFI engines (Reference ECMGuy's ASDZ hac), and it appears that it's different again for the V6 pfi. I'm looking at chapter 4 of Greg Banish's book on Google Books. The answer is probably there if I can find time to read through it.

[EagleMark]

I haven't had time to look at this but it's not the same for PFI engines (Reference ECMGuy's ASDZ hac), and it appears that it's different again for the V6 pfi.

Well differant but same as I compared TBI and PFI.

TBI
; Val = 1461.5 * (VOL/RATE)

PFI
; Val = 365.375 * (VOL/RATE) 365.375 x 4 = 1461.5 so 4X as many injectors!

Reading Greg Banish book is making my head hurt! But your right, I think it's there.

But one of the first things in calculation is AFR. 1461.5 / 100 = 14.615 AFR... so right there BPW/BPC calculation would need some adjustment to handle 12.5 AFR.

Still working on it but I really think that BPW/BPC could be used as a calculation of Injector Duty cyle at which static with RPM could be used as a guidline. It should come out to 100%, but we know that 85% of that is usable as we need off time or static is an issue...