BPW and Injector math

[chevmasta]

That could be another hack!

But the purpose here was not to figure out how once tuned, it was to avoid it by calibrating a tune in the first place. I still think theres a clue in BPW (now known as correct BPC) as far as RPM. Seems a smaller number BPC will rev higher, increase pressure, recaculate BPC and number gets smaller was my first clue, then seeing $85 7.4L BPC at 142? Higher then other BB with bigger injectors at BPC 134... well it's hitting %85 Injector Duty Cycle at 3500 PRM and BLM go lean.

I think BPC of 135 is 13.5 Msec

Here's some convincing numbers. In another thread a 7.4L engine with 74 (75.55)lb/hr injectors and BPC set to 142 from factory.

He is having BLM issues at higher RPM and reported 7.05 Msec at 3600 RPM. Using the formula in calculator
TBI IDC (%) = [4 * IPW (ms) * RPM]/1200
7.1 M/sec, 3600 RPM set to TBI is 85.2% Injector duty cycle.

If you use the MPFI formula
MPFI IDC (%) = [2 * IPW (ms) * RPM]/1200
on his BPC of 142 which I still believe is M/sec so 14.2 at 3600 RPM guess what? Same 85.2% Injector Duty Cycle!

Is it because of TBI injector firing strategy compared to MPFI strategy that this works? What cuts BPC in half for injector duty cycle? 2 injectors? Firing? DRP? Could also be the reason these systems go Async after it runs out of Sync fuel for higher load needs?

If you use a lower BPC of 125 = M/sec 12.5 at 3600 RPM it goes down to 75%...

To get to 5000 RPM and stay under 85% Injector Duty Cycle you need a BPC of 102! Seems to be working...

So it's looking like calculating BPC for higher fuel pressure is not only correct, but the lower BPC is also needed for higher RPM?

If he's at 85% IDC then the injector is too small. BPC, VE, BARO, CTS correction... all work together to come up with injector "on time." If the BPC is wrong, then during tuning the VE tables will "automagically" get adjusted wrong in the other direction and make up much of the difference. At the end of the day, tuning for AFR and best timing, no matter how you play with the variables you'll hit the same wall. 85% IDC. It's just robbing Peter to pay Paul.

BPC may work out to a time value. You can use Excel to see required BPC for a specific engine size, VE, and AFR. This one also helps visualize IDC.

What's strange about 7.4L $85/$D8 bins is BPW was 134, then later 142! I beleive this was the year 7.4L changed from 80 to 76 lb/hr injectors. This is also the mask that was $85 BCC Axxx that was all superceeded to $D8 Bxxx.

Mark is talking about this thread:
http://www.gearhead-efi.com/Fuel-Inj...y-math-7060-85

This is where I'm at 85% IDC. Which seems weird considering the truck engine is mostly stock minus headers and an exhaust. Fuel pressure has ben adjusted from 12psi to 16psi.
I'm currently going to try to raise my BPC to 150 and retune my VE table and see where I end up.....up top. Mind you im not "that" lean. Up top a BLM of 131 (BUT im only at 58% throttle.......I just realized). In PE mode I sure would think it would run out of fuel...........but I don't have any knock counts at WOT thank God.

Mark,
Just for clarification. I'm using the $85 mask with a BANC.bin
Should I be using the $D8 mask becuase of the superceed? Will it make a difference?