BPW and Injector math

[dave w]

I protected everything so people would not change the cells that were not in White! There is no password, in Open Office just click Tools, Protect Document and Sheet to turn off protection. I assume Excell is similar?

The Excel message I posted states how to disable write protection. I was hoping to help others who might encounter the same write protection message.

I'm wondering where the formula for injector bias was sourced from? Some TBI definitions don't show an injector bias ($85, $OD, $OE, $E6, $31 $61, $62)? I think $42 usually has 454 injector bias @ .276 and 350 injector bias @ .375? I'm wondering if changing injector bias will affect injector flow?

dave w

[EagleMark]

Added to the total BPW. used to compensate for injectors that open slowly. As the fuel pressure is increased the injector takes longer to open. If you add fuel pressure this should be increased.

Injector bias is in $42, $85, $61 and all the older ECM that have a BPW/BPC. Notice these do not have Cubic Inch or any displacement or Injector size? It is calculated by engine size in liters / 8 so volume of one cylinder, divided by rate of injector at grams/second = vol/rate * 1461.5 = BPC

TBI 5.7L 61 lb/hr = 61.2 lb/hr $0D states injector size is 61 lb/hr
 Val = 1461.5  * (VOL/RATE)
 VOL  = Vol of 1 Cylinder in liters, (0.7125l)
 RATE = Injector flow in gms/sec
 5.7l = 7.71 gms/sec (61.2#/HR)
 (VOL/RATE) = 0.0924 l/gm/sec
 = BPW 135

It is not in newer PCM like $0D which does not have BPW/BPC. The starting BPC is calculated by injector size and Engine displacement that those masks have.

But there's much more to BPW/BPC that I have not figured out the math for yet!!! If you stick to the guidelines this is not an issue. But you know yourself and I and others have had this discussion on cheating and adding BPW/BPC for more fuel. My tests showed that to work to some extent but fall apart at higher RPM/Load if raised and I found out why. Injectors go static at about 150 BPW/BPC at 3600 RPM!!! Base Pulse Width is time injector is open compared to how much time is available at RPM, so back to why Duty Cycle is so important, Injector Pulse Width in percent of time open in M/Sec changes with RPM = time gets shorter. But if we could do the math we would find a direct correlation from Injector Duty Cycle to BPW/BPC.. constant.

[1project2many]

But if we could do the math we would find a direct correlation from Injector Duty Cycle to BPW/BPC.. constant

Say this differently... what are you looking for? The BPC alone won't give you IDC because injector pulsewidth is a function of the air calculation. Are you trying to estimate how much the BPC can be adjusted without going static?

[EagleMark]

Are you trying to estimate how much the BPC can be adjusted without going static?

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

[RobertISaar]

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), and then you SHOULD hit 100% DC in the otherwise identical condition, assuming the O2 sensor doesn't cause a significant reduction to base fueling.

[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.

[chevmasta]

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

It's going to be different for every engine (atleast modified). The biggest variable will probably be Volumetric Efficiency (and engine displacement). Then also RPM.

VE
Cubic Inches
BSFC
RPM
Injector size (flow)
Bias or latency
NA or Boosted

soooo many variables!

[dave w]

I've been thinking Marks spreadsheet is a guideline, to help figure out an appropriate injector for a given engine application. IMHO, the practical limit for two 90 lb TBI injector is about 400 HP +/- 5%. I think the calculations in Marks spreadsheet help both the engine builder and EFI Tuner understand what would be required from the EFI system on a performance engine. The old saying, "Do the Math", IMHO Marks spreadsheet does the math!

dave w

[EagleMark]

I've been thinking Marks spreadsheet is a guideline, to help figure out an appropriate injector for a given engine application. IMHO, the practical limit for two 90 lb TBI injector is about 400 HP +/- 5%. I think the calculations in Marks spreadsheet help both the engine builder and EFI Tuner understand what would be required from the EFI system on a performance engine. The old saying, "Do the Math", IMHO Marks spreadsheet does the math!

dave w

Woo Hoo!

Usually I make a fool of myself first shot!