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Thread: UGH! need to squash SFI/batch fire argument

  1. #1
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    UGH! need to squash SFI/batch fire argument

    Pretty simple question....

    *disclaimer* I dont believe this is true but cant find docs to back it up

    Does all GM ECM/PCMs drop SFI above ~3000 rpm and switch to batch fire?

    Im certain its a big NO but again, no docs to back it up.

    *edit* did not specify but meant if the ECM/PCM is SFI

    Thanks!
    Last edited by 34blazer; 07-24-2013 at 11:52 PM.
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    Technically, no. Why? Because direct injected engines are SFI and the injection pulse has to occur at the correct time to work with direct injection. Everything I've read regarding injection into the intake indicates that SFI mode stops somewhere around 3000-3500 rpm.

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    Ok well that settles it. I remember someone stating(RBob?) that sequential, I think the LC2 platform fires the injectors 2 per 2 revolutions no matter what RPM.
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    Are we gonna win a bet?

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    Quote Originally Posted by 1project2many View Post
    Are we gonna win a bet?
    LOL i wish!
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    i've never seen it happen, but i don't have a scope either, let alone a dual channel one.

    i've also never found any code that would imply it to happen.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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    http://www.fiero.nl/forum/Forum2/HTML/075502-13.html

    Curious myself.
    So it does drop SFI and go batchfire or it doesn't?
    (not counting direct injection,only port/intake injected.)
    I had never heard this till I saw it in above thread i've been following and remembered this thread.

  8. #8
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    A GM engineer told me the SFI in an LT1 was only done for emissions, did not need it in 94 but 94 would pass 96 emissions tests... and do to time it is batch fire by 3000-3500 RPM. In the code it has to do with Sync and Async fueling as well as time available per revelution, as RPM increase this time decreases and there's not enough time to SFI at that point. Wish I had the proof...

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    Mark, you have a dual channel scope?
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    RIP EagleMark's Avatar
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    Yes but nothing to test it on... plus I'm in Portland for who knows how long...

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  11. #11
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    We are kidding ourselves if we think that SFI is accurate when engine speeds are high. 6000rpm has 1 cylinder's intake valve opening on every other rotation of the crank or 50 times per second. I'd bet the farm that there is some injector duty cycling overlap independent of intake valve position. I have heard the rumour too. I didn't know if it was an actual change in fueling strategy or simple lack of time to perform sufficient injection in the time allowance. DI is a completely different animal. Its injection occurs during different positions of the piston, no matter where the valve is, very precise.

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    Those speeds are certainly low enough for modern electronics to work with. Electronically activated diesel injectors have to to stay sequential and both Benz and Isuzu/GM run their diesels in the same RPM range as GM gas jobs. What are the Indy and F1 cars using for injection control? They run to 14k or so.

    By switching to batch mode you reduce the dynamic range required of the injectors and greatly reduce complexity and cost of the system. With no significant benefit from SFI at higher rpm, is the added cost to stay SFI worth it??

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    Quote Originally Posted by Hog View Post
    We are kidding ourselves if we think that SFI is accurate when engine speeds are high. 6000rpm has 1 cylinder's intake valve opening on every other rotation of the crank or 50 times per second. I'd bet the farm that there is some injector duty cycling overlap independent of intake valve position. I have heard the rumour too. I didn't know if it was an actual change in fueling strategy or simple lack of time to perform sufficient injection in the time allowance. DI is a completely different animal. Its injection occurs during different positions of the piston, no matter where the valve is, very precise.
    to me, reverting to batch fire just sounds like more code to do the same thing. I do think the PCM's are more then capable of keeping up though, it's not moving faster then the speed of light or something. I don't know the exact numbers, but an injector firing 100 times a second doesn't seem absurd for an electro-mechanical device.

  14. #14
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    The PCMs are capable of performing the processing and electrical functions (triggering) of the injectors in an SFI scheme well above the useable RPM range of an engine.

    The issue is not the PCM, it's at the other end of the wires, well a little beyond that actually. The higher the RPM gets the less time the valves are open, allowing less time for fuel to enter along with the air. When going to batch fire, some injectors will be injecting fuel when the valves are closed, this is not a bad thing. When they are pointed correctly, that being at the back of the intake valve, the heat from the valve created by the combustion process causes the fuel to vaporize and remain suspended until the next time the valve opens, at which point the vaporized fuel along with any current fuel being injected can be pulled into the cylinder.

    This is the main distinction between port injection and direct injection. With DI, there is more time for fuel to be injected into the cylinder, since there is no mechanical restriction or valve to get in the way. DI can and has to stay "SFI" for the entire RPM range in order to operate correctly.

    F1 engines are a different animal, and are designed very differently than passenger car engines. They idle at a much higher RPM than a passenger car engine, and have a very different placement for the injectors as well. In an F1 engine the injectors are mounted above the intake runners, in what would be the plenum. This allows there to be an abundance on fuel in the runner and remain a suspended mixture, to aid in high RPM running.

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  15. #15
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    I was under the impression that SFI didn't necessarily mean the injectors fired when the valve was open, but just that they fired one after the other instead of groups.

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