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Discussion Starter #1
Anyone understand the (simplified) theory of Mass Air Meter measurement?

As I understand it there are 3 items that effect air density: Temperature, Baro Pressure, and Humidity.

Now does the method of operation of a hot wire MAF sensor account for all three of these?

The reason I ask is I have observed that on dry days, not hot, just dry, I see some lights on the Safeguard. At virtually the same temperature but on a moist day I will get no lights.

My understanding is that all equal, dry air is supposed to be denser, increasing load. But if that were the case shouldn't it be more effective at cooling the hot wire and simply be read as more mass and therefore more load?

For that matter, wouldn't that also be the case for Barro?

If so, then consequently less timing should be run by the ECU.

So can someone get me straightened out on the theory here?
 

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damn rod u need a girl lol

J/K bro i wish u lived closer it would be cool to hangwith someone thats like tuning too
 

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Discussion Starter #3


That's the thing, been married for almost 18 years... got time to think about things like this
 

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My understanding is that all equal, dry air is supposed to be denser, increasing load. But if that were the case shouldn't it be more effective at cooling the hot wire and simply be read as more mass and therefore more load?
From a little physics that I know - water is a good energy conductor and thus the more water (humidity) you have in the air, the faster it will cool off the wire - so the exact opposite of what you're saying there.

For that matter, wouldn't that also be the case for Barro?
With pressure, you are correct, the higher the pressure (and thus density) the more energy the air can absorb - it can cool the wire faster - thus higher reading.


If so, then consequently less timing should be run by the ECU.

So can someone get me straightened out on the theory here?
To sum it up real quick:
Higher temperature -> lower maf count reading (this one is pretty intuitive)
Higher pressure -> higher maf count reading
Higher humidity -> higher maf count reading

Note: I got all of this from my engineering physics college classes which was quite a few years ago, so please correct me if I'm wrong.
 

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You know, I've thought a lot about this myself, and my guess was always that the stock MAF meter probably doesn't account for humidity very well, if at all. Like Petik said, it seems that humid air would cool the hot wire better than an equal mass of dry air, so humid air would appear to the PCM as having more mass than it actually does.

Humidity hurts power, but it also acts an anti-detonant. So, in theory, you should be able to run more timing on a humid day vs. a dry one to regain some of the lost power. But I don't see how to use the MAF meter to make that adjustment automatically. My conclusion was always the same - you want to tune your car on a cold, dry day, so that you've got it set up properly for the highest potential airflow. Then if you get a humid day, you might be down on power a bit, but the tune will be safe. A hot day might still be trouble, though, since higher intake temps can lead to detonation, but thankfully the PCM measures intake temps directly.

BTW, this is yet another reason why I ordered a J&S Safeguard, since it's the only practical means of optimizing my timing for slight changes in climate. But my main reason for buying the J&S is for safety, rather than to eke out every last bit of power. Honestly, I doubt it would make a huge difference even if we could tweak the tune based on daily humidity changes.

Rod - I say tune for the typical humidity levels where you live, and keep an eye on the Safeguard whenever the weather is "unusual". Of course, it sounds like that's what you're already doing.
 

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Discussion Starter #7
Here were I live there is high humidity almost all the time, probably 10 months a year.... of course most of the time is down right rain or mist. In any case I normally tune during wet weather as that is what we get here.

I had my timing maxed out, but with no lights on the Safeguard, then boom, very dry day, the second of two in a row... but temp was the same, around 70F. I got lights up to 6 degrees retard. Today is moist out again and back to normal.

So it seems ideally there should be a humidity sensor talking to the ECU.

Anyhow it looks like the lesson here is that tuning should be done on a dry day. I know that at 6 degrees Safeguard retard I would have had crazy deto had it not been in place... that is why I just can't see running blind without one.

It is almost never dry here unless it is mid 60s or higher. I don't want to pull timing for the dry and leave power and response on the table during our predominantly wet weather. So I think I will compromise and compensate by increasing retard rate with ACT at all points from about 70F on up, maybe 1.5 degrees. Then maybe 1 more off the BKT.
 

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I have no input here, but I am watching hoping some cool info comes out of this! I honestly have no idea why that would happen Rod. My thoughts on it are just like yours.
 

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What has more "timing power" to the ecu?... o2 sensor or Maf?
 

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Discussion Starter #10
Base timing is determined by load which is computed by airflow via MAF readings. O2 adjusts closed loop fueling.
 
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