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Thread: BPW and Injector math

  1. #1
    EFI GearHead ! EagleMark's Avatar
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    BPW and Injector math

    Found some interesting things in another thread.
    http://www.gearhead-efi.com/Fuel-Inj...ll=1#post19832

    Maybe we can find out why? Could use someone with actual math skills as well as spreadsheet skills to apply what we find. I have plenty of starter spreadsheets but some are off, some are incomplete.

    Here is what I found in hacks.

    Code:
      
      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 set in $42 bin file and correct
    
    
     TBI 7.4L 76 lb/hr = 75.55 lb/hr
     Val = 1461.5  * (VOL/RATE)
     VOL  = Vol of 1 Cylinder in liters, (0.930L)
     RATE = Injector flow in gms/sec 7.4L = 9.52 gms/sec (75.55#/HR)
     (VOL/RATE) = 0.09768 l/gm/sec
     = BPW 142 set in $85 bin file and correct.
    
    
     TBI 7.4L 80 lb/hr = 80.50 lb/hr 
     Val = 1461.5  * (VOL/RATE)
     VOL  = Vol of 1 Cylinder in liters, (0.930L)
     RATE = Injector flow in gms/sec 7.4L = 10.144 gms/sec (80.50#/HR)
     (VOL/RATE) = 0.091679 l/gm/sec
     = BPW 134 set in $42 bin file and correct.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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    any mask that uses a BPC/BPW is really a fudge to being with... trying to use that conversion for MPFI engines doesn't work at all, last time i remember trying to do it.

    example: 3.1MPFI 60V6 = 16.7 lb/hr injectors(6 of them, obviously), factory BPC at 0% EGR is 224.

    .517 L/cyl volume
    2.1 grams/sec per injector

    .517/2.1 = 359.8

    i really can't come up with a reason for a BPC of 224 using that calc... making the injectors "appear" as a TBI setup could be done by multiplying flowrate by 3, so that it will seem like 2 50.1 lb/hr injectors, the calculated BPC comes out to 119.9....

    if you then try to account for the injector firing scheme(alternating TBI(two injections from all injectors per revolution???) vs double-fire MPFI(one injection from all injectors per revolution)), i guess you could multiply the 119.9 by 2 to come up with 239.8, which is at least somewhat close to the factory BPC of 224, it's off by 7%, which is within O2 correction range.



    in any case, i just take known values and multiply or divide depending on difference in displacement or flow... tends to work out pretty well.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  3. #3
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    and that conversion is curious.... move the decimal place over twice and it's almost the exact stoich ratio for E0 gasoline...
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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    EFI GearHead ! EagleMark's Avatar
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    Problem with the old calculator was it read wrong if fuel pressure was same in both? I spent the day learning Open Office version of Excell... have it working properly except a way to get new injector flow from old pressure new pressure? Can't figure out how it was done?

    =ROUND(SQRT(new pressure/old pressure)*injector lb/hr;2) What does ;2 do? Or what would be another way to do this?



    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
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    Super Moderator dave w's Avatar
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    The attached file (Excel) is what I use to figure BPW. I use injector flow and engine displacement as the variables.

    dave w
    Attached Files Attached Files

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    That is the calculator I used (from dave w) to calculate my 120 in the other thread....

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    Super Moderator dave w's Avatar
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    Quote Originally Posted by dave w View Post
    The attached file (Excel) is what I use to figure BPW. I use injector flow and engine displacement as the variables.

    dave w
    An example pic:

    I'm often challenged with BPW. It seems it's apples, oranges, and bananas...what injectors are rated to flow (apples), what injectors actually flow (oranges), and what is programmed into the .bin (drives me bananas).

    dave w
    Attached Images Attached Images
    Last edited by dave w; 02-22-2013 at 06:25 AM.

  8. #8
    EFI GearHead ! EagleMark's Avatar
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    I got the numbers to add up to what GM starts with, problem is 2 fold.

    Liter per cylinder
    5.7L is 5.73L
    7.4L is 7.44L
    So now when you convert engine liter to cylinder liter it adds up.

    Injectors
    61 lbhr is 61.2 lb/hr
    74 lb/hr is 75.55 lb/hr
    80 lb/hr is 80.50 lbhr
    SO now when you convert to grams/second they add up.

    My problem now is what pressure did these start with? We can assume 13, 12 may be better. But I don't think adding pressure on top of that a calculating by percent of increase is accurate?

    This was in one of my spreadsheets.
    =ROUND(SQRT(new pressure/old pressure)*injector lb/hr;2) What does ;2 do? Or what would be another way to do this?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  9. #9
    EFI GearHead ! EagleMark's Avatar
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    And Greg has corrected us from following internet protocol. This has always been mis titled even in hacks back to 90s as BPW when it is the BPC.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  10. #10
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    http://witchhunter.com/injectorcalc1.php

    i just use the bottom calculator to determine new flowrate... but since you're not 100% on the original pressure, that throws a rather large wrench into the machine.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  11. #11
    EFI GearHead ! EagleMark's Avatar
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    That, like so many MPFI calculators doesn't work. Change the old pressure to anything and old flow rate never changes?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  12. #12
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    well, no, old isn't supposed to change... you're supposed to put in known flow and pressure ("old") and a new pressure that you want to run them at, and it will spit out a new flowrate.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  13. #13
    EFI GearHead ! EagleMark's Avatar
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    Ah.. I see, they should move the calculate button down, you need to fill in three spots, not 2... OK. I'm not real brite when it comes to something set up to confuse in first place.....

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  14. #14
    EFI GearHead ! EagleMark's Avatar
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    Now that I can use it correctly it does calcaulate to same numbers I'm getting in spreadsheet.

    Still don't know what the ;2 does in equation?

    =ROUND(SQRT(new pressure/old pressure)*injector lb/hr;2) What does ;2 do? Trying to learn Excel...

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  15. #15
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    i've never used a semi-colon in an equation in excel or it's equivalents... i've got no idea.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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