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Thread: Humidity spark tables

  1. #21
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    I can confirm that on the 2017+ Z06's and ZL1's the Relative Humidity is what is used in the Humidity multiplier. And the column axis of 0.0 to 4.0 is indeed 0%-400% humidity. I'm not sure why there is a value over 100% but GM is GM lol. I built a custom math parameter to determine what the missing amount of timing is on a 2017 Z06. It exactly follows the multiplier as described above when compared to the Relative Humidity PID.
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  2. #22
    So what?s the general consensus? Unless spliting hairs on a certain day just leave this table stock? On my 2020 L87 compared to a 2021 L87 in Australia there was a odd 2* advance in the timing down low on the table I found that kind of odd. Think gm would do the same across the board on these trucks?

  3. #23
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    I typically leave the table stock or slightly modify it in the WOT area so that I can better control timing. It seems most tuners zero out the table. I'm not trying to start a consensus, I am probably in the minority here.
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  4. #24
    Thanks I’ll maybe take your advice and play around with the wot area. Also curious on why that 2021 from Australia had more timing in certain areas but hard saying.

  5. #25
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    I only wish there was a PID that made sense for this table. I take back my earlier comment that it is definitely 0-400% humidity. I can't find and PID that makes sense here.
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  6. #26
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    @TriPinTaZ I think I might have figured this out. I think the GM "Humidity (%)" is the actual % of water that the air is comprised of. In other words, it's the mass of water/mass of air x 100. This makes a lot of sense for a couple of reasons. First, in the research I've done, all parties seem to agree that knock tendancy is a function of absolute humidity/mixing ratio/dew point, not relative humidity. Secondly, the ranges match up well with typical ambient conditions. The truck file range is 0-2.3 and SC car file is 0-4. The record high dewpoint in the US is 90 deg F, which converts to 31 g/kg or 3.1%. A more typical high dewpoint in the gulf coast region is more like 75 deg F, which converts to 1.9% water in the air. So basically, the truck scale covers a very wide range of humidity and the car scale covers the extremes. In order for the car file I'm looking at (2018 CTSV) to actually add timing, you'd need a dew point over 77 deg F, which is very rare even in South Fl. The truck file just needs a dew point over 58 deg to get a timing adder.

    I'm actually building a humidity correction device for Ford since they don't have a stock sensor or logic to do this, which is what led me here to begin with....

  7. #27
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    So I used a calculator for todays weather here in FL.

    Temp: 79

    Relative Humidity: 58%

    Dew Point Temperature: 62.9?F (17.2?C or 290.3K)

    Water Vapor Pressure : 1,963 Pa

    Saturation Water Vapor Pressure: 3,385 Pa

    Absolute Humidity: 14.2140 g/m3

    Moisture Volume Concentration: 19,374 ppm (1.9374%)

    Moisture Weight Concentration: 12,048 ppm (1.2048%)


    Would I be correct in assuming that GM is looking for % of water in the air by weight rather than volume? Which would then mean the value is about 1.2% and the Humidity Table on an LT4 car would put that right in line with what I see on the scanner today. IF it is by weight, that would be accurate.


    This thread should be sticky'd somewhere because this would solve the humidity table multiplier confusion. Great info, sir! Humidity spark is important to maximizing the performance through the seasons/weather.
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  8. #28
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    I checked your math and get 1.2% as well. I would assume it's by weight, and that falls right into the correct scale range for the column axis. All of the research I've done uses mass ratio, typically g/kg, to analyze knock impact. Volume ratio would get needlessly complicated. This is a fascinating subject, as I've been wondering why humidity wasn't more widely used as a spark correction...not knowing GM has been doing it for years until a fellow Ford guy pointed me this direction.

  9. #29
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    Most tuners seem to disable humidity spark. But I've started using it to accommodate for weather changes so that the car isn't dangerous in the winter when it needs less timing and the car doesn't slow way down in summer when it needs more timing due to increased humidity. 99% of the cars I tune are in FL or Texas so this logic applies well.

    Thank you a ton for your contribution here. You've answered a question that many of us have had for quite some time.
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  10. #30
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    No problem...I'm glad to help.

    The Ford tuners tend to zero out the things they don't understand as well, such as lambda timing correction, cam timing spark correction, even charge temp correction. It's a huge time saver but it's dangerous and technically incorrect as well.

    I noticed last summer (75 deg dew point constant for months) that I could run a lot more timing at any given load. Then, as soon as fall arrived, I started getting knock. Someone pointed me in the direction of humidity and I was able to find tons of documentation and testing that supported humidity as having a significant effect on knock. However, Ford doesn't account for it, I'm assuming because of a relatively good working knock sensing system. But when your cylinder pressures are double what they were stock, I don't know that Ford can adequately protect it using just knock sensors. Like I said earlier, I now have a functioning external controller that measures humidity and applies a correction to the charge temperature to create a humidity timing offset.

  11. #31
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    Quote Originally Posted by TriPinTaZ View Post
    Most tuners seem to disable humidity spark. But I've started using it to accommodate for weather changes so that the car isn't dangerous in the winter when it needs less timing and the car doesn't slow way down in summer when it needs more timing due to increased humidity. 99% of the cars I tune are in FL or Texas so this logic applies well.

    Thank you a ton for your contribution here. You've answered a question that many of us have had for quite some time.
    So you're doing it how the OE does it with adding timing and taking it out. How much are you "extending" this out or in other words how much are you roughly increasing the stock settings? On trucks I can see the factory settings working just fine for the amount needed to pull when dry.
    2010 Vette Stock Bottom LS3 - LS2 APS Twin Turbo Kit, Trick Flow Heads and Custom Cam - 12psi - 714rwhp and 820rwtq / 100hp Nitrous Shot starting at 3000 rpms - 948rwhp and 1044rwtq still on 93
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  12. #32
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    Fwiw the ctsv seems to call 77 deg F (25 c) dew point the zero-correction baseline. The 5.3 truck calls 44-58 deg F (6.5-14 C) dewpoint the baseline.
    Last edited by engineermike; 05-02-2023 at 03:42 PM.

  13. #33
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    Quote Originally Posted by GHuggins View Post
    So you're doing it how the OE does it with adding timing and taking it out. How much are you "extending" this out or in other words how much are you roughly increasing the stock settings? On trucks I can see the factory settings working just fine for the amount needed to pull when dry.


    Actually I zero out any positive numbers in the multiplier table so that it doesn't add any timing. I also slightly adjust the negative multiplier depending on the setup. Since I always add timing when tuning a car, I set the car up so that it has a theoretical maximum before multipliers, including humidity. AT WOT during the most humid times, it might pull half a degree up top and zero in the peak torque area. Then when winter hits, it will pull anywhere from 1-1.5 degrees.

    Example: LT4 in summer months regularly hits 26-27 degrees peak on E60+. But in the in the winter when its cold and the humidity drops, you will see 25-26 degrees peak on E60+. In FL and Texas it could be either one of these scenarios in "winter time" lol.


    I usually pull the spikes out of the WOT area and maybe even the part throttle areas in the Humidity spark table. On some setups I just put 2 degrees in the whole table if I have a really different timing strategy. But in a nut shell, I tune the cars spark advance for maximum performance in humid air, and use the humidity spark to pull spark only, never add. The stock humidity spark table is pretty decent for stock heads/cam cars and I leave part throttle alone and make the WOT areas smooth. Some cars have random rows of 3 degrees at WOT. I lower those to match the cells next to them so I can have a stead amount of timing removed.
    Last edited by TriPinTaZ; 05-02-2023 at 03:59 PM.
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  14. #34
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    Quote Originally Posted by TriPinTaZ View Post
    Actually I zero out any positive numbers in the multiplier table so that it doesn't add any timing. I also slightly adjust the negative multiplier depending on the setup. Since I always add timing when tuning a car, I set the car up so that it has a theoretical maximum before multipliers, including humidity. AT WOT during the most humid times, it might pull half a degree up top and zero in the peak torque area. Then when winter hits, it will pull anywhere from 1-1.5 degrees.

    Example: LT4 in summer months regularly hits 26-27 degrees peak on E60+. But in the in the winter when its cold and the humidity drops, you will see 25-26 degrees peak on E60+. In FL and Texas it could be either one of these scenarios in "winter time" lol.


    I usually pull the spikes out of the WOT area and maybe even the part throttle areas in the Humidity spark table. On some setups I just put 2 degrees in the whole table if I have a really different timing strategy. But in a nut shell, I tune the cars spark advance for maximum performance in humid air, and use the humidity spark to pull spark only, never add. The stock humidity spark table is pretty decent for stock heads/cam cars and I leave part throttle alone and make the WOT areas smooth. Some cars have random rows of 3 degrees at WOT. I lower those to match the cells next to them so I can have a stead amount of timing removed.
    Guess that would be better. A lot of my guys log at night, to work and then home. The weekend mid sunny day logs are nearly always the ones that will show any knock. I was wondering about this for the guys towing as ECT and the IAT tables don't help - already been down that attempted road. You can have one run really good with an empty truck and then throw 9000lbs behind it mid day and she'll always show 4 degrees of knock being pulled - also why I do PE settings differently now - again - more OEish. I was even thinking about it in this instance to add a degree for the really dense or foggy mornings heading out.

    Do you change the percentage axis any? I started running a 6.2 trucks in everything truck related which seems to divide things up more. I just haven't played with it in the degree you have. In fact a fraction from the sound of it as I've only recently started leaving it mostly stock
    2010 Vette Stock Bottom LS3 - LS2 APS Twin Turbo Kit, Trick Flow Heads and Custom Cam - 12psi - 714rwhp and 820rwtq / 100hp Nitrous Shot starting at 3000 rpms - 948rwhp and 1044rwtq still on 93
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  15. #35
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    Quote Originally Posted by GHuggins View Post
    Guess that would be better. A lot of my guys log at night, to work and then home. The weekend mid sunny day logs are nearly always the ones that will show any knock. I was wondering about this for the guys towing as ECT and the IAT tables don't help - already been down that attempted road. You can have one run really good with an empty truck and then throw 9000lbs behind it mid day and she'll always show 4 degrees of knock being pulled - also why I do PE settings differently now - again - more OEish. I was even thinking about it in this instance to add a degree for the really dense or foggy mornings heading out.

    Do you change the percentage axis any? I started running a 6.2 trucks in everything truck related which seems to divide things up more. I just haven't played with it in the degree you have. In fact a fraction from the sound of it as I've only recently started leaving it mostly stock
    I haven't adjusted the axis at all but I may going forward now that I think we know what the column values are referencing. I pretty much knew weather played a roll in which direction the multiplier table went and I would desensitize the column value to my needs. I have it figured out where it will pull 0-.5 degrees when humid, pull 1-1.5 when its dry or cold out and I give it no ability to add timing. I definitely see the use case you are speaking of. I don't do a lot of trucks or deal with towing so I haven't had a use case for that yet. Most of what I tune are Camaros and Corvettes.
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  16. #36
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    Definitely following this thread... great info in here. Thanks all for sharing the knowledge on this one!

  17. #37
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    I found a quick chart to reference the "GM Humidity". This chart is listed in decimal percentages so just multiply the values by 100 to get the value that lines up with the GM Humidity multiplier table.

    Example, if it is 80F outside with 70% humidity then the "GM Humidity" aka Moisture Concentration by Weight = 1.54%. In the Humidity Multiplier table you can see the value aligns with -.30

    Screenshot 2023-05-10 222713.png

    Screenshot 2023-05-10 223340.png

    I think there is a way to build a formula in the scanner with user math that would spit out the Moisture Concentration by weight as it can be derived from Specific Humidity, which GM cars have available as a PID to monitor.
    Last edited by TriPinTaZ; 05-10-2023 at 09:44 PM.
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  18. #38
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    Very insightful read. I was thinking the following would hold true. Let me know what you think of these scenarios.

    (Leaving the Humidity Base and Multiplier tables alone)

    1. You tune the car for max Hp/Tq on the dyno on a "high humidity" day (multi table = 1) and
    (Base table "ADDS" up to 3 degrees of timing). The tuner does his regular tuning and make X amount of Hp/Tq.

    2. Without making any changes to the tune; car comes back on a day of lower humidity when the
    (Multi table = 0) and the Base table adds 0 timing. The car makes X amount of Hp/Tq on this day.

    If the tuner then tried to make more power on that same "low humidity" day; He shouldn't be able to if GM set the tables up correctly.

    So with that scenario I have a few questions...

    1. Do you think the tuner could add more power via spark to the "lower humidity" tune or is the tune still optimized?

    If the answer is no he can't add more, then it would make since to leave it alone while tuning and it doesn't matter the humidity on the day you're tuning as the tune will automatically compensate for those changes which is the whole point in having it there in the first place.

    If the answer is yes, then how? (If GM set the tables up correctly)

    Someone above said something like one day they were getting 28 degrees and then the next only like 26.5 as the weather had changed and it appeared to frustrate them. I would say to that you shouldn't be concerned with the "particular number" you're getting but the "best number" you can get on that particular day.

    Agree? Disagree?

    Your thoughts on this please.
    Last edited by Clutch; 05-14-2023 at 06:28 AM.
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  19. #39
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    I think you have it backwards. In dryer air (which happens when cold out) you need less timing and the engine will tolerate less timing. In humid air (usually when warm) you need more timing. You can search the internet for these studies that explain the details why. GM did set it up properly.

    It's up to the tuner how they want to use humidity spark. I've already shared my logic and its done well for me.
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  20. #40
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    Man these modern controls really mess up the dyno correction factors. But TriPonTaZ hit the nail on the head. You can get most of your humidity-lost power back in high humidity by running more timing and the knock limit is increased.