P Mack

iTrader: (6)

Why does everyone assume speed density and/or the maf can't respond to changes in weather conditions? They can't compensate for humidity but they can for pressure and temperature.

Technical, if the trims are at zero in speed density, but they change when you plug in the maf, are you saying you think the error is with the ve table?

technical

Has anyone been able to determine the latency of the MAF?
Could there be a bigger problem with determining instantaneous rate of change with respect to airflow especially after a head and cam swap or lid change?

Maybe everyone is scaling their MAF to accomodate higher slopes in airflow curves as apposed to correcting a particular point on the chart. That could be the unknown that causes the need for a MAF recalibration when instinct dictates MAF output per airflow should not be altered. I'm thinking out loud, but a comparison in MAF recal's could be helpful to analyze how they were changed in conjunction with VE.

technical

In an attempt to approach this problem from a logical perspective, yes... If and only if the factory MAF calibration is indeed accurate per rate of airflow. If the factory MAF calibrations are not accurate, then no the VE could still be correct but the MAF is fooling the PCM.

TAQuickness

iTrader: (28)

not the case. the PCM samples barometric pressure. humidity has a significant effect on barometric pressure. humidity also changes the airs ability to transfer heat.

In both cases, SD or MAF, humitidy is accounted for.

WS6FirebirdTA00

iTrader: (33)

has anyone determined if the amount of air flowing through the pcv system throws off the maf calibration? i would think the ariflow though the pcv lines would accound for an amount of air that would not be considered negligable, esp at low throttle settings

joecar

The PCV source and destination points are after the MAF (and so PCV flow is already accounted for unless the block "leaks" air).

TAQuickness

iTrader: (28)

agreed. however, altering the breathing characteristics of the motor may alter the flow charicteristics of the PCV system, creating yet another variable in the "real world" that makes it necessary to recalibrate the MAF.

P Mack

iTrader: (6)

The fact that humidity has an effect on pressure and heat transfer is exactly why the pcm can't account for it. The map can't tell how much partial pressure is due to water vapor, so as far as it knows it is just denser air. The maf can't tell how much of the heat transfer is due to water vapor, so it just sees it as more massflow. Ultimately the pcm wants to know how much oxygen is going in the engine, not water vapor. Make sense?

P Mack

iTrader: (6)

The air entering and leaving the pcv will actually be different if the pressure in the crankcase is changing. Also all the massflow from blowby will create a difference in air entering and leaving the pcv. So in some cases it's not accounted for, but it's not much either.

TAQuickness

iTrader: (28)

No, your post does not make sense to me.

When humidty increases, and altitude and temperature remain constant, air density decreases.

Other points to ponder:

The molecular weight of dry air is greater than water.

Dry air will transfer more heat than humid air.

Humidity has the least effect on air density as compared to changes in altitude and temperature.

The PCM doesn't measure oxygen content until after the combustion process in the exahust pipes.

P Mack

iTrader: (6)

I didn't realize dry air transfers more heat than humid air. But anyway I think you're missing my point. Which has more mols of oxygen, 1 cubic foot of air at 100 kpa and 50 degrees with 0% humidity, or 1 cubic foot of air at 100 kpa and 50 degrees with 100% humidity?

WS6FirebirdTA00

iTrader: (33)

yeah they are AFTER the MAF so any air that comes in through there will not pass through the maf sensor, therefore it is unmetered air

joecar

Fresh air is sourced from [downstream of MAF] upstream of throttle blade, thru crankcase, thru PCV valve, and dumped downstream of throttle blade; this air was part of the air that flowed thru MAF and is already measured, except that:
a. P Mack pointed out that PCV gets extra material by drawing blowby gases from the crankcase. Blowby gases mildly dilute the intake air with combustion by-products (and also oil vapour), and the MAF does not account for this.
b. If crankcase has air leak (e.g. if oil cap o-ring leaks air) then there is definitely unmeasured airflow.

WS6FirebirdTA00

iTrader: (33)

ok this air was accounted for at one point in time, but im talking about instantaneously accounted for, and it is not. i feel there is more to it than that and i dont feel the scale is linear in any way to throttle position, it may be but i have always thought this to be an area for error.

WS6FirebirdTA00

iTrader: (33)

the one in the tb is where the crank case gets air in and manifold is where it gets it out right? my main reason for concern would be low throttle sections. like i said though i could be be 100% wrong

joecar

WS6FirebirdTA00,
Good point, the PCV airflow is time-lagged compared to main airflow thru TB, and MAF definitely does not account for this;
I wonder what fraction of the total airflow this would typically be...?
if it's a significant fraction, should we toss [into the trash can] the MAF (run SD) or the PCV (run breather oil cap)...?

technical

I don't believe the MAF is responsible for knowing exactly how much air *just* entered the engine so much as it just reports the rate of flow. Air coming from the PCV in non-idle conditions can't force it's way into the engine. Therefore it's entering the engine at the same rate as freshly metered air. This way the PCM knows just how much load the engine is under (using MAP, TPS, and O2's as well).

WS6FirebirdTA00

iTrader: (33)

you would think inlet and outlet rate woudl be the same, in staedy state, but in quick transitions from no throttle to some and all that, you would think there would be some error, just in that range. which i have read about guys that have had problems tuning the maf for those areas. just a though, i would assume steady motor speed it is all properly accounted for.

TAQuickness

iTrader: (28)

The cube with 0% humidity will have more moles of O2, because the air is 0.4% more dense than the cube with 100% humidity. You could achieve the same result by leaving the humidity as a 0% constant and raising the temperature by 2.35*F

This site has a nice air density calculator. I used 0' altitude, 50*F temp, 29.9212 inHg Baro, and 0 or 100 for relative humidity. The formulas are also available on this site.

So, back to the original question/statment of the PCM accounting for humidity. Whether you are running SD or MAF, the PCM does account for humidty because of the air's ability to transfer heat.

MAF:
The MAF itself if not measing air flow or density. The MAF is measuring how much voltage it takes to maintain a constant temperature across the heating elements. Because dry air will remove more heat from the heating elements than humid air, the MAF will show a greater voltage delta. The PCM then reads the output from the MAF and looks up the appropriate value in the MAF table.

SD:
The IAT sensor outputs a temp to the PCM. As dry air passes across the IAT sensor, it will remove more heat causing the IAT sensor to output a cooler temp to the PCM.

RedHardSupra

iTrader: (1)

water has a MUCH greater cooling capacity than air, that's why we things like radiators also, water injection also been known to be a great help with fighting these high IAT's. so with that said, wouldn't humid air be better at cooling off the Hot Wire element than dry air?

so if humid air cools the heating element better, is it proportional to the way it cools the mixture in the engine?

MAF table is frequency vs flow. if the humid air cools it better, would we see same flow at lower frequency, or more flow at the same frequencies? or does it scale together in such a way that we'd never notice from this relationship, unless we'd start mixing it in with rpm or map?