Tuning BPW (BPC) on 7747

[EagleMark]

In the 1227747 there are 2 ways to tune BPC althoaugh it's called a BPW in this ECM
"! Is a constant "BPW CONSTANT FOR EGR OFF" Seems the SBC and BBG have simalar settings in several chips.

What does the number refer too? Adn how far should it be adjusted to get BLM within reason before going to VE table.

The second way is a main table "BPW CONSTANT FOR EGR ON" Since this conversion motor does not have an EGR it is disables in constants. So these numbers should not mean anything?

[RobertISaar]

are you running rich or lean before adjusting anything?

and are the injectors stock sized or displacement changed?

[EagleMark]

Stock injector to size engine.

Running max lean.

[RobertISaar]

hmm... on wife's car, with a bone-stock motor, i was running near max rich with the factory VE tables...

assuming the O2 is good, then i would just start with the VE tables. if you can't get enough adjustment out of them(should, though rarely a case will come up where it won't), then start playing with the BPC.

i assume it's running lean globally? if so, your high kPa VE cells near max torque might not be able to supply enough adjustment without playing with the BPC... in that case, you should start with the BPC and use the VE table(s) for fins-adjustment.

[EagleMark]

Thanks for the insight... Yeah it's globally lean, O2 sensor checks OK. I may have to pull these injectors and see what they really flow. It may be they swapped in the newer BBC injectiors when they did the rebuild. I know it had the 80lb ones when it went in...

[EagleMark]

From a book I have.

BPW constant is the most important paremeter that you can adjust in a GM TBI system. It's a universal fuel rate and rasing it globally increases fuel while lowering it globally decresases fuel.

To calculate you would use the size of one cylinder in liters and the flow rate of one injector in grams per second.
BPW = 1461.5 x engine size in liters/ number of cylinders/injector flow rate in grams per sec.

If you record data and are globally lean increasing this BPW constant will increse fuel globally, then when you are closer to 128 go to Main VE table.

[RobertISaar]

that sounds like quite a useful book....

here's a little except from a GM document i got ahold of:

CONSTANT = 1461.5 * (EGR FACTOR)(LITERS/CYL)/(INJECTOR RATE)

INJECTOR RATE IS IN GM PER SEC

and for an example value(0% EGR), a 91 W-body 3.1 LH0 with 16.7 lb/hr injectors comes out to d224, which is 0.153.

and on a 91 W-body 3.4 LQ1 with 22.5 lb/hr injectors comes out to d174, which is .119.

[EagleMark]

Looks like this thoery is used throughout GM ECMs. All the SBC and BBC were 125 to 134. Some were identical.

[RobertISaar]

odd.... this equation seems to work perfectly for all of the TBI stuff, but i can't get it to come out correctly on the MPFI V6 motors.... i get ~362 in place of 224 on a 3.1 with 16.7 lb/hr and ~287 for a 3.4 with 22.5....

both are roughly ~60% higher than what the factory value is.

[Six_Shooter]

MPFI is a different equation, well different value in the equation.

IIRC, you replace 1461.5 with 730.75.

[Six_Shooter]

This is what I have in my notes for MPFI calculations:

BPC calc for PFI:

BPC = 730.75 * (VOL / RATE)

; VOL = Vol of 1 Cylinder in liters
; RATE = Injector flow in gms/sec
;
; gms/sec = (lbs/hr / 3600) * 453.6

[RobertISaar]

that equation cuts the value in half...

so, now 181 calculated, but factory value of 224 for a 3.1...

at least now it's within the limits of the table, but still considerably off.

i decided to look up the 8F value as well, since the turbo 3.1 should give some kind of reference.... it has a value of 105, yet the injectors are only 22 lb/hr, not the near doubling that the two values would indicate....

non-linear equation?

[Six_Shooter]

This is why I don't use these equations.

I just move the BPC/BPW until it is around 14.7:1 AFR at idle and tune from there.

At times I will adjust the BPC, depending on how the tune is going overall. Like if I can't get a low enough idle PW, I will reduce the BPC/BPW until I do.

[EagleMark]

Maybe it was for TBI only?

See how they drop in BPW for EGR on?

[RobertISaar]

some of the MPFI masks use more or less the same table, and a GM document for $A1 (91-94 3.1MPFI) even states the same equation as originally posted, yet i've never been able to get it to come out correctly...

[EagleMark]

OK S o what's the math to convert PPH to GMSps?

This book says 66lbs/hr x 453.6 gms/lb then x 1hr/3600 seconds... but mine does not come out close? Supposed to come up with 8.316 gms/sec...

I'm trying to convert 83.1 LBS/hr x 453.6 gms/lb then x 1hr/3600 seconds.

[RobertISaar]

type this into google:

convert pounds per hour to grams per second

and put whatever PPH value you have in the appropriate place and it will do the math for you.

[EagleMark]

Yeah but it's hard to learn math that way? I am building a little math cheat sheet so I have the math I need in the field.

[EagleMark]

Googled has evry calculater but that one.

I have the math but I am doing it wrong on a calculater...... got to remeber I went to high school before calculaters were made in the 70's so it ws stored in my head and not looked for since then,..

[RobertISaar]

well, if you type "convert 1 pound per hour to grams per second" into google, it will spit out a multiplier for you. using that you can convert back and forth easily.