I realize that atmospheric changes are reflected in the map reading...for the intake side. If the pcm sees 80 kpa, it doesn't care if it's at w.o.t. in colorado, or 50% throttle at sea level, it will use the same ve cell in either case. However in colorado there will only be 80 kpa for the exhaust to push against but at sea level there will be 100 kpa. Don't you think the change in backpressure would cause a change in ve that the pcm can't account for?

 

P Mack - Intriguing observation.

Chris - can you shed any light on how the PCM is using the GM.BARO parameter? It seems evident the PCM is in fact not using it for direct fueling/density calculations. However, the PCM does have a couple of tables available for fine tuning the BARO. I'm curious now if it might be using it to address P Macks querry.

 

imo, the effect on fueling from baro changes in the exhaust are negligible since the relative pressures are much higher.

 

In SD mode maybe, but the PCM will still see MAF and O2 readings that won't jive with the VE and can then trim from there.

I am curious since we're talking about it, what LTFT numbers are guys at higher elevations seeing on stock tunes/cars? Does the manufacturer take this into account?

 

When does the PCM sample BARO pressure (is it only at key on engine off...?)...?

 

Joe - I think it varies from OS to OS - but take a look at b0301, b0302, b0303, b0304, & b0305.

 

the initial sample is done at key on, then during throttle transients the PCM will check to see that it doesn't see a higher value that what it already has. If it does it will increase Baro (that's why it is important to disable this mechanism for boosted cars).

 

Chris - why does the PCM do this?

 

Yeah, I should have said it can't account for it in sd except with fuel trims.

 

By the way, when driving from Pikes Peak to Colorado Springs to San Antonio I didn't see a real change in fuel trims. But I wasn't in speed density at the time either.

 

i'm gonna be doing a trek from chicago to california in < 2 weeks, should we do some experiements? should i just scan trims or more? sd or maf?

 

how bout SD one way and MAF the other?

 

a lot of things in relation to engine operation have Baro modifiers to make the effect of something greater or less depending on air density (eg. cat protection)

Also, a lot of things are calibrated in relation to Vacuum so an accurate Baro reading is important.

 

Here's where my head starts spinning...

Seems like direct fueling calculations are based on MAP, IAT/ECT, & MAF (if applicable). However, I can't seem to shed the idea that BARO has an indirect influence on fueling.

 

Fuel calcs also use IFR table which is looked up by (BARO - MAP) i.e. vacuum.

Edit: oh, I'm with you, the PCM does use BARO or (BARO - MAP) other than for IFR, but I'm not sure how or what at the moment.

 

So I read a bunch of this, and one question still remains. Are aftermarket MAFs recommended?

 

Depends on who you ask.

I say no.

The goal of this thread was to help us understand the "real-life" dynamics that effect the MAF and why it needs to be calibrated to the engines airflow.

 

Unstickying thread... this thread is still linked in the main help sticky

 

cool. thanks for the sticky time

 

This is one of the most informative, adult and professional threads in the history of the internet. Im new to the tuning world and have learned alot, thanks guys.

FYI - some things to consider. There was some discussion with regards to humidity. One thing to keep in mind, given the same exact volume of air at both 0 and 100% humidity, its important to realize one thing. The water in the air is not magically stuck inbetween the N2 and O2 molecules, it *displaces* them.

Think of it as a box. You got a toy box, filled with ***** of different sizes and shapes. About 78% of the box is full of red ***** Nitrogen) which for the purposes of this discussion is inert. About 21% are yummy green ones (oxgyen). The remaining 1% is argon, co2 and other things that we dont care about. Lets assume our little toy box is full and has no water in it, just N2 and O2. Given the exact same volume of air, if we dump a bunch of water in it (up to 100 humidty), whats going to happen? The N2 (who cares) and the O2 (OMG!) will be displaced to make room for the water molecules. So, ignoring all other factors, given a certain volume of air, 100% humidty will have LESS O2 in it. This sounds bad. The good news is however, the max H2O that can be held in the air(in human livable tempatures) is less than 4%. This means we have at maximum 1% less O2 to burn. 1%? where did I get that number? 4% max water displaces only 21% oxygen remember! The rest is displaced N2 which we dont care about. In the big scheme of things, this is nothing. This is roughly equivilant to changing your altitude (atmospheric PSI) by about 250 feet. Whoopdee doo!

Im not saying humidity has no role...im just saying that its a minor one, and for the purposes of tuning our vehicles, we should be FAR more concerned with tempature/altitude and other factors.

One caveat - I dont know anything about MAF's yet, just what I learned in this thread. Different humidities might have a larger effect on the cooling effect of the wires. Remember, a water molecule is much better at conducting heat than air, air has more insulative properties. A hot object would theorehtically cool faster in humid air. You call BS! Of course you do. You went from dry and arid pheonix to hot and muggy houston, and sure as hell feels hotter! it FEELS hotter to human beings because sweat. We sweat to induce evaporation, which cools our skin. Sweat won't evaporate in an environment already saturated with humidty, will it? So we feel hotter. Comparing human cooling (active water evaporation) to a peice of hot metal cooling is NOT the same.