Borderline Spark Tuning - Humidity Effects

[engineermike]

@Cobra Jet and @K4fxd this is what the GUI looks like:

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[engineermike]

Last night, Mercury Marine introduced a new 500 hp outboard. In the literature, they say:

"Humidity Compensation Technology
To promote consistent performance in a broad range of atmospheric conditions the V8 500R outboard is equipped with a new sensor located in the intake attenuator to measure the humidity level of the incoming air. This allows the engine ECM to combine reported humidity with air pressure and air temperature data to determine the ideal spark timing for prevailing conditions to maintain the most aggressive calibration possible to optimize performance under virtually all conditions. Boaters will notice a significant gain in power in challenging, very humid conditions – up to 30 additional horsepower than would be available without humidity compensation."

 

[Chris Barnes]

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.

Ran across this today regarding the new Mercury 500hp outboard engine. Sounds quite similar to the humidity compensation you have been discussing here. Fast forward to the 2m40s mark in the video

 

[Chris Barnes]

Nevermind. You beat me to it!

 

[Midwestracer]

how's this going interested

 

[engineermike]

how's this going interested

The device has been installed on two cars and run in normal daily driver use for over a year now with no issues. However, both cars are now running flex tunes and mostly e85, so knock isn’t really an issue. Doing spot checks of mct vs dew point shows it’s still working.

I did switch to a 3.25 pulley on true 93 some time back, which basically no mainstream tuners recommend. It worked perfectly fine, as much as 18+ deg timing on high humidity moderate temp mornings and as low as 12 deg on hot days with low dew points.

 

[Midwestracer]

Sounds great

 

[Cheatham]

I know my comment may not have much to do with this but back in the 90s when I ran dirt late models with methanol on foggy nights after the races were over we would discuss how much different the engines would run, the conclusions were perhaps the moisture in the air cools the air going into the intake and perhaps the moisture is taking up space in the chambers causing a compression increase, all we knew was the engines ran stronger, I also remember back in the street racing days back in the 80s on late nights in the fog or when dew was setting in the car seem to have a little more power, never thought much more about it.

 

[Dana Pants]

Nature’s water injection…

My 2013 VW is so stupid that it throws a code sometimes in heavy rain because the spark advance algorithm reaches the limit and detects no knock… which brings up the point that motors really like the rain. Good luck putting the power down.

 

[Timbuck]

Really interesting.

It’s really perfect example effects of dry air , and how water injection can benefit NA engines not just boosted.

I’d love to see port injection water/meth on NA coyote to see if you could gain that 2 or 3 degree’s back…maybe more.

 

[robvas]

Which makes more power, 25% humidity or 90% humidity with whatever extra timing you can put in it?

 

[engineermike]

Which makes more power, 25% humidity or 90% humidity with whatever extra timing you can put in it?

I think it might just break even.

 

[Angrey]

I think it might just break even.

Interesting stuff. @engineermike few ???'s.

Do you think the additional knock protection is thermal or some sorta ablating properties of the water vapor keeping pre-ignition down?

What's interesting is that there is oppo convo's (even right now) about humidity and water grains and their effect on dragy times.

There's decent empirical evidence that our cars run slower in humid conditions. I'm curious as to whether that's simply additional risk to the motor in dryer conditions (where it's making more power with the same or similar timing) or something in the OE tune parameters which hampers the output when it's humid.

I haven't done the research to see whether SAE and STD correction factors take into account humidity, but this paired with the observed effects from humid track days and dragy measurements would suggest that without some sorta hygrometer inputs, it's probably best/safest to tune the motor on a dry day and suffer the power loss with humidity than to tune it on a humid day and experience more knock risk when it's cold and dry conditions.

What are you thoughts?

 

[engineermike]

I think I read some time back that absolute humidity slows the combustion process, so with high humidity it makes less power and peak cylinder pressure is lower. So increasing timing really just gets peak cyl pressure crank angle back where it’s supposed to be.

 

[K4fxd]

Basically it is natures water injection.

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