How the MAF really works
08-10-2005, 06:54 PM
#21
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Originally Posted by RedHardSupra
5. VE table is absolutely fundamental. It must be perfect, and not just <4krpm, but the full thing, even if you're going back to MAF.
The PITA with the MAF is that you have to deal with reversion (mostly with cammed cars), and airflow bias (when you replace the lid or other intake-rekated parts). Plus I think there are quirks in the way the MAF works at low airflow that the engineers factor into the VE table.
08-10-2005, 09:08 PM
#22
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Originally Posted by Another_User
The real PITA is that I can't figure out what temperature (and other factors) the VE table is tuned at.
However, simply put, the PCM massages the values in the VE table based on IAT, ECT (weighted factor), and barometric pressure. Because the PCM does this, it doesn't matter what constant's GM used, you will be adjusting VE values referenced to GM's constants.
08-10-2005, 09:13 PM
#23
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Originally Posted by TAQuickness
Still thinking about the rest of you post... But, number 2 really grabbed me.
I got a really good log of stagnant traffic. Litteraly took me 45 minutes to travel 2 miles. During this time my IAT reached a peak of 133*F (average during this time was 125*F). I would have expected my WB-AFR to go rich, however, it went lean.
more on this later - it's beer thirty
I got a really good log of stagnant traffic. Litteraly took me 45 minutes to travel 2 miles. During this time my IAT reached a peak of 133*F (average during this time was 125*F). I would have expected my WB-AFR to go rich, however, it went lean.
more on this later - it's beer thirty
The timing the PCM pulls is part of the reason for you seeing the car go lean. I seen this happen also and I think its because of an incomplete burn when the exhaust valve opens. The WB is reading the O2 content in the raw fuel which is misleading (looks like things are leaning out). Its really not lean at all just an incomplete burn being read as such but it isnt.
Hope this makes sense because at first it didnt to me either and i was expecting the same results as you to my surprise it went the other way
I also tested this theory with no fueling tables changed i pulled -2* of timing and everyhting went 4% lean. Flashed back +2 and everthing feel into place
08-11-2005, 03:34 PM
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Originally Posted by TAQuickness
I just finished reading RHS doc at www.allmod.net/hpt. I missed any reference to how a bone stock MAF with a bone stock MAF table would incorrectly report air flow by the addition of a cam, heads, LT's, etc...
I run a stock MAF table with mostly VE, PE, and spark modifications and some other minor modifications for driveablitly. IMO, I see no need to modify the MAF table for a stock MAF. LTFT's are your friend.
08-11-2005, 03:46 PM
#26
Originally Posted by technical
Unless I missed something myself, didn't he say he had a Granatelli MAF? That would definitely require modifications to the MAF table.
I run a stock MAF table with mostly VE, PE, and spark modifications and some other minor modifications for driveablitly. IMO, I see no need to modify the MAF table for a stock MAF. LTFT's are your friend.
I run a stock MAF table with mostly VE, PE, and spark modifications and some other minor modifications for driveablitly. IMO, I see no need to modify the MAF table for a stock MAF. LTFT's are your friend.
08-11-2005, 05:29 PM
#27
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Originally Posted by technical
Unless I missed something myself, didn't he say he had a Granatelli MAF? That would definitely require modifications to the MAF table.
Originally Posted by TAQuickness
how a bone stock MAF with a bone stock MAF table would incorrectly report air flow by the addition of a cam, heads, LT's, etc...
08-11-2005, 08:57 PM
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Originally Posted by RedHardSupra
great thread, first let me share some newer discoveries about the MAF behaviors, that I haven't included (yet) on my MAF tuning page.
2. After doing #1, idle trims got better, but what I've been noticing lately that the trims go significanly richer (as much as 10 points!) as the car warms up, and I don't mean just ECT's going >180F, I mean like 15 mins of driving warm. Any clue why so much and why just the idle trims are temp sensitive?
4. we need data filtering in HPT badly! Lately i've been gathering larger samples than usual and trimming them, all kinds of sudden transitions, or deceleration, and unusually large/small values get kicked out. and after i'm down trimming all this junk out, i end up with a much cleaner dataset, even though there's less data. look at it on raw y(x) graphs and you'll see how patterns become simpler and data isn't spread anywhere near as much.
This is a broader thing, but it applies very visibly to MAF/DynAir data.
5. VE table is absolutely fundamental. It must be perfect, and not just <4krpm, but the full thing, even if you're going back to MAF. Why? Because Dynamic Air is a function of IAT, MAP, RPMs, displacement and VE. The first three are kinda just there and you measure them, displacement is hopefully a constant, but VE is what we alter and now we must make the PCM aware of the changes we've made. so the list of dependencies goes:
VE->dynamic air->MAF->effective AFR
2. After doing #1, idle trims got better, but what I've been noticing lately that the trims go significanly richer (as much as 10 points!) as the car warms up, and I don't mean just ECT's going >180F, I mean like 15 mins of driving warm. Any clue why so much and why just the idle trims are temp sensitive?
4. we need data filtering in HPT badly! Lately i've been gathering larger samples than usual and trimming them, all kinds of sudden transitions, or deceleration, and unusually large/small values get kicked out. and after i'm down trimming all this junk out, i end up with a much cleaner dataset, even though there's less data. look at it on raw y(x) graphs and you'll see how patterns become simpler and data isn't spread anywhere near as much.
This is a broader thing, but it applies very visibly to MAF/DynAir data.
5. VE table is absolutely fundamental. It must be perfect, and not just <4krpm, but the full thing, even if you're going back to MAF. Why? Because Dynamic Air is a function of IAT, MAP, RPMs, displacement and VE. The first three are kinda just there and you measure them, displacement is hopefully a constant, but VE is what we alter and now we must make the PCM aware of the changes we've made. so the list of dependencies goes:
VE->dynamic air->MAF->effective AFR
3.
this is why I hand smooth everything and try and get a mental picture of where the graph wants to go and what the engine is asking for I just throw away the bullshit values that seem to crop up spuradically. Unfortunately this takes a huge amount of time and study to get it the way it should be.
4.
Ive been having problems with pe not commanding what it should and it has lead me to the realization that in a perfect world the ve table in SD would command 14.7 exactly and so when I enter pe with my perfect ve table I could just do math to get my wot afr. 14.7/1.22137=13.1
So I think pe values are a good indication of how much the ve table is off at wot. If youve got values like 1.32 at 5600 rpms and are seeing 13.1 you know ve is not right.
and this leads me to the question of trims and exactly what the number means in relation to its accuracy and stoick afr. Is there a certain range of inaccuracy that is excepted ( I think there is) and what is that range?
08-12-2005, 12:25 AM
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Originally Posted by TAQuickness
Changing the MAF, porting, descreening, replacing with other than like-in-kind definately requires re-calibrating the MAF. However, you did miss something, the original question.
I'm of the belief that the MAF was calibrated from the factory to produce a frequency per flow of air. After engine or even lid/intake mods the MAF should still accurately report the amount of air entering the intake. This is of course dependant on the accuracy of the initial factory calibration.
08-12-2005, 12:37 AM
#30
you know, i was really reluctant to recalibrate the maf myself (afterall we don't recalibrate thermometers when we put it in different environment, do we?), but two things convinced me:
1. when the car runs perfect in SD, and i reenable stock config for a MAF, the whole trim table goes crazy, not only power but throttle response as well as shifts all go bad. why not map that MAF to the settings that we know that work (VE/SD/DynAir)? I tried it and it worked. this way the engine measures the same amount of air it expects to get.
2. when the car runs like a champ with MAF, but in SD it barely moves, and you get VE done, and then recalibrate the MAF and you end up very close to the config you've had already. doesn't that mean MAF was right, and now you also have VE right?
1. when the car runs perfect in SD, and i reenable stock config for a MAF, the whole trim table goes crazy, not only power but throttle response as well as shifts all go bad. why not map that MAF to the settings that we know that work (VE/SD/DynAir)? I tried it and it worked. this way the engine measures the same amount of air it expects to get.
2. when the car runs like a champ with MAF, but in SD it barely moves, and you get VE done, and then recalibrate the MAF and you end up very close to the config you've had already. doesn't that mean MAF was right, and now you also have VE right?
08-12-2005, 12:44 AM
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That's probably because VE needs to be calibrated at a specific temp and elevation to work automatically with the MAF. It's not like there's a VE vs. IAT table...that's where the MAF comes in.
08-12-2005, 10:23 AM
#32
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I concur with RedHardSupra,
Been through the whole "reconnect MAF, trims go way positive" thing. At this point, I've bumping the MAF table the same way I'd bump the VE table and it is working wonderfully. Just like sanding a piece of wood to get the high spots out.
John.
Been through the whole "reconnect MAF, trims go way positive" thing. At this point, I've bumping the MAF table the same way I'd bump the VE table and it is working wonderfully. Just like sanding a piece of wood to get the high spots out.
John.
08-12-2005, 10:40 AM
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Here's a question for the guys that do modify the MAF calibration: When you adjust the MAF and get your trims zero'd, do you have to go back later on and modify the MAF ###'s again when the weather/ambient temps change significantly?
e.g. You tune on a nominal day 70*, setup the MAF table, trims ~0. What happens when the temps go up to >90*? negative trims? or is there no adjustments needed?
e.g. You tune on a nominal day 70*, setup the MAF table, trims ~0. What happens when the temps go up to >90*? negative trims? or is there no adjustments needed?
08-12-2005, 11:18 AM
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Originally Posted by technical
Here's a question for the guys that do modify the MAF calibration: When you adjust the MAF and get your trims zero'd, do you have to go back later on and modify the MAF ###'s again when the weather/ambient temps change significantly?
e.g. You tune on a nominal day 70*, setup the MAF table, trims ~0. What happens when the temps go up to >90*? negative trims? or is there no adjustments needed?
e.g. You tune on a nominal day 70*, setup the MAF table, trims ~0. What happens when the temps go up to >90*? negative trims? or is there no adjustments needed?
08-12-2005, 12:02 PM
#36
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Originally Posted by technical
Here's a question for the guys that do modify the MAF calibration: When you adjust the MAF and get your trims zero'd, do you have to go back later on and modify the MAF ###'s again when the weather/ambient temps change significantly?
e.g. You tune on a nominal day 70*, setup the MAF table, trims ~0. What happens when the temps go up to >90*? negative trims? or is there no adjustments needed?
e.g. You tune on a nominal day 70*, setup the MAF table, trims ~0. What happens when the temps go up to >90*? negative trims? or is there no adjustments needed?
08-12-2005, 12:19 PM
#37
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Tuning the MAF has been very well covered by RedHardSupra and TXHorns.
By design, the MAF compensates for climate changes. In laymans terms The nuts and bolts
By design, the MAF compensates for climate changes. In laymans terms The nuts and bolts
08-12-2005, 12:27 PM
#38
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TX & RHS - Both of you, as well as many others, have stated that once the VE table was dialed in, you reconnected the MAF and then LTFT's went all over the place. Was this true for all of your FTC's?
In other words, did your FTC's during a steady state run condition (i.e. 3000 RPM constant) look reasonable?
In other words, did your FTC's during a steady state run condition (i.e. 3000 RPM constant) look reasonable?
08-12-2005, 12:35 PM
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Nope, I tuned mine back when the temps were in the 50s and 60s. I did an hour log the other day when it was 95 degrees 80% humdity and they still look good(0 to -2).
So tuning the MAF is analogous to forcing the PCM to see actual airflow equivalent to the *desired* airflow that made the most power while tuning in SD. Thus allowing the MAF to adjust for variations in climate from that baseline. This assumes that the VE calibration is more accurate than the factory MAF calibration itself. As opposed to correcting VE to the correct efficiency that matches the factory MAF calibrations to minimize trim values which assumes the MAF calibration to be the most accurate measure of aerodynamics within the intake.
If the MAF calibration is indeed accurate from the factory, it would seem to me that if trims went awry after reconnecting the MAF post SD VE tune then perhaps the VE calibrations aren't truly dialed in for every portion of the map.
08-12-2005, 12:59 PM
#40
hotter temps will go negative, as does my SD tune. and thats the same error i saw. before hand i saw swings equal to or more than the sd tune with maf on, before i went sd. error in the maf when set up on the cars how it is, is too much of a difference to see any benefit from it. when it comes down to it in my opinion the diff in sd and maf tune would be removing the maf and it would not restrict the airflow. just thinking about airflow and condition changes, i feel the maf as more areas where it will give screwy results. maybe some calibration is being missed but from what i have read, there are certain areas with maf that just dont get right. although they may be worked out by now, i havent followed up much, but want to becuase others may want to keep their maf after i tune their car