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Thread: Not what I expected

  1. #1

    Not what I expected

    In my proccess of learning timing. I have been taking other people's tunes from the forum and punching them into a calculator to see others strategies and to find common trends. I have seen some people have well over 150% btdc in the low mm3 higher rpms. Can some one explain the over 100% and why I might want to do that?
    Thanks again

  2. #2
    Advanced Tuner Jim P's Avatar
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    Throw the calculator away is my advice rather than confusing yourself over injection percentages before top dead center and the who, what, where and why of it all.

  3. #3
    Advanced Tuner Jim P's Avatar
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    Now as for why are some running further advanced timing in the low mm3 high rpm area, as rpm increases your window speaking in the matter of time decreases, thus timing increases with rpm. In the low mm3 you are injecting much less fuel, you aren?t going to be putting a whole lot of internal stress on the rotating components with the increased timing at the high rpm.

    Now, switch it around so it?s high mm3 and low rpm. You are injecting large volumes of fuel at a low speed, you got a much larger window in the matter of time to inject that fuel, you don?t need to advance it much. In fact, if you advance it and advance it too far you can create very high stresses on the internal rotating components, enough to cause damage.

    If you advance timing a couple degrees at a time in even mid fuel, say 45-75mm3 and 1000-1600rpm, eventually you?ll begin to hear what sounds like a rattle or knocking. Some call this injector rattle, some call it timing rattle. Whatever you want to call it, what it is is a sign your timing is becoming too high. Think of it as pre-detonation in a gasoline engine. It?s the exact same thing happening. The fuel is igniting too early and very rapidly in the engine cycle, time to back off the timing some. Go too far advanced and the stresses can become so great you could push a connecting rod out the oil pan.

    If you must have a percentage and are just absolutely dead set on having a percentage... not going to get one. But, with that being said, in general for optimal timing, the duration of the combustion cycle, very important words here... combustion cycle, NOT injection duration but combustion cycle, the middle should occur at around 2 degrees or so AFTER top dead center.

    Some factors that affect this, coolant temperature, air temperature, cylinder temperature, fuel pressure, fuel quality, air density, fuel temperature, hydraulic delay. All of these plus many other factors affect what?s called ignition delay, the time from when the injector begins to spray fuel to when combustion begins to occur. You have no real way to measure all this and determine exactly where you should set your timing tables at for optimal timing.

    This is why that calculator shouldn?t be used to try to figure out timing. It?s nothing more than a tool to show some numbers for the curious of mind on some stuff like how many crank angle degrees the injectors are open for at cells. That calculator does nothing to help anyone with anything and no one should use it to try to understand how to do tuning or why something is done in tuning.

    If you want a real understanding, start reading lab studies on thermodynamics in a diesel engine, affects of pilot timing and pilot quantity, affects on fuel pressure and things of that nature that apply to diesel engines. Then you will begin to understand.

  4. #4
    Thanks Jim! The reason I like the calculator is because I can see how changes effect things. I can load all my factory maps, then as I change for example duration I can see how that effects timing. I’m not looking for a program to time my truck. But I am trying to get my mind wrapped around the concept and see what other people have done. Which I think the calculator helps me see it easier.
    And now off to dive into your previous post! Thanks again Jim. By the way sent you a pm several months ago, did you ever receive it?

  5. #5
    Advanced Tuner Jim P's Avatar
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    That calculator is good for seeing how things like increasing or decreasing rail pressure or duration affects injector open time within cells but it won?t explain why someone chose a specific start of injection for a cell.

    Sorry I didn?t see you had sent a pm.

  6. #6
    Tuner MAIDENCR's Avatar
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    What year?

  7. #7
    It's an 05 six speed

  8. #8
    Tuner MAIDENCR's Avatar
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    Ok ok...04.5 run at very low timing compared to 03-04

  9. #9
    Advanced Tuner Jim P's Avatar
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    Different emissions requirements at time of production has a lot to do with the differences between 03 to early 04 and 04.5-07 if looking at stock tunes. If looking at modified tunes it has a lot to do with intended results and hardware used.