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Thread: 7727 singal or dual pulse at WOT?

  1. #1
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    7727 singal or dual pulse at WOT?

    At WOT when running a 7727 ecm with $8D loaded, will the batch system pulse each injector once or twice per cycle?(2 rotations of the crank) I remember reading both over the 2 years of working on this car and I am trying to figure out if I have hit the limit of my injectors. Currently it shows a max of 12ms pulse width which seems rather short. Thanks in advance
    Last edited by space387; 07-10-2020 at 05:21 AM.

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    As I remember it, when you are not in asynchronous mode the 7727 with $8D should be triggering eight injectors with each reference pulse. There is another state often seen at idle with big injectors called synchronous double fire where all injectors fire with every other reference pulse but it should not affect WOT.

  3. #3
    Fuel Injected! 84Elky's Avatar
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    Expanding on what 1project said:
    Here's what the code does. Batch Fire, Single Fire or Double Fire mode is determined by examining Preliminary Fuel Out BPW (after o2 corrections if any but before low PW and voltage compensations):
    ---Double Fire Mode used if Preliminary Fuel Out BPW > 1.1ms
    Double Fire = 1x the calculated PW, fired twice per one 720 degree Engine Cycle (or one, 1x firing per 360 crankshaft degrees)
    ---Single Fire Mode used if Preliminary Fuel Out BPW < 0.854Us
    Single Fire = 2x the calculated PW, fired once per Engine Cycle (one, double the calculated PW, fired every 720 crank degrees)

    After the above firing mode decision is made, low PW and voltage compensations are added and the Final PW is sent to the ECM to fire the injectors.

    Certainly won't affect WOT, but the mode determination thresholds can be changed in the calibration at these addresses:
    0x3EB: .byte 0x00,0x48 ;=0072, [1.1ms] 1098 uSec, SINGLE FIRE DISABLE THRESH if PW >
    0x3ED: .byte 0x00,0x38 ;=0056, [0.854ms] 854 uSec, SINGLE FIRE ENABLE THRESH if PW <

  4. #4
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    so looking at WOT if the BPW is reporting say 12ms it is giving a total injection time of 24ms per cycle? If I am understanding this correctly attempting to run this nearing 6k rpm is a 100% duty cycle

  5. #5
    Fuel Injected! 84Elky's Avatar
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    Quote Originally Posted by space387 View Post
    so looking at WOT if the BPW is reporting say 12ms it is giving a total injection time of 24ms per cycle? If I am understanding this correctly attempting to run this nearing 6k rpm is a 100% duty cycle
    There have been so many questions and so much discussion about this, it needs to be cleared up. Maybe Dave W. can sticky this.

    This discussion applies to Batch Fire Double-Fire-Mode in ECMs with only one injector driver (I believe all 86-92 Multi-Port Fuel Injected engines except Turbo GN Buick). All injectors are fired at the same time once every crankshaft revolution. That means they must be opened and fully closed within that time. That time is engine speed in revolutions per minute. We don't need to be concerned with anything else like 2 firings per cycle, etc. We only need to deal with the smallest time period during which the injectors must go from fully closed to again fully closed. And exactly that cycle must precisely occur or the ability for the injector to properly function is violated (violation = "static").

    So let's determine that time period. We'll use 6000 RPM as an example. The formula for Maximum (100%) Base Pulse Width (BPW) at 6000 RPM is:
    Code:
        1 min          60 sec        1000 ms
    -------------  x  ---------  x  ------------  =  10 ms
     6000 Revs         1 min         1 sec
    Said another way: 10ms is the maximum amount of time available for an injector to be opened and fully closed during one 360 degree crank rotation at 6000 RPM. Or another way: An injector's pulse width cannot theoretically exceed 10ms at 6000 RPM -- it's theoretically maxed-out (gone "static"). This leads to a general formula for Maximum (100%) Theoretical Available BPW at any RPM:
    60000 / RPM

    What is BPW? It's the calculated time the injectors need to be held fully open (note "fully open"), to provide fuel for the current engine state. But it takes time to go from closed to fully open, and time from fully open to fully closed. That time is not in the BPW calculation. The code doing the BPW calculation knows nothing about the physical limitations of the injectors' opening and closing characteristics.


    This brings us to the concept of Duty Cycle % (DC%). DC% is nothing more than BPW expressed as a percentage. It's understood that something less than 100% DC is required. Allowances need to be made for opening and closing time, and the injectors need down-time without voltage applied. It's generally accepted that a DC% should be 80-90% of theoretical maximum. The actual DC% depends upon a variety of factors beyond the scope of this discussion. So in the 6000 RPM example with 10ms equalling 100% DC, if we assume a safe DC of 85%, that means the maximum allowable BPW at 6000 RPM is 8.5ms (10ms * 0.85). Above 8.5ms, the injectors are considered static.

    But that's not the end of the process. All masks for the above engines/ECMs only report BPW in the ALDL data stream (logs). That BPW is NOT the "Final PW" the ECM sends to fire the injectors. A voltage compensation from a calibration table is added to BPW to produce the Final PW to "compensate" for the opening and closing times, among other things. It's from this Final PW that at DC% must be calculated because that's the time the injectors will be commanded to be held open by the ECM. And this is not an insignificant amount of time despite it being in units of microseconds (uSec).

    Let's assume the calculated BPW is 8.0ms. Using Factory $8d voltage compensations for 22# Multecs (Table at 0x3FA), a voltage compensation PW of somewhere between 580 and 793 uSec will be added at an ignition switch or fuel pump voltage between 12.8 and 14.4. If we assume operating at 13.5 volts, approximately 700 uSec (0.7ms) will be added to BPW to arrive at Final PW. In the example, this means the Final PW will be 8.7ms (8.0 + the 0.7 voltage compensation). Oops, we've just exceeded 85% DC of 8.5ms.

    This is why BPW cannot be used to compute DC%. Voltage compensations must be added. And this is the reason v6 of S_AUJP was modified to report Final PW. It allows a true DC% to be determined.


    I realize this is a long way around answering your question, but at 12ms and 6000 RPM, your injectors are way past 100% DC.

    HTH, Elky

    See also this concise summary 1project may have forgotten:
    http://www.gearhead-efi.com/Fuel-Inj...ll=1#post13624

  6. #6
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    Thanks a lot for the clarity. I am familiar with opening times from using megasquirt but I had the benefit of 2 or 8 injector drivers depending on what model I was working on. With that being the case I didn't have the limitations caused by a single driver and the extra opening time.

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