As for an explanation for how reported duty cycles can exceed 100% (which is a physical impossibility of our universe -- nothing can be "on" for a longer duration than "all of the time"), the answer lies in the simple fact that you are referring to a calculation. It's not something physical, and as such, is not numerically limited to 100 (even if the physical injector duty cycle is so limited). Doubt the PCM even knows what "duty cycle" means -- it has no need to calculate it.

For example, the formula for duty cycle = [(injector pulsewidth * 100) / (120000/RPM)]. This simply boils down a ratio of two integers: commanded injector pulsewidth divided by the period of the combustion cycle. If the engine RPM is 1000, it takes approximately 120ms to complete a combustion cycle. For the calculated duty cycle to equal 100%, the commanded injector pulsewidth would also have to be 120ms. For the calculated duty cycle to exceed 100%, the commanded injector pulsewidth must exceed 120ms, but again, this is just an integer -- it's not what the physical injector is doing.

So where does the discrepancy between "calculated" and "physical" duty cycle come from?

First guess off the top of my head would involve latency -- the "dead time" needed for the injector to open and close in response to the coil being energized/de-energized. The electromagnetic field driving the injector takes time to propagate, and the moving components of the injector, no matter how small, have mass and take time to move. Different injectors have different latencies, and the electromagnetic field driving the injector is proportional to the current flowing through the coil -- thus different voltages will affect how quickly the injector behaves. This latency time is added to the base commanded pulsewidth as a correction factor, as programmed in the injector offset vs. battery voltage table.

Presumably, operating at a reported 100% duty cycle should still include this latency period, so that would mean the injector still technically isn't physically open 100% of the time at a calculated 100% DC -- it's still allowing the injector to fully open and close, but it's at its extreme limit. When the commanded pulsewidth finally exceeds the combustion cycle period, my guess is that the latency period begins to get overlapped. The injector may even continue to increase fuel flow as the injector begins to hang open. It's at that point that you've lost stability of the control system -- what GM engineers painstakingly tried to avoid. PCM is little more than a paperweight that point.

Of course, this is presuming that the injector hasn't already malfunctioned before that point. Wires in real life are not ideal, and while approximately ≤1 amp of current courses through the internal coil over time, that coil wire will heat up. As it heats up, it becomes more resistive. As it becomes more resistive, less current can flow. Less current means a weaker magnetic field driving it, and the injector malfunctions.

I could toss out some other ideas, but I'm tired. I haven't proofread this, so there might be some errors, and someone more versed in automotive control systems may be able to correct me. If anyone has a better explanation, I'm all ears.

 

1: I don't know who bigcat is. If his NA 350" stock compression, stock valve, port volume & stock throttle body 3300lb car is half a second faster that mine, then I'm a fan.

2: I can't provide dyno graphs from four years ago. Honestly, I don 't care if you believe me or not. I've only met a couple of engineers you could tell anything anyway.

3: I was tuning these things before any of this software existed. I've been tuning cars since 1962, long before computers in cars. Same things make power, we just use a key board for a lot of it now. Have not tweaked a carburetor for over 30 years now.
I used DOS PCs with a hex editor a long time before there was point & click "tuning software". Myself, and the few others doing this back then (mid-late 1980s) had to reverse engineer the calibrations in those old chips. I still use a hex editor at times.
Engineers are needed.
I still enjoy racing them.

 

Correction: BigCat is running 8.90s. His avatar must be outdated.

With that, I think I'll step out of the thread and leave everyone to their own devices. Feel free to tear my posts apart -- that's why I posted, that's how people learn.

Keyboard jockey's taking a break...

 

The subject of injectors and throttle bodies has been beat to death over the years. It's making me tired.

 

I am sure it has been. tried searching 5 different ways, and the search function on this site is so crappy, none of the results that came up were at all relevant to the info I was looking for. but this thread really didnt cover what I was looking for either, Too Many keyboard jockeys to busy trying to get respect from each other to add viable info to help another member out. I'm Out

 

It appeared from post #18 that you'd made your decision.
I don't see anything since then that should change that.

 

Post #19 shed light in a different direction, sending research elsewhere, post #36 was the final decision, it was only the first several posters that actually added viable content to the original question. I would say read the thread, but honestly it isnt worth reading. unless you just enjoy reading arguments.

 

Post 19 wasn't "light". Regarding your original question, it was more like chaff.
See Post 39 for the explanation.

 

I do not see it that way, I have read #39, and if it reduced emissions and allowed the computer to better control the injectors, then it increased efficiency, reduced emissions = more efficiency. there again it's a win win situation as it would lead to better economy and a more reliable system, IMO, also a dry system is what I am considering, as it is less complicated, So in my decision what you posted also influenced the decision. and I happen to have 3 fully functional sets of the factory 24# injectors. but feel for the reasons I explained here it would benefit me to upgrade to 30# injectors, prior to attaining a tune to complete my project and get it fired up and driving.

 

It's your thread; the last word is yours
I certainly agree with your decision since you've clarified dry kit.

 

I just wish the thread could have been more informative, with less pissing matches.

 

OEMs measure things like emissions and efficiency at far smaller increments than can matter to an enthusiast, and that benefit would be at idle will be hurting launch performance at the track in a measurable way.