Computer Diagnostics and Tuning Technical discussion on diagnostics and programming of the F-body computers

MAF sensor functioning

Old 04-04-2007, 06:16 AM
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MAF sensor functioning

I am starting to get further into part throttle tuning on my car and I think at this point I am ready to start looking at the MAF tables. A question came to mind as to the function of the MAF though, I know it uses a hotwire to relate the density of air to temperature changes, but I am curious about its output to the PCM.

Say you have a given engine on a 100 degree day, and at peak hp rpm it is pulling 340 gm/s. Is it possible to correlate that to a 50 degree day (same baro and humidity)?

I would imagine that the AFGS would increase due to the engine injesting more air in the cooler, more dense, charge.
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Old 04-04-2007, 10:56 AM
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This is the advantage of a MAF versus speed density system. MAF sensors have the ability to directly measure air flow, regardless of temperature or density, into the engine whereas speed density uses a calculation with the inputs of the absolute intake manifold pressure, inlet air temperature and volumetric efficiency of the engine to *estimate* how much air is flowing into the engine. For stock, or mostly stock vehicles, MAF systems are, in most cases going to produce more precise and consistent fueling under a variety of operating conditions then speed density systems will.

So, yes, for a given operating range of the vehicle, the AFGS will be higher when the air is colder. When tuning MAF tables I always worry about how the car is going to run when its warmer out (or at higher elevation), because that portion of the MAF tables are inaccessible through driving alone.

I believe the best course of action is to scale the MAF table in a linear fashion through the areas of the table that are inaccessible given the weather conditions of the day that you are tuning it and revisit it again, if you can, when the air is warmer, and less dense.
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Old 04-04-2007, 06:03 PM
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Originally Posted by VinceTrifecta View Post
So, yes, for a given operating range of the vehicle, the AFGS will be higher when the air is colder. When tuning MAF tables I always worry about how the car is going to run when its warmer out (or at higher elevation), because that portion of the MAF tables are inaccessible through driving alone.

I believe the best course of action is to scale the MAF table in a linear fashion through the areas of the table that are inaccessible given the weather conditions of the day that you are tuning it and revisit it again, if you can, when the air is warmer, and less dense.
What "portion of the MAF tables" are you referring to? I was under the impression that the sensor works independently of factors which affect air density (temperature, barometric pressure), so you don't need to calibrate it for different temperatures.
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Old 04-04-2007, 07:43 PM
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Originally Posted by evilundisguised View Post
What "portion of the MAF tables" are you referring to? I was under the impression that the sensor works independently of factors which affect air density (temperature, barometric pressure), so you don't need to calibrate it for different temperatures.
The MAF tables need to be calibrated for the sensor, some portions of the MAF tables (very low frequencies or very high frequencies) are hard to hit in normal driving due to the fact that the stock MAF maxes out at 471.51 grams per second and has a minimum of 2.45 grams per second. Right now my car sucks in about 7 grams per second, thats ~2512Hz, how do I get it all the way down to 1488Hz that 2.45 grams requires? Hes talking about using a wideband and a few other calculations to insure the MAF tables are properly calibrated to the MAF sensor that is on the car, so that when its at 1488Hz it actually reads 2.40 and not 2.45 grams because the stock calibration was wrong (Just an example not necessarily accurate).
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Old 04-04-2007, 08:55 PM
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Yes, I was referring to the MAF frequency calibration tables.
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Old 04-04-2007, 09:07 PM
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Originally Posted by LiENUS View Post
The MAF tables need to be calibrated for the sensor, some portions of the MAF tables (very low frequencies or very high frequencies) are hard to hit in normal driving due to the fact that the stock MAF maxes out at 471.51 grams per second and has a minimum of 2.45 grams per second. Right now my car sucks in about 7 grams per second, thats ~2512Hz, how do I get it all the way down to 1488Hz that 2.45 grams requires? Hes talking about using a wideband and a few other calculations to insure the MAF tables are properly calibrated to the MAF sensor that is on the car, so that when its at 1488Hz it actually reads 2.40 and not 2.45 grams because the stock calibration was wrong (Just an example not necessarily accurate).
You shouldn't reach the limits of the sensor during normal (stock-ish) operation, that would be a bad system design. I'm not quite following you; tiny miscalibrations like you're mentioning are corrected via closed-loop feedback from the O2 sensors.
I don't think you're thinking what Vince was thinking anyway, since he seemed to be geared toward temperature variation induced miscalibrations.
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Old 04-05-2007, 12:24 AM
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Originally Posted by evilundisguised View Post
You shouldn't reach the limits of the sensor during normal (stock-ish) operation, that would be a bad system design. I'm not quite following you; tiny miscalibrations like you're mentioning are corrected via closed-loop feedback from the O2 sensors.
I don't think you're thinking what Vince was thinking anyway, since he seemed to be geared toward temperature variation induced miscalibrations.
You misunderstood my post. When you switch MAFs or do something such as descreening/porting the MAF there are very large variations, Much larger than you want the O2 sensors to even attempt correcting (plus theres open loop startup mode). You recalibrate the MAF tables to handle those changes.
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Old 04-05-2007, 02:13 PM
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Originally Posted by LiENUS View Post
You misunderstood my post. When you switch MAFs or do something such as descreening/porting the MAF there are very large variations, Much larger than you want the O2 sensors to even attempt correcting (plus theres open loop startup mode). You recalibrate the MAF tables to handle those changes.
I understand that. I just don't understand how it related to the questions/ideas presented by TQdrivenws6 and Vince Trifecta.

TQdrivenws6 you're asking if cooler air temperature can be related to an increase in measured AFGS? Well, it's observed by seeing a certain engine produce more power with cooler intake temps. I've never seen a direct correlation between temperature and AFGS, that won't be so easy to calculate as flow characteristics are affected by many things. Maybe it could be roughly approximated by the percentage increase in air mass between two temperatures, given constant volume and pressure, calculated by the ideal gas law:

PV=nRT

or rearranged and removing constants,

n (is proportional to ) 1/T, where n = number of atoms and T = temperature in Kelvin

So with your proposed temperature conditions (100F = 311K, 50F = 283K)
Change in n between the 2 conditions is 9.9%
That'd be roughly 25-30hp on a stockish-bolton LT1. Sounds a little optimistic to me... maybe it's not the best approximation after all
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Old 04-05-2007, 10:43 PM
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The sensor measures the mass air flow, so it picks up the change in density due to the change in air temperature. If you are pulling 340 gps on a 100*F day (560* absolute), AND NOTHING BUT THE TEMPERATURE CHANGES, you will pull 373 gps on a 50*F (510* absolute) day:

(560 / 510) X 340 = 1.098 X 340 = 373.3 gps.
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Old 04-05-2007, 10:45 PM
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Thanks. That's what I was attempting to say in my post above.
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Old 04-05-2007, 11:31 PM
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Originally Posted by Injuneer View Post
The sensor measures the mass air flow, so it picks up the change in density due to the change in air temperature. If you are pulling 340 gps on a 100*F day (560* absolute), AND NOTHING BUT THE TEMPERATURE CHANGES, you will pull 373 gps on a 50*F (510* absolute) day:

(560 / 510) X 340 = 1.098 X 340 = 373.3 gps.
Mass is proportional to # of atoms, right?
What scale is your absolute temperature on?
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