MAF Accuracy....when intake changes...

Question: Since the MAF, by definition measures airflow, when testing an intake /cai etc. Higher Hz should mean better airflow correct? Esp since g/s, lb/min even cyl airmass is all calculated by Hz.

Certainly there will be some variation on different days/temp/humidity, but say you test out several intake configurations on the same day?

Would it make sense that the best flowing one will show higher Hz than one capable of less flow? Meaning same car/engine, same day, higher Hz = more power from more air correct?

MAP is also a good simple measure of intake restriction. Higher MAP at WOT means you have less restriction. It is also an indicator if the MAF itself is becoming the restriction.

If you are talking about cold air kits as "intakes" where you are changing parts upstream of the MAF sensor, then be careful. Many of these kits change the way air approaches the MAF sensing element and can result in a different output Hz at the same actual airflow (g/s). This can be misleading if you're trying to use this to quantify "which intake is better."

Any time the relationship between physical airflow and output frquency changes, the PCM MUST be recalibrated to account for this. If this is done properly for each intake, then you can simply compare MAF g/s between the systems to see which allowed the engine to flow best. As Chris indicated, the highest flowing unit will probably also have the least pressure drop and result in the highest MAP reading.

Not only what Chris and Greg said...

Should the intake diameter at and near the MAF sensor change, any reliance on MAF Hz readings are not only irrelevant, but useless until scaled to reflect the change.

IOW, if the tubing at the MAF is larger (typical), the Hz value will be lower, due to lower velocity across the MAF. Opposite is true for a smaller pipe.

MAP under high load, high RPMs is the best indicator of overall intake restriction and resulting flow.

A good manifold may drive MAP down rather than up. It all
depends on the tug-of-war between the runners (draw down)
and the TB opening / plenum (fill up).

Airflow bias and reversion both can skew a reading. Straight
long tube and remote from the engine are better than close
in and bent hoses. A well run test would keep from any
significant plumbing changes.

Thanks guys!

Ok so if I understand correctly...

So I am testing air intakes, that is different "plumbing" between unfiltered air and Throttle body. So yes, the airflow to the MAF is changed.

That means:
MAF frequency may change, ie. read lower even if I am moving more air correct?
MAP is a better indicator of better flow (MAP of 97 kPA means better flow than MAP of 95 kPA at same RPM (say 5500).

Quote:

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but useless until scaled to reflect the change.

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Wouldn't STFT/LTFT relfect this? Meaning if the air intake is flowing better the trims would become more positive because more air is entering than measured, thus creating a lean reading and causing the PCM to add more fuel?

So say I installed a "new" intake, would it be reasonable to say if my trims starting going positive it would be a sign of more air flow at the same MAF Hz?

Quote:

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Originally Posted by johnh

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Wouldn't STFT/LTFT relfect this? Meaning if the air intake is flowing better the trims would become more positive because more air is entering than measured, thus creating a lean reading and causing the PCM to add more fuel?

So say I installed a "new" intake, would it be reasonable to say if my trims starting going positive it would be a sign of more air flow at the same MAF Hz?

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No, as said already in this thread, their change is a useless unit for measure since you jacked up the measuring device in the process...

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the measuring device in the process...

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But the measuring device for the Fuel Trims is the O2 Sensor...the MAF readings become "off" and the O2 sensor readings drive a "correction"

Only at low RPMs where you are closed loop. In
the areas where performance is interesting the
MAF (stock OS) is the primary airflow instrument
and any correction applied, is only an assumed-
relevant (at best) residue from other operating
points.

It's not necessarily that the airflow changed, to
see more Hz; the distribution of airflow across
the MAF face is a key embedded assumption in
the calibration and if it's skewed, so is the scale
factor between "sensible air" and total air flow.

to put it another way, the MAF is +/- 10% accurate, the MAP sensor is *A LOT* more accurate. The problem with MAP is that when you have two or more intakes that give good readings (very possible given most decent intakes with a decent throttle body should give minimal pressure drop). In that case the answer is found on the engine dyno or flow bench.

Quote:

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Originally Posted by BennyHHR

Not only what Chris and Greg said...

Should the intake diameter at and near the MAF sensor change, any reliance on MAF Hz readings are not only irrelevant, but useless until scaled to reflect the change.

IOW, if the tubing at the MAF is larger (typical), the Hz value will be lower, due to lower velocity across the MAF. Opposite is true for a smaller pipe.

MAP under high load, high RPMs is the best indicator of overall intake restriction and resulting flow.

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Hi, I'm dealing (or to be honest "fighting") with MAF scaling issues. I installed a Procharger P1SC1 in my Bellair with a LS3/525 crate engine, E67 ECU. I replaced the 3,5" intake air kit (K&N) with the 3" tubing provided by ATI. I relocated the stock MAF Sensor in the new tubing (taking in account recommended distances, position, etc.) without connecting the Procharger, so the only change that I made is to reduce the intake air tube.
I applied 0,85 to MAF vs Output Fqcy Table. As soon as the engine starts it revs up to 2.000 RPM and stay there forever. I roll backed and install the K&N intake kit again and the problem dissapeared. Could you give me some clue to understand why the idle speed increase?

Quote:

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Originally Posted by BennyHHR

Not only what Chris and Greg said...

Should the intake diameter at and near the MAF sensor change, any reliance on MAF Hz readings are not only irrelevant, but useless until scaled to reflect the change.

IOW, if the tubing at the MAF is larger (typical), the Hz value will be lower, due to lower velocity across the MAF. Opposite is true for a smaller pipe.

MAP under high load, high RPMs is the best indicator of overall intake restriction and resulting flow.

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Hi, I'm dealing (or to be honest "fighting") with MAF scaling issues. I installed a Procharger P1SC1 in my Bellair with a LS3/525 crate engine, E67 ECU. I replaced the 3,5" intake air kit (K&N) with the 3" tubing provided by ATI. I relocated the stock MAF Sensor in the new tubing (taking in account recommended distances, position, etc.) without connecting the Procharger, so the only change that I made is to reduce the intake air tube.
I applied 0,85 to MAF vs Output Fqcy Table. As soon as the engine starts it revs up to 2.000 RPM and stay there forever. I roll backed and install the K&N intake kit again and the problem dissapeared. Could you give me some clue to understand why the idle speed increase?