engineermike
Well-Known Member
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- #1
As most have accepted, the challenge of making power with a supercharged Gen3 Coyote on pump gas is maximizing spark timing while avoiding knock. The rule of thumb is that 1 deg is 20 hp, but this gets worse the lower you go. I would qualitatively say that the goal should be to stay around 18 deg in most cases, and the losses really start stacking up under 16 deg.
When I was tuning the spark curves on my car, I was guided by an OEM engine calibrator to stay at least 2 deg below sensed knock when the humidity is high, and it should get minor knock at low humidity. I initially had my doubts because I had never heard of humidity being a significant factor before, but later noticed that I could run significantly higher timing without knock at 75 deg dew point than I could at lower humidity levels. I did some technical archive searching and found that it's a very well-established phenomenon that humidity has a significant effect on knock-limited spark advance. Spark humidity corrections are on the order of 4-5 deg during typical variations but can go as high as 10 deg(!) in extreme conditions. The biggest difference is on the humid end of the range at dew points of 70+.
To demonstrate, the two following logs were at nearly identical ambient temperatures, same tune, but one was at 68 deg dew point (humid) and the other was 45 deg dew point (dry). Commanded "borderline" timing was 16.7-16.8 plus 2 deg limited knock advance, for a total of about 18.7 deg at 6000 rpm. In the humid case, 3 cylinders knocked, resulting in 1-1.3 deg retard (.4 deg average retard). In the dry case, all 8 cylinders knocked, resulting in .2 - 2.5 deg of retard (1.5 deg average retard), some cylinders experiencing more than one knock event.
68 deg dewpoint:
45 deg dewpoint:
I do find it surprising that Ford accounts for timing adjustments based on load, speed, cam timing, lambda, coolant temp, and air temp, but not humidity. I did learn, however, that one OEM will soon be adding humidity sensors with spark timing corrections to their strategies. Now that I think about it, I'm starting to wonder if most of the reports of knock due to a bad tank of gas or winter gas are actually just the effects of atmospheric humidity changes, especially considering the absolute humidity is almost always lower in the winter. I have over 700 logs of my car and have never experienced unexpected knock that couldn't attributed to low humidity at the time.
Luckily, I have a close friend who builds circuits and programs microchips for fun. He designed and built a circuit that adds an external humidity sensor, intercepts the Manifold Charge Temp (MCT) signal, and modifies it such that timing is pulled or added as a function of humidity. The final MCT signal is a combination of both charge temp and humidity such that both can be used to adjust borderline timing. MCT was chosen because it has little other effect on the overall tuning other than just for timing adjustments. The following log data were taken within minutes of the above logs on their respective days, but with the humidity timing correction active:
68 deg dewpoint, humidity correction applied:
45 deg dewpoint, humidity correction applied:
You can see that the commanded spark timing (borderline plus knock advance) went from 18.7 deg without the humidity correction to 17.1 deg with correction for 68 deg dewpoint and 15.7 deg with correction for 45 deg dewpoint. You can also see that only one cylinder had incidental knock on each pull. At a 75 deg dewpoint, there wouldn't be any correction at all and the full 18.7 would be commanded even with the correction on. The end result is less power loss on hot, humid days because the timing will be more advanced, and safer spark timing on dry days.
I'm pretty excited about this because it will allow me to run the same tune year-around and take advantage of an extra couple of degrees of spark timing in the summer while maintaining the same knock safety margin.
I think the way most tuners handle it is they command a low borderline timing and let knock advance slowly feed in more timing. This will "sneak up" on knock, but then knock events will cause timing retard and the result is it will stay near optimal. I never liked this because you are guaranteed to have knock on every WOT pull, and you have to wait too long to reach a good timing number after going WOT.
When I was tuning the spark curves on my car, I was guided by an OEM engine calibrator to stay at least 2 deg below sensed knock when the humidity is high, and it should get minor knock at low humidity. I initially had my doubts because I had never heard of humidity being a significant factor before, but later noticed that I could run significantly higher timing without knock at 75 deg dew point than I could at lower humidity levels. I did some technical archive searching and found that it's a very well-established phenomenon that humidity has a significant effect on knock-limited spark advance. Spark humidity corrections are on the order of 4-5 deg during typical variations but can go as high as 10 deg(!) in extreme conditions. The biggest difference is on the humid end of the range at dew points of 70+.
To demonstrate, the two following logs were at nearly identical ambient temperatures, same tune, but one was at 68 deg dew point (humid) and the other was 45 deg dew point (dry). Commanded "borderline" timing was 16.7-16.8 plus 2 deg limited knock advance, for a total of about 18.7 deg at 6000 rpm. In the humid case, 3 cylinders knocked, resulting in 1-1.3 deg retard (.4 deg average retard). In the dry case, all 8 cylinders knocked, resulting in .2 - 2.5 deg of retard (1.5 deg average retard), some cylinders experiencing more than one knock event.
68 deg dewpoint:
45 deg dewpoint:
I do find it surprising that Ford accounts for timing adjustments based on load, speed, cam timing, lambda, coolant temp, and air temp, but not humidity. I did learn, however, that one OEM will soon be adding humidity sensors with spark timing corrections to their strategies. Now that I think about it, I'm starting to wonder if most of the reports of knock due to a bad tank of gas or winter gas are actually just the effects of atmospheric humidity changes, especially considering the absolute humidity is almost always lower in the winter. I have over 700 logs of my car and have never experienced unexpected knock that couldn't attributed to low humidity at the time.
Luckily, I have a close friend who builds circuits and programs microchips for fun. He designed and built a circuit that adds an external humidity sensor, intercepts the Manifold Charge Temp (MCT) signal, and modifies it such that timing is pulled or added as a function of humidity. The final MCT signal is a combination of both charge temp and humidity such that both can be used to adjust borderline timing. MCT was chosen because it has little other effect on the overall tuning other than just for timing adjustments. The following log data were taken within minutes of the above logs on their respective days, but with the humidity timing correction active:
68 deg dewpoint, humidity correction applied:
45 deg dewpoint, humidity correction applied:
You can see that the commanded spark timing (borderline plus knock advance) went from 18.7 deg without the humidity correction to 17.1 deg with correction for 68 deg dewpoint and 15.7 deg with correction for 45 deg dewpoint. You can also see that only one cylinder had incidental knock on each pull. At a 75 deg dewpoint, there wouldn't be any correction at all and the full 18.7 would be commanded even with the correction on. The end result is less power loss on hot, humid days because the timing will be more advanced, and safer spark timing on dry days.
I'm pretty excited about this because it will allow me to run the same tune year-around and take advantage of an extra couple of degrees of spark timing in the summer while maintaining the same knock safety margin.
I think the way most tuners handle it is they command a low borderline timing and let knock advance slowly feed in more timing. This will "sneak up" on knock, but then knock events will cause timing retard and the result is it will stay near optimal. I never liked this because you are guaranteed to have knock on every WOT pull, and you have to wait too long to reach a good timing number after going WOT.
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