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What does %100 duty cycle REALLY mean?

This is a discussion on What does %100 duty cycle REALLY mean? within the Computer & Tuning forums, part of the LSx Technical Help Section category; I bought the Predator and like it. It monitors all kinds of good and useless crap, but there is no ...

  1. #1
    Member ericwilloughby's Avatar
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    What does %100 duty cycle REALLY mean?

    I bought the Predator and like it. It monitors all kinds of good and useless crap, but there is no duty cycle. Only pulse width. How do you convert PW and RPM into duty cycle?

    I'm having a major problem with the math on this because I don't know if %100 duty cycle means the injector never turns off, stays on through all 4 cycles, or if it means the injector stays on through the entire intake stroke only???

    I'm starting to think it means something completely different because this is a stock 05 GTO and the pulse width stays 19 ms from 4,000-6,500. Since the intake stroke at 6500 only takes 18 ms to occur

    at 6,500 RPM.
    Only half the revolutions contain intake strokes. 3250 strokes per min.
    \ by 60 to get strokes\sec.
    54.16 strokes per sec.
    1\x, or invert it to get from strokes\sec to sec\stroke.
    018 sec per intake stroke = 18 ms

    Either the factory setup is at %100 at 6,500 and almost %200 at 4,000 or I just don't understand what a duty cycle is. Someone explain it and why this math is wrong?

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    at 6,000 RPM with PW of 10msec, you have 100% DC. It takes 10msec for the crank to turn once at 6,000 RPM. So, if the PW is 10msec at 6,000 RPM, you have defined 100% Duty Cycle.

    at 3,000 RPM, PW = 20msec, DC = 100%
    at 4,000 RPM, PW = 15msec, DC = 100%
    at 5,000 RPM, PW = 12msec, DC = 100%

    At 100% DC, your fuel injectors is said to have gone "static" (they are open all the time). You want to avoid this.
    '99 Camaro SS; 6-spd, LS2 402 stroker short block, PP stage 3 heads, GMS MAF, 244/248 cam, FLP headers, Borla catback, 4.10 gears, LS6 intake, 39#/hr injectors, Pro 5.0 shifter, subframe connectors, Comp Cams 1.85 rr,
    Best ET: 12.23sec (2.000sec 60'), trap spd:117.5mph, with the LS1 engine.

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    Member ericwilloughby's Avatar
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    Yep, I learned a lot about this subject this weekend. Like:

    The injector is not just on during the intake cycle but through all 4 when at %100.

    6500 RPM is = to .0001538 min. pre revolution. Just 1\x to get it.
    x 60 to go from min to sec. = .0092 sec per revolution or 9.2 ms
    That 9.2 ms for a revolution.
    2 strokes take place in any 1 revolution.
    An intake, or any other stroke, is half the time of a revolution. Thus
    4.6 ms

    Since there are 2 revolutions in a combustion cycle the whole combustion cycle takes.

    18.4 ms at 6500RPM.

    To get the max possible duration of a pulse width at any RPM.
    120\RPM

    7,000 is 17ms
    6,500 is 18.5ms
    6,000 is 20ms

    So when your pulse width gets there you are maxed out at %100. Cool.

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    Yep, you are correct, 20msec at 6,000RPm is 100% DC.

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    Member koolaid_kid's Avatar
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    The experts say not to exceed 85%.

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    Member ericwilloughby's Avatar
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    I keep hearing that also but the stock factory setup has my pulse width at 17 at 6,000. Thats %85. Weird thing is that when I go over 6000 I can see the pulse width getting shorter which leads my to believe that I am actually at %100.
    Last edited by ericwilloughby; 11-19-2006 at 04:21 PM.

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    Member Chad97z's Avatar
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    So what does all this mean? Sounds like to me that the faster your engine turns, the injectors reach a point to where they can't keep up with the on and off triggering.

    For example, all integrated circuits have propagation delay. And that is the amount of time it takes for the output to change states after the input has been changed. There is a clock (a/c signal, usually square wave)on each integrated circuit and when the clock changes state, the input of the IC is analyzed and the output is modified if necessary.

    If the ECM tries to clock the injectors faster than the injectors can change state, then you will get a 100% duty cycle because the injector cannot keep up, so it stays open.

    So the clock that operates the injectors must be faster than the input signal from the ecm, which tells the injector to change states and the physical attributes of the injector must be able to change states that fast as well. Whatever that rate is. milli seconds.

    As a side note, our cars get a/c current from the alternator. So, their is a peak frequency the alternator is capable of producing. And the period of any circuit on our cars cannot be shorter than the highest frequency the alternator can produce. So, who knows, if you want a faster injector, and of you want to do 7000 rpms on a fuel injected system, you might need an alternator upgrade. =)
    Last edited by Chad97z; 11-18-2006 at 07:35 PM.

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    I just did a little calculation. 20ms is the period of a 50HZ signal. And that is pretty much the industry standard for an a/c circuit in america. So I doubt you could ever get a better alternator. Of course, even if you could the whole system would have to be completely re-engineered. But I have heard of 60 HZ systems. I think our wall outlets are 60HZ.


    also not sure exactly what your trying to figure out, but the duty cycle is the pulse duration divided by the pulse period. Maybe what you call the pulse width is the same as the pulse duration.
    Last edited by Chad97z; 11-18-2006 at 08:14 PM.

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    Chad, it's not a matter getting faster injectors, it is a matter a getting injectors that will flow enough flow for your needs at your max RPM without exceeding 85% DC.

    In other words, if you cant get enough flow out of 26.5#/hr injectors when you hit max RPM (say 6500 RPM) when the injectors have gone over 85% DC, then you need to install higher flow rated injectors, perhaps 36#/hr injectors.

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    Member Chad97z's Avatar
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    ya that too can help. Increasing flow can reduce duty cycle. makes sense.. =)

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    Member koolaid_kid's Avatar
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    Not sure about car alternators, but the American standard for AC is 60 Hz.
    The European standard is 50 Hz. Just my two cents worth.
    And the aircraft standard is 400 Hz.

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    Member ericwilloughby's Avatar
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    "So what does all this mean? Sounds like to me that the faster your engine turns, the injectors reach a point to where they can't keep up with the on and off triggering."

    Not the case. It's not a matter of keeping up. They just stay on all the time. Which is not a problem for them. Just means you are not getting any more fuel than that and it starts to go lean. Now that I can log data I'm putting the NOS on this car just to see what happens to my AFR with the spray. I think I'm already at %100 stock. Will know for sure with a 50-100 shot.

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    Member ericwilloughby's Avatar
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    Quote Originally Posted by Chad97z View Post
    As a side note, our cars get a/c current from the alternator. So, their is a peak frequency the alternator is capable of producing. And the period of any circuit on our cars cannot be shorter than the highest frequency the alternator can produce. So, who knows, if you want a faster injector, and of you want to do 7000 rpms on a fuel injected system, you might need an alternator upgrade. =)
    What? AC? It's DC dude. 4 way bridge rectifier.

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    Yea Eric , I'm with you on this AC crap, we are dealing with only DC.

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    DC does not have a frequency. Frequency is made up of up and downs on the cycle. DC does not have these. That is one of the reasons we went to A/C power in our homes back in the .... early 50s i think

    There are other reason too but lets stick to the subject.



    My question though... If the injector is being controlled be electricity it should be able to turn off and on as fast as any other injector out there. The electricity flows at the speed of light so the only limiting factor on the injector is putting out enought fuel in other words a bigger squirt in the same period of time... that could effect the Duty Cycle... yea .. yea that makes sense.. Duty cycle is the amount of time it is on in a set time frame. so if it has to stay on longer in a set time to put out more you increase the duty cycle...
    you can have a half in hole on for 1 minute or a 2 inch hole for 30secounds but you get the effectively same amount total

    Well not sure if that is exactly what you all were saying but if not i hope it helped in some way.

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    We are wasting our time talking about AC or how fast an injector "turns on".

    In the fine tuning of our PCM, there is a thing called injector offset, I think that this parameter is used when we need large injectors, such as, those needed for power adders.

    You need to only gauge the size of the injector according to how much Hp you want to make at the crank, its a simple equation.

    inj size = (crank Hp) * BSFC / [(# of inj) * (DC)]

    Typically BSFC for LS1 engines is 0.45; use 0.6 for power adders; set DC to 0.85. If you want to make 500 Hp, then you need a 33#/hr injector. So pick the next size up from 33, and set the injector constant to the correct value in your tuning efforts.

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    Since the subject of injector DC is on the table I have a question. How would fuel pressure be considered in this equation? Case in point. My buddies c-5 was bought already modded with heads, cam, and pretty much everything else. The original owner put an adjustable FPR instead of larger injectors and bumped the fuel pressure. I've calculated his duty cycle and it's 80ish%. The problem is his cold idle has owned me since he's had the car. I've tried numerous things and still can't get the cold idle right. It's ok, but not as good as it should be. I'm almost convinced that the bumped fuel pressure is why I've had so much trouble with it ( I've also blamed his descreened MAF ). I'm trying to talk him into going to bigger injectors and putting the fuel pressure back to stock. It's currently at 68psi if I remember correctly. I do see where the original owner was coming from...kinda. He's getting more flow out of the stock injectors by bumping the FP up but I think it's creating more headaches in other areas.

  18. #18
    Not to exceed 85%? heh. So what do they say about 104%?

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    Member ericwilloughby's Avatar
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    Quote Originally Posted by 0rion View Post
    Since the subject of injector DC is on the table I have a question. How would fuel pressure be considered in this equation? Case in point. My buddies c-5 was bought already modded with heads, cam, and pretty much everything else. The original owner put an adjustable FPR instead of larger injectors and bumped the fuel pressure. I've calculated his duty cycle and it's 80ish%. The problem is his cold idle has owned me since he's had the car. I've tried numerous things and still can't get the cold idle right. It's ok, but not as good as it should be. I'm almost convinced that the bumped fuel pressure is why I've had so much trouble with it ( I've also blamed his descreened MAF ). I'm trying to talk him into going to bigger injectors and putting the fuel pressure back to stock. It's currently at 68psi if I remember correctly. I do see where the original owner was coming from...kinda. He's getting more flow out of the stock injectors by bumping the FP up but I think it's creating more headaches in other areas.

    If it has a FPR then set it to stock pressure when there is a vaccum and boosted FP when at WOT.

  20. #20
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    An increase in fuel pressure increases the flow rate of the injector,, there is an equation for that also. For example, on my 3rd gen car, where I am running an AFPR, I am running 52psi fuel pressure with 24#/hr injector (rated at 39psi) therefore the actual fuel rate for me is about 27.5#/hr.

    So, I would use 27.5 in the equation above. That is how a bump in fuel pressure is handled.

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