I've been steering clear of other people's spreadsheets, because I'm not sure of my own thoughts on the subject, let alone other people's understanding of it. I should probably make a spreadsheet of my own. It would be very handy for seeing the curve through the RPM range, I'm sure.
I have also revised my thinking on this since I first commented on this subject in this thread. My original understanding (and that of practically all of us) was that raw gas was being wasted out the exhaust valve during overlap. After discussing the Pentacorn theory with Paul, I've come away with a different outlook. I now share his opinion that at idle, the combustion chamber and exhaust system are under higher pressure, while the intake manifold is under a vacuum. This leads me to believe the last little bit of the exhaust stroke is actually pushing inert exhaust gases up the intake valve, diluting the intake charge. This is a problem too, of course, but a different problem than what we thought. And it probably has about the same solution.
If we are able to delay idle and part throttle injection start points until after the exhaust valve closes, we may be injecting it into an open intake valve, but the fuel is still hitting the hot valve on it's way in. Reversion from the cam is still happening, and inert gases are still being pumped into the intake manifold, diluting the air charge. But at least the fuel is not part of that mess. We can delay the fuel injection until after that. By doing so, we take the fuel issue away, and are only left with the other fuel issue... the air charge being diluted. Just like when we use EGR, we need less incoming fuel because we are now pumping less (diluted) incoming air. In the end, we just let more fresh air in by increasing idle airflow (and fuel flow matches that increase).
One sign of this theory of mine being right would be the amount of fuel and deposits on the back side of the throttle plate. If there is fuel in the reversion, it will get scattered all over the intake manifold, but will tend to collect as deposits on the throttle plate. Or at least moisture instantaneously. If we fix the fuel problem by delaying injection time, then clean the throttle body good, perhaps we won't see any more fuel on the back side of the throttle plate, indicating we're making progress.
The other thing is, as we waste less fuel, we may find that the same amount of fuel now creates a rich condition. Therefore, I wouldn't be surprised to see idle fueling/VE be reduced slightly as we fix the overfueling issue with injection delay.
I'm troubled by the -784 thing. There's only 720 degrees of duration in a cycle, so I'm not sure what the -784 number is supposed to represent. If it was -64, I would understand it a bit better, and that's how I have been treating it. We know that the cam sensors job is to tell the PCM what stroke the engine is on, compression or exhaust. This is used to start the firing order or injector firing order off on the right set of cylinders, for example 1843 or 6572 (SBC firing order example). But it's the crank sensor that tells us from there, exactly where we are in engine rotation, and which cylinder is up to bat next. Both the cam and crank sensor need to be advanced a bit, meaning that we need to read the position before the engine actually gets there. So there's gonna be a delay between when the sensor sees it's mark (say TDC compression on the cam sensor, for example) and when we actually hit TDC compression. The sensor must read it first, so there is enough time for the PCM to react to this piece of data.
When somebody figured out -784, that might be very handy. However, was it the cam sensor or the crank sensor? And was that sensor "advanced read" position already factored by the computer, or do we need to manually factor it in our injector delay timing? Let's say the sensor reads 60 degrees before TDC, for example. Does the PCM know that, and add 60 degrees for us, so our tables make sense? Or did GM tell the calibrators the amount of sensor delay, so they could build that into the tune? I don't know. And I don't think anybody else knows either. To truly know, we would need the engine on an engine stand with a perfectly installed degree wheel that's large (18"). We would need to measure where these sensors hit their marks. Then, we would need to read the stock tune settings for injector delay. Then we would need to see this same engine run in a car, with a really good digital storage oscilloscope (like my Picoscope) show us the injector on times on one channel, while we simultaneously log cylinder pressure of that same cylinder on another channel.
I also have an in-cylinder pressure transducer for my scope, that can do just that actually. We can find the two TDC compression points of an engine revolution or two, mark the exact center of the two TDC points, then mark the number of degrees between those points as 720 degrees. Now that we have our engine rotation degree marked, the next step is to overlay the injection timing events, and see where the PCM is actually turning the injector to that cylinder on and off. With all this madness, we would finally be able to watch a stock car do it's thing, compare that to the tune data, and know what all this means FOR SURE.
If you would like to read a bit more about Inertia Supercharging, I wrote an article on it a while back on my Drag Radial Performance facebook page. Here's a link:
https://www.facebook.com/DragRadialP...510359875510:0
There's a bunch of other articles in there for tuner type guys as well. They're often centered around fox bodies, but tuning is tuning. All engines work the same. The software is just different.
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Originally Posted by pontisteve
