IAC Math Fun
04-26-2019, 05:57 AM
#1
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IAC Math Fun So I've been working on an understanding of EXACTLY what the steps in the IAC table correlate to, including how a step change and the amount of air it changes as a result. Obviously bigger throttle bodies will cause different numbers, and I have a way to show this. The PCM does not know this number though, but it can be assumed that when GM programmed the PCM initially, it was well known, so I've used stock values in many assumptions.
Also known, is that the stock TB output hole for the IAC is approximately 310 cubic millimeters, according to the table, and if you calculate the size of the hole, it's relatively close to this number, assuming a square hole, which it is not. Using 25.4mm (1 inch) x 12.7 (1.2") you get 322 mm2... I'd say that's fairly close, baring any other design interference from shape. This happens to be about 0.065% of the total TB blade area, using 310mm2. So that would assume a few more things.
If you take the max MAF calc and divide that into the max TB area of a 78mm TB, you get about 10.8 mm2 per g/s at WOT. Again, assuming the GM numbers. Removing PE from this also makes a change, and this number gets much bigger.
Here is the excel I've been playing around with, using stock data and some of my own from my car, including injector changes. Feel free to experiment with it, I'm still trying to figure these things out in detail because it is one of the few things I still do not fully understand.
I also have injector average and conversion on there. Top row is stock LS1 and bottom is mine, or whatever you plug in. On the top you can see how fast the derivative is too, lol, and now we know it takes about 200 ms for the STIT to react according to that, and the rate it adds air too, over 190-200 ms.
Also known, is that the stock TB output hole for the IAC is approximately 310 cubic millimeters, according to the table, and if you calculate the size of the hole, it's relatively close to this number, assuming a square hole, which it is not. Using 25.4mm (1 inch) x 12.7 (1.2") you get 322 mm2... I'd say that's fairly close, baring any other design interference from shape. This happens to be about 0.065% of the total TB blade area, using 310mm2. So that would assume a few more things.
If you take the max MAF calc and divide that into the max TB area of a 78mm TB, you get about 10.8 mm2 per g/s at WOT. Again, assuming the GM numbers. Removing PE from this also makes a change, and this number gets much bigger.
Here is the excel I've been playing around with, using stock data and some of my own from my car, including injector changes. Feel free to experiment with it, I'm still trying to figure these things out in detail because it is one of the few things I still do not fully understand.
I also have injector average and conversion on there. Top row is stock LS1 and bottom is mine, or whatever you plug in. On the top you can see how fast the derivative is too, lol, and now we know it takes about 200 ms for the STIT to react according to that, and the rate it adds air too, over 190-200 ms.
Last edited by ChopperDoc; 04-26-2019 at 06:05 AM.
05-04-2019, 09:41 AM
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I was hoping for some folks to take a crack at it too, and maybe if we put our heads together we can come up with a solid answer as to what the values mean. The other way I was doing it was taking the root of the mm2 and converting that to grams. I'll get back to working on this at some point. I think I'm headed in some sort of direction, not sure if it's the right one lol. I think the biggest thing to do is port out the IAC on the 102's which makes a huge difference from what I've read on that over on the HPT forums. I'm gonna give that a try when I get back stateside and report back on it.
05-04-2019, 04:50 PM
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I meant to post up. I was trying to see the math behind 1% throttle stuff. Because the blade is not linear. 5% open does not mean 5% of the area is flowing.
05-04-2019, 04:59 PM
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Yes this is true, it would be more of an acceleration based on pressure delta. So basically a slope rate based on how big the area becomes and in what amount of time. I get that, but at or near closed, it won't change too much so I used a simpler model there.
05-04-2019, 05:27 PM
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Originally Posted by ChopperDoc
Yes this is true, it would be more of an acceleration based on pressure delta. So basically a slope rate based on how big the area becomes and in what amount of time. I get that, but at or near closed, it won't change too much so I used a simpler model there.