Is the injector duty cycle just a function of how long the injector is open? So as an example if you have a 80% duty cycle than your out of injector? When people say their at 114% is that possible or at 100% is the injector just staying open?
Is the injector duty cycle just a function of how long the injector is open? So as an example if you have a 80% duty cycle than your out of injector? When people say their at 114% is that possible or at 100% is the injector just staying open?
Yeah, it's just the computer saying that to get the fuel that it wants, the injector flows 14% (actually more) too little.
2000 GMC 2500 2-bar SD
Stock LM7, LTs, TBSS intake manifold
OK, so when your at 100% duty cycle that's it? It's not a calculation that can have it's data manipulated? I've always questioned if it was just a calculation and if your at more than 100% the data was inaccurate.Originally Posted by MikeGyver
Duty cycle is accurate, the percentage is the actual time that the injector is being held open, or the PCM is trying to hold it open. The reasons that it tries to keep it open that long is where the manipulation is.
My experience comes from low fuel pressure. When your fuel pressure drops because of an inadequate pump or other, the computer does not know that. So even though the injectors can flow enough, the engine isn't getting enough, and the duty cycle goes up.
Still, the injectors are being held open 100% of the time, and the PCM is trying for 114%.
2000 GMC 2500 2-bar SD
Stock LM7, LTs, TBSS intake manifold
Thanks, my 60lb injectors are at 90%, fuel pressure and volume is good with twin walbros. I wanted to double check and make sure the duty cyle is correct before making any other changes.
You know you are already 10% over the recommended duty cycle, right?
2000 GMC 2500 2-bar SD
Stock LM7, LTs, TBSS intake manifold
Yep, I wanted to verify that injector duty cycle is a accurate measure before going with larger injectors and a return system with afpr. Stepping up to 80's is a whole different animal. I'll have to lower the base pressure to give me a large enough envelope of tuning to get a good idle.
Thanks for your help
In line with this question - I have a 515 rwhp LS2 w/L92 heads/LS3 intake/injectors. My understanding is the LS3 injectors max out around 500 rwhp. Yet in my logs, I show the max duty cycle is about 78% at redline,(7100). I want to verify there aren't any settings I might have changed by accident which are showing incorrectly in the duty cycle in the logs? We did change the injector/Kpa settings, and if the others have an impact - I can post if they were different.
Bottom line - what does HPT use to determine Duty cycle? As I understand it, and is posted here, it purely MSec opening time vs avail time for that rpm? I know at 7000 rpm, there is approx 17.1ms of avail time, 24ms at 5000, etc.. So if at 7000 rpm, mine are being commanded to open for 13.4 MS, 13.4/17.1 = 78%? That matches my logs. That says that the limit for LS3 injectors is right about where I am - I'm at 515 rwhp, so maybe 530? 600-650 flywheel hp?
Along with this question - at what point do the injectors go static - 90%? It would seem 100%, but I've read,(and we know how good the internet is), that when folks have had over 90%, their injectors effectively go static as they can't close and re-open in the 10% remaining time. This would? drop the AFR,(until you exceed that 100% fueling requirement).
I imagine you have allready tuned your afr with a wideband but just in the rare case you got this far without one if you happen to be running too rich leaning it back out to proper afr should gain you back some headroom on the duty cycle
PB's 1/4 mi 12.21 117.75 trap ,1/8 mi. 7.779 93.99trap , 1.949 short time (FWD W body)
Does anybody know the size of injectors that the L76 truck engines use? (2008 Silverado Vortec Max) Somebody told me they were 42 lb from factory.
29 in a 2009 5.3
37 in a 2011 5.3
at least that's what the tune shows around 58psi.... i don't know what they actually flow, though.
I don't know if that's the L76 though, is that the 6.0?
Yes, the L76 is a 6.0. I've just got bolt ons and around 5800 rpm, and above, I'm seeing 95% duty cycle.
Cool, well I know the difference now between the 2009 and 2011 injectors I posted about are Flex Fuel for 2011. Needs a bigger injector in case you pour in E85.
Hi Bob.
Internet math will tell you that you should not exceed 80% DC, but it never gives a logical explanation for that limit.
Probably because one does not exist.
What we are concerned with is that we stay within the linear operating range of the injector. Within this linear operating range, the output vs pulsewidth, when plotted on a graph, will follow a straight line.
If our injector data is correct, this straight line will exactly match that of the line defined by our offset and IFR values. As long as the output of the injector lies on this line, the fuel delivered to the motor will be as commanded.
The chart below shows the flow vs pulsewidth of an actual injector in blue, with the straight line as defined by the offset and IFR values in white.
This test was performed at 100Hz, which would be equivalent to 12,000 rpm on a 4 stroke engine.
Using the same calculations Bob used above, we can arrive at 10milliseconds for a total time per cycle which makes 10msec on the chart equal to 100% duty cycle, 8msec equal to 80% duty cycle, etc.
As the duty cycle exceeds 90% the flow vs pulsewidth curve gradually deviates from a straight line as it delivers more fuel than is defined by our IFR and offset values.
There are two reasons for this increase in fuel flow.
1. The magnetic field of the coil has not fully collapsed, leaving it precharged for the next cycle, resulting in faster valve opening.
2. The valve has not closed fully before being hit with the next pulse.
Because the injector is delivering more fuel than is defined by the offset and IFR values, the engine runs rich. The tuner will then tweek either the VE, or MAF which results in an incorrect air mass for spark calculations.
As the pulsewidth increases further, the flow deviates more sharply from the straight line, and at some point we need to draw a line in the sand and decide just how much non linearity we can tolerate.
2% is reasonable, and we find this point at about 9.25 milliseconds, which means that we need to let this injector "rest" for .75 milliseconds between pulses if we are to maintain linear output.
This "rest" period is called the recovery time, and remains constant regardless of rpm.
Those of you with a calculator handy will realize that at 100Hz, or 12,000RPM, the .75msec recovery time limits our maximum duty cycle to 92.5% as seen on this chart.
At 7,000RPM, where we have a cycle time of 17.1 milliseconds, we can run this same injector to a 95.6% duty cycle and still stay within 2% linearity.
The injector shown in the test is not a stock LS3 injector, but we have tested them and they also have a recovery time of .75 milliseconds.
The short story is, you can run that injector quite high without problems, and we have in fact run these and other Bosch EV-14 injectors above 90% duty cycle on every shift in 6, 12, and 24 hour races without issue.
Just don't assume you can do the same with any old injector! Some of the older EV-1 style Bosch injectors need a full 2.2msec to breathe, and go static with anything less than 1.75msec.
The point is to know what you've got, and give yourself a reasonable margin of error. If your initial tuning was done at sea level in cold weather you could potentially run this injector over 90% duty cycle without concern.
On the other hand, if you tuned it in Denver in the middle of summer, and plan to drive down the hill to race at sea level you better give yourself a lot more headroom.
And finally, there is one anomoly on the graph I need to point out to avoid confusion. The injector does not go static at 9.5 msec as it appears.
The driver in our test equipment will always turn the injector off for a minimum of .5msec, regardless of the reported pulsewidth, much like your PCM will show a duty cycle above 100% which is of course impossible.
So there's a really long answer to your short question.
Hope it helps.
Paul Yaw
Injector Dynamics
I BELIEVE the limitation is the stock injector drivers, Greg Banish or other please correct me if I'm wrong.
From my understanding if they are run at their max capacity for any length of time they can fail so it not necessarily an injector limitation but more a physical limitation of the injector drivers in the stock pcm.
Last edited by Bill@HPTuners; 03-11-2011 at 12:18 AM.
It doesn't have to be perfect, it just needs to be done in two weeks...
A wise man once said "google it"
Do yourself a favor and forget wasting your tuning time with those 80's injectors. Get some ID1000's and never look back.
Last edited by Z06SUPERCHARGED; 03-11-2011 at 12:18 AM.
The 80% "rule" has been around forever, not as a result of a limitation in a stock GM ECU.
Having said that, I admit to not having considered that the stock GM injector drivers may have their own limitations.
Has anyone ever toasted an injector driver?
Paul Yaw
Injector Dynamics
The 80% recommendation comes from a combination of several things, all of which have already been mentioned on here:
1) Safety margin for additional air mass. Like Paul said, if you tune in the summer or above sea level, there's always the potential that the customer will later drive the car in denser air which will require more fuel mass to maintain the same AFR under load. I'll push this to about 90% on occasion, but I'm certainly not happy seeing 100% when I know there's another case where the car will certainly need more.
2) Injector driver protection on the PCM. The injectors are driven by transistors similar to a car stereo amplifier. Just like a stereo amplifier, the PCM usually has some form of cooling fins to help shed the heat. If they're run full tilt for an extended period of time, they can overheat. You won't overheat the injector itself since that's liquid cooled.
3) Recovery time for the coil. Again, Paul is on target here. This varies with injector design. The nice thing is that if you've left enough safety margin in #1 above, you should have this covered by default.
super informative posts Paul & Greg.
thank you for sharing your knowledge
PB's 1/4 mi 12.21 117.75 trap ,1/8 mi. 7.779 93.99trap , 1.949 short time (FWD W body)
Yes, I've had several people that have had to replace their pcm due to injector driver failure.
It doesn't have to be perfect, it just needs to be done in two weeks...
A wise man once said "google it"
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