SoarerCentral: MAF vs MAP

SoarerCentral General Soarer Chat MAF vs MAP

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Rehan Bandara
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Friday, February 29, 2008 - 01:10 pm, by: Rehan Bandara (Parsec)

I've always wondered what the advantages of a MAF over a MAP system are. As far as I can tell, MAF costs more and is less versatile (e.g. all of the air that flows past the MAF must enter the engine or it will run rich).

Why do so many cars use a MAF sensor?

I'm asking because I'm helping a friend fix his hyundai. It seems the last person he bought it off has removed the MAF entirely, and it's metering the fuel solely off the TPS and EGO sensor.

Cihan Aday
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Friday, February 29, 2008 - 10:29 pm, by: Cihan Aday (Cihan)

MAF = much more accurate than MAP.

Little need for acceleration enrichment as the incoming stream is metered before it makes its way into the cylinders.

Better for fuel economy and can be more responsive as a result.

Rehan Bandara
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Monday, March 03, 2008 - 04:30 pm, by: Rehan Bandara (Parsec)

ah, yeah, that makes sense. I wouldn't have thought it would take long for a map system to respond though, since a manifold pressure reading can be taken very quickly, and since the injector period can be easily changed for the next stroke

surely the ecu could calculate the appropriate injector pulse period given the manifold pressure during each intake stroke?

Cihan Aday
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Monday, March 03, 2008 - 07:42 pm, by: Cihan Aday (Cihan)

Resolution or frequency isn't the problem, accuracy is.

When the throttle body snaps open the additional molar content of oxygen that comes with increased induction flow for that split second isn't metered as a pressure increase and you get a lean condition. It takes time to fill the intake manifold so the MAP sensor can take an accurate reading. To fill the intake manifold at low RPM's takes longer so the engine demands more enrichment based on changes in throttle.

Another negative of MAP sensor implementations is that manifold pressure doesn't always represent exact fueling requirements - you can still achieve the same manifold pressure at a lower throttle opening in which case you get the over-fueling and increase emissions i mentioned in my first post.

There is a reason why Toyota moved to air flow meters in their turbo engines

Rehan Bandara
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Tuesday, March 04, 2008 - 06:01 pm, by: Rehan Bandara (Parsec)

Why doesn't the increased induction flow get metered as a pressure increase when the throttle snaps open? I would assume that if you're measuring pressure at the intake manifold it would have to be sensed.

I can understand why there would be inaccuracy due to the manifold pressure not being representative of the cylinder pressure (unless the pressure reading is taken very close to the intake valve, or even in the cylinder- they've got temperature sensors built into spark plugs, why not pressure transducers?).

I would assume that if you achieve the same manifold pressure at a lower throttle position, then if you're transducing the pressure in the cylinder or very close to the cylinder, you ought to be adding the same amount of fuel anyway.

I know it's a dynamic system though, and when the flow rate through the manifold increases, so does the pressure drop across it, which in turn makes the pressure disparity between the intake manifold and cylinder greater.. but i would have thought they could factor that out with some sort of empirical lookup table, much like they do for MAF sensors. Since you need the engine RPM anyway to determine the mass of air flowing into the engine, you could cross correlate it with the manifold pressure to make an assumption about the pressure drop between the MAP sensor and cylinder, and compensate the MAP reading accordingly.

I get your point though, I'm just nitpicking

Daniel Clarke
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Tuesday, March 04, 2008 - 08:51 pm, by: Daniel Clarke (Dieseltrain)

I dunno fellas But how about sharing some of that

Please Rehan!!!

Cihan Aday
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Wednesday, March 05, 2008 - 03:28 pm, by: Cihan Aday (Cihan)

Rehan Bandara wrote on Tuesday, March 04, 2008 - 06:01 pm:
Why doesn't the increased induction flow get metered as a pressure increase when the throttle snaps open? I would assume that if you're measuring pressure at the intake manifold it would have to be sensed.

Already answered this. The problem is specifically - that the MAP sensor doesn't necessarily represent oxygen intake vs its input reading and rpm with complete accuracy. As you pointed out, depending on its location and the volume (hence design) of the intake manifold accuracy/response time will change. There is also resonance/pressure waves creating inaccuracy's in its readings under operation, so any more compensation is .. well, fighting a lost cause.

Rehan Bandara wrote on Tuesday, March 04, 2008 - 06:01 pm:
I get your point though, I'm just nitpicking :-)

Yes, you are

Rehan Bandara wrote on Tuesday, March 04, 2008 - 06:01 pm:
Since you need the engine RPM anyway to determine the mass of air flowing into the engine, you could cross correlate it with the manifold pressure to make an assumption about the pressure drop between the MAP sensor and cylinder, and compensate the MAP reading accordingly.

IMO you're probably better off implementing a TPS vs MAP lookup and removing fuel like that.

Still not as good as a MAF setup with respect to subsequent emissions and compatibility with drive by wire implementations.