Maf vs map vs DP - Ford Mustang Forums : Corral.net Mustang Forum
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post #1 of 28 Old 11-05-2018, 11:59 PM Thread Starter
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Maf vs map vs DP

So ford and all the other have used maf , map, separately and together for engine management. maf tells the eec what coming and map tells it's there. Both work well together and separate. Why didn't they use a differential pressure transmitter across the throttle body as well? Just my 2cents

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post #2 of 28 Old 11-06-2018, 10:18 AM
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Well the question is, what problem is that solving? If SD or MAF can put you on a mixture target, what more is needed?


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post #3 of 28 Old 11-23-2018, 07:51 PM
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why would you need to know diff press across the TB?

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post #4 of 28 Old 12-23-2018, 03:06 PM Thread Starter
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The maf tells you kg/hr in front of the T.B. The BAP tells you pressure / vacuum behind the T.B. Correct.

So why does ford and GM use both?

I understand either will both will aid in fueling.

Why would D.P. help. If the T.B is closed the motor is in vacuum. Correct?Atmospheric conditions will differ. Use a BAP sensor with the known engines demands. What happens. You tell me.
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post #5 of 28 Old 12-23-2018, 04:49 PM
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MAF tells you both actually. The fluid moves as a column, it isn't getting past the MAF unless it's moving into the cylinders. It doesn't matter what is in between. As long as the system is sealed up tight, there's nothing you can do to make the MAF measurement not represent the cylinder fill which means there is no other measurement you can make to enrich the data further.

A MAP based strategy does need to know the differential but it doesn't need to be exactly at the T.B. Typically it will either use a MAP from the manifold plus a BAP from the engine compartment, or it will use the MAP at key on before the engine starts to take a one-time BAP reading and assume that reading doesn't change much thereafter. This is normally fine but can cause problems if you're going up or down a mountain.
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post #6 of 28 Old 12-23-2018, 05:07 PM
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Quote:
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The maf tells you kg/hr in front of the T.B. The BAP tells you pressure / vacuum behind the T.B. Correct.

So why does ford and GM use both?

I understand either will both will aid in fueling.

Why would D.P. help. If the T.B is closed the motor is in vacuum. Correct?Atmospheric conditions will differ. Use a BAP sensor with the known engines demands. What happens. You tell me.
first you need to know, absolute pressure is used, NOT gauge

manifold is pressure, not vacuum

second, fuel is delivered by mass, by the correlating measured or calculated air mass

why are both maf and map used in some applications? really that is something that oems developed from some need, be it mileage or emissions constraints

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post #7 of 28 Old 12-23-2018, 07:46 PM Thread Starter
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Maf measurers what? weight right

bap logs what. pressure right. so there is no vacuum with your statement. So there is still a differential in pressure. Right.

Please explain
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post #8 of 28 Old 12-23-2018, 10:10 PM
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MAF is measuring the mass. The volume is fixed. Pressure is just how much mass vs. that fixed volume. Using pressure and temperature is a way to derive the mass of air based on the known volume via Boyle's law.

Using a MAF you simply measure the mass directly. With MAF you don't even need the temperature because the measured mass is absolute. As the gas temperature rises reducing mass, the MAF will directly read that reduced mass.

The pressure delta across the throttle body, or across any other point along the intake is irrelevant. The only thing that matters is the air mass entering the cylinder, because that is the air mass you are mixing with your fuel mass.

The circumstance where you care about the pressure delta is trying to determine how much resistance there is to the air column moving at that point, but measuring what it is doesn't actually tell you anything about the air mass at that point unless you know some other data as well. If you get all that other data you're just reinventing speed density but measuring vacuum at a different location. There isn't anything special about the location.
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post #9 of 28 Old 12-24-2018, 09:18 AM
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Quote:
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Maf measurers what? weight right

bap logs what. pressure right. so there is no vacuum with your statement. So there is still a differential in pressure. Right.

Please explain
what do you want to be explained?

maf is a measure of MASS AIR FLOW

the ecm, delivers, MASS FUEL FLOW

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post #10 of 28 Old 12-24-2018, 09:44 AM Thread Starter
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Thanks Seijirou.

So GM uses SD and MAF on some models. Do they use a BAP. If so why. Which one controls what?.

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post #11 of 28 Old 12-24-2018, 10:57 AM
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Sorry I'm not familiar with the GM strategies so I can't really comment on that. Both are capable of doing everything needed but they might like the characteristics of one over the other for some things. Perhaps they find that acceleration enrichment works better with manifold vacuum in the solution. It could simply be another data point for more accurate self diagnostics.
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post #12 of 28 Old 12-26-2018, 06:30 PM
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...So GM uses SD and MAF on some models. Do they use a BAP. If so why. Which one controls what?.
Everything they do is to meet federal gov't emissions requirements.
Fuel economy & performance are secondary.

Some manufacturers have eliminated the MAF on some models, probably due to cost, it being more expensive than MAP or BAP sensors.
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post #13 of 28 Old 12-26-2018, 06:35 PM
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CAFE has fuel economy requirements imposed on OEMs
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post #14 of 28 Old 12-26-2018, 09:47 PM
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CAFE has fuel economy requirements imposed on OEMs


The answer to almost everything oem’s do. I don’t think most people have any idea how reaching federal standards are.

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post #15 of 28 Old 01-04-2019, 10:51 AM
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GM blends MAF with MAP to have a better predicted VE. Most GM tunes stock use MAF only at 4,000+ RPM since MAF measures actual airmass its more accurate and can compensate for any performance improvements or diminishes that would cause lean out or enrichment since speed density can only deliver x amount of fuel at 0 vac / 0 boost.

Simply put, in order to have accurate fuel control you have to calculate an accurate airmass. The rest is simple math i.e. x airmass * y(air fuel ratio) = x:y Air fuel ratio.

MAF gives more accurate fuel control since your measuring the actual airmass.

AM = Actual AM from MAF Sensor

Speed density (MAP) you are assuming at a given RPM and pressure that in order to achieve x:y ratio you MUST be inhaling x amount of air its simply guess work and assumes airmass doesn't change for a given condition (rpm and pressure) which is defined in the VE table.

therefore simply put,
AM = Looked up AM from VE table

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post #16 of 28 Old 01-04-2019, 12:21 PM
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I didn't say it directly earlier but I figured they used MAP because it responds more quickly to transients than the MAF meter hanging off the periphery. This would be more prominent at lower RPM (so it kind of jives that they don't use it at 4,000+ RPM like Decipha pointed out). I'm inferring then that there's an economy gain in that response time, which is to Indy's point. That's completely conjecture though and I don't know it to be true.

Also I believe at least some of the current Ford Ecoboosts are strictly MAP.
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post #17 of 28 Old 01-07-2019, 07:24 PM
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With Megasquirt & a MAF, VE table is only used for trim. It actually makes it a bit easier to understand, since VE table starts at 100 for all cells, i.e. 100%. You then increase or decrease cell depending on the volumetric efficiency for that cell condition (load vs. rpm).

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post #18 of 28 Old 01-08-2019, 02:41 AM
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So is SD preferred over MAF for turbo application?
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post #19 of 28 Old 01-08-2019, 01:31 PM
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As a rule? No. Consider this, "boost" is nothing more than naturally aspirated in a thicker atmosphere.

Check these out if you haven't seen them, good primers on the different strategies. Alpha-N isn't really used much but still helpful to start there as a baseline to understanding SD and MAF.

Theory: Alpha-N « Moates Support
Theory: Speed-Density « Moates Support
Theory: Mass Air Flow « Moates Support

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post #20 of 28 Old 01-08-2019, 06:32 PM
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So is SD preferred over MAF for turbo application?
When the amount of air required to make power is limited by the MAF meter, it is time to go SD

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post #21 of 28 Old 01-09-2019, 03:22 AM
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No sensor is perfect. Different types of sensors are better at taking measurements than other types of sensors are under some conditions.

One of the limitations of a Hitachi style MAF sensor is that it can't measure the DC air mass. It measures the AC air mass. If the intake system has any reversion in it, then the MAF measurement will be incorrect. The intake system is much more likely to have reversion at lower rpms or at high loads. Another limitation to a MAF is this. The larger the intake manifold plenum volume is compared to the engines displacement, the greater phase lead there is between the measured air mass and the actual air mass going into the cylinder head. The acoustics of the air flow system need to be modeled to compensate for this.

The EcoBoost engines don't use a MAF. They do use three pressures sensors in the air inlet system. Two of these are used to determine the differential pressure across the TB. The ECU has a map of differential pressure across the TB versus blade angle versus resulting airflow. This is part of how it knows what the airflow going into the engine is as well as determining what angle to position the throttle blade too, to achieve a certain torque output.
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post #22 of 28 Old 01-10-2019, 01:00 AM
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So is SD preferred over MAF for turbo application?
When the amount of air required to make power is limited by the MAF meter, it is time to go SD
That's a good point! What's is a typical limit of a MAF?
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post #23 of 28 Old 01-10-2019, 06:45 AM
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That's a good point! What's is a typical limit of a MAF?
I suppose it depends on the size. How many cfm of air does a 110mm MAF flow? I do not know. Another issue that crops up is placement of MAF sensor and the length of tubing required to accommodate it and the resulting packaging issues. The longer the plumbing gets the more restricted the flow will become unless the tubing diameter gets larger, which again will compromise packaging, making everything fit. The throttle body on my race car flows 2,250 cfm. Not going to get a MAF that can flow that much and fit within the confines of the engine compartment without sacrificing air delivery.

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Best 1/8 mile time 5.339 @ 133.46 mph with a 1.29 60'
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post #24 of 28 Old 01-10-2019, 07:33 AM
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That's a good point! What's is a typical limit of a MAF?
I suppose it depends on the size. How many cfm of air does a 110mm MAF flow? I do not know. Another issue that crops up is placement of MAF sensor and the length of tubing required to accommodate it and the resulting packaging issues. The longer the plumbing gets the more restricted the flow will become unless the tubing diameter gets larger, which again will compromise packaging, making everything fit. The throttle body on my race car flows 2,250 cfm. Not going to get a MAF that can flow that much and fit within the confines of the engine compartment without sacrificing air delivery.

Wow what size TB is that? A 90mm flows about 1300..
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post #25 of 28 Old 01-10-2019, 09:36 AM
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Wow what size TB is that? A 90mm flows about 1300..
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Mammoth 168MM | Kenne Bell

I have the one on the right in this link. 2,350 cfm

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post #26 of 28 Old 01-11-2019, 08:24 AM
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Wow what size TB is that? A 90mm flows about 1300..
Make sure you know the pressure drop so you can compare apples to apples too.

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post #27 of 28 Old 01-11-2019, 12:44 PM Thread Starter
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Thanks Jack, When I started this post all I was wanting to know is if and why anyone would use a dp reading. I also understand air mass determined by the maf. I just wanted to know why a BAP and a dp cell wouldn't work togather for the same result. Looks like the eco boost does this .
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post #28 of 28 Old 01-11-2019, 12:49 PM
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Clearly there's a use for it but I'm not sure what it is. If DP across the TB + TB angle tells you air mass, just like a MAF alone tells you air mass, the MAP isn't needed. We already know a MAP alone can work in SD.

The only thing I can figure is the addition of throttle by wire is creating a question that MAP alone can't answer so the DP is needed but the other answer is there's just something I'm not aware of to consider.

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