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Thread: BPW and Injector math

  1. #31
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    Quote Originally Posted by EagleMark View Post
    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!

  2. #32
    Fuel Injected! gregs78cam's Avatar
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    Quote Originally Posted by RobertISaar View Post
    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.
    1978 Camaro Type LT, 383, Dual TBI, '7427, 4L80E
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  3. #33
    Super Moderator dave w's Avatar
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    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

  4. #34
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    Quote Originally Posted by gregs78cam View Post
    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.
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  5. #35
    Fuel Injected! gregs78cam's Avatar
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    Quote Originally Posted by RobertISaar View Post
    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.
    1978 Camaro Type LT, 383, Dual TBI, '7427, 4L80E
    1981 Camaro Z-28 Clone, T-Tops, 350/TH350
    1981 Camaro Berlinetta, V-6, 3spd
    1974 Chevy/GMC Truck, '90 TBI 350, '7427, TH350, NP203, 6" lift, 35s

  6. #36
    EFI GearHead ! EagleMark's Avatar
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    Quote Originally Posted by dave w View Post
    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
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  7. #37
    EFI GearHead ! EagleMark's Avatar
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    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.

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  8. #38
    Fuel Injected! gregs78cam's Avatar
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    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.
    1978 Camaro Type LT, 383, Dual TBI, '7427, 4L80E
    1981 Camaro Z-28 Clone, T-Tops, 350/TH350
    1981 Camaro Berlinetta, V-6, 3spd
    1974 Chevy/GMC Truck, '90 TBI 350, '7427, TH350, NP203, 6" lift, 35s

  9. #39
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    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.
    Last edited by 1project2many; 02-26-2013 at 06:16 AM.

  10. #40
    EFI GearHead ! EagleMark's Avatar
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    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...

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  11. #41
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    TBI
    ; Val = 1461.5 * (VOL/RATE)

    PFI
    ; Val = 365.375 * (VOL/RATE) 365.375 x 4 = 1461.5 so 4X as many injectors!
    Seems easy. But when I use Robert's numbers for the 3.1 from page 1 and work backward the constant works out to something like 906. 1461 / 3 does not equal 906. So maybe not so easy...

    That book is definitely crammed full of engineer speak. Maybe he thinks he's making it easy for us... :)

    You're right... we could calculate the maximum rpm available to the injector based on the number of times the injector is turned on. But that doesn't tell us where the injector will actually go static based on VE table.

  12. #42
    EFI GearHead ! EagleMark's Avatar
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    That book is definitely crammed full of engineer speak. Maybe he thinks he's making it easy for us... :)
    Sure makes himself look as smart as he is! Hard for a normal GearHead Tuner to convert...

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  13. #43
    EFI GearHead ! EagleMark's Avatar
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    Quote Originally Posted by 1project2many View Post
    Seems easy. But when I use Robert's numbers for the 3.1 from page 1 and work backward the constant works out to something like 906. 1461 / 3 does not equal 906. So maybe not so easy...
    TBI V8 engine 1461.5

    PFI V8 engine 365.375 which just happens to be 1461.5/4

    PFI V6 engine 1461.5/3 = 487.166

    3.1L engine (vol=.5231/rate=1.23) =.425284 * 487.166 = 207.12 which is pretty close to his 224 BPC! Figured at 43.5 PSI fuel pressure.

    But if you use 48 PSI which I beleive is what MPFI pressure is without regulater it's even closer! 217 BPC to his 224!

    Then consider the injector LB/HR I used are closely guarded secrets but accurateto calculations. Roberts 16.7 LB/HR may not be accurate, none of the TBI injectors are, but there's 10% possible difference as internet rumor...

    Now if his injectors flowed actual 16.2 at 48 PSI and you do the calculation... guess what you come up with? 223.54 = his BPC of 224!

    That book was worthless 2 years ago when I bought it... now it is starting to make sense. The answer to what 1461.5 is? AFR! BTW all those calcualtions are based on an AFR of 14.615

    Which brings me to my next issue! Changing Stoich AFR in cal to match AFR of E fuel? Guess what? Need to recaculate 1461.5 before we do BPC/BPW...


    SO if AFR of E fuel is 13.97 then the calculation would be:
    TBI V8 = 1397.
    PFI V8 = 349.25
    PFI V6 = 465.66


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  14. #44
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    Are the TBI flow ratings wrong, or was a different pressure spec'ed for engineering when setting up the calibrations? Did GM / Delco flow them using gasoline or another fluid, such as Stoddard Fluid? This same road was travelled 15 years ago and IIRC injectors were checked. Final decision for me was to work out the flow rating from the calibration. I do as previously mentioned and scale the value whenever I can.

    All I'm going to say about the math right now is this:
    Picking different numbers to make the math work just isn't fair. Maybe Robert can chime in and give us the application that engine / cal is from and I can try with Delphi's published flow ratings.
    No need to recalculate constant when AFR is a variable in the calibration.
    It may be as simple as dividing constant by 3 or 4 but until I work out the whole equation I don't feel this has reached a conclusion. After all, the 7427 PFI conversion was supposed to be as simple as a jumper or two and a bit change but look how much effort's gone into straightening that one out.

  15. #45
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    Quote Originally Posted by 1project2many View Post
    Picking different numbers to make the math work just isn't fair. Maybe Robert can chime in and give us the application that engine / cal is from and I can try with Delphi's published flow ratings.
    example injector is a 91 pontiac grand prix w/3.1. was used on a LOT of 3.1 applications, probably some aluminum head 2.8 as well. GM defined the injector flowrate in a couple of locations in the BIN, two of which agree with the 16.7 lb/hr rating:

    Injector Constant for Alpha-N Idle (KINJXBC5) = .2375 seconds per gram. (!!!) converting that into lb/hr = 33.42 lb/hr, but since the mode apparently is based off of single-fire calulcations, divide it by 2 to equal 16.71 lb/hr.

    Injector Flowrate in Gallons per Hour (KDISFS) = 2.67 gallons per hour. converting that into lb/hr gives 16.70 lb/hr.



    also, the 60V6 engines ran 43.5PSI at full pressure(until 99, i think, in 2000, they bumper up to ~52 or 55, i forget which)... pressure dropped from there with vacuum.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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