Air Mass Flow Reading on Turbo Cars

[LazyDog]

From the little that I know.
The Mass Air Flow reading on a none turbo at operating temperature: The reading in grams/ second should be close to engine size like a 2.9L reading 2.86g/seconds.

What about turbo cars, any good rules to follow??
My 2.3L turbo reads 4.3 g/second.

 

[tessaseveride]

The MAF g/sec ratio is only applicable while idling at operating temperature. Smaller engines (from my experience, below 2.5L, particularly Ford for whatever reason) are sometimes a 2:1, not a 1:1 ratio. You should not be making boost at idle, therefor your MAF reading should be pretty close to 1:1 or 2:1 depending on which engine you have and how the ECUs are programmed to read the MAF in your particular vehicle.

 

[LazyDog]

The MAF g/sec ratio is only applicable while idling at operating temperature. Smaller engines (from my experience, below 2.5L, particularly Ford for whatever reason) are sometimes a 2:1, not a 1:1 ratio. You should not be making boost at idle, therefor your MAF reading should be pretty close to 1:1 or 2:1 depending on which engine you have and how the ECUs are programmed to read the MAF in your particular vehicle.

Thanks for the info provided.

 

[twX]

The 2.0 L turbo GDI on my Subaru has a MAF reading of around 3.2 g/s when idling at 800 rpm with a warm engine with the A/C off. An engine with a higher idle speed will draw more air. An engine that is less efficient due to a lower compression ratio will require more airflow to idle, and this is usually the case with turbo engines.

 

[gathermewool]

The 2.0 L turbo GDI on my Subaru has a MAF reading of around 3.2 g/s when idling at 800 rpm with a warm engine with the A/C off. An engine with a higher idle speed will draw more air. An engine that is less efficient due to a lower compression ratio will require more airflow to idle, and this is usually the case with turbo engines.

Good point -it’s not just about the size of the engine, but CR and GDI vs MPI. VVT scheme should also be considered, since it can effectively change dynamic CR.

 

[tessaseveride]

Everyone is correct. The correlation between size of engine and MAF reading at idle is only for a quick diagnostic check to make sure the MAF is functioning and somewhat in range. There is not golden rule exact MAF ratio across all engines between engine displacement and g/sec reading, however if you're reading for example 12 g/sec on a 4L at idle, there's obviously something wrong.

 

[DriveHard]

ok, wait a second...

Why are folks looking for a ratio between a MASS flow rate of GRAMS/sec as compared to a VOLUME of an engine displacement? Does it just so happen (coincidence) that those values line up at idle?

For a throttle controlled engine you are coming NOT EVEN CLOSE to filling the cylinders completely at idle speed...that is what the throttle is for, to restrict flow into the engine.

Mass air flow is a function of throttle position, engine speed, and volumetric efficiency of the intake system (and valves), as well as the temperature, pressure, and humidity of the air the engine is being operated in. It is complex, and the reason the MAF sensor was developed...to better understand how much air was actually getting into the cylinders.

 

[tessaseveride]

ok, wait a second...

Why are folks looking for a ratio between a MASS flow rate of GRAMS/sec as compared to a VOLUME of an engine displacement? Does it just so happen (coincidence) that those values line up at idle?

For a throttle controlled engine you are coming NOT EVEN CLOSE to filling the cylinders completely at idle speed...that is what the throttle is for, to restrict flow into the engine.

Mass air flow is a function of throttle position, engine speed, and volumetric efficiency of the intake system (and valves), as well as the temperature, pressure, and humidity of the air the engine is being operated in. It is complex, and the reason the MAF sensor was developed...to better understand how much air was actually getting into the cylinders.

There is a correlation, and it is not coincidental.

 

[DriveHard]

There is a correlation, and it is not coincidental.

any chance you care to show the math (explain)?

 

[tessaseveride]

any chance you care to show the math (explain)?

I asked the same question when I was taking a class at Lexus. It's been a few years, but from what I remember, the instructor said it has to do with stoichiometrics. He said 14.7:1 or slightly rich AFR is the ideal and engine designers and engineers calibrate the ECU to the MAF sensor, intake volume, throttle body, etc. There is no forced induction, so all air is being pulled naturally by the motor and combustion process. Larger engines will naturally try to hit the ideal AFR by taking more air, so the MAF readout in g/s will track linearly with displacement.

I think we are on the same page, just different definitions of "coincidental".

It's not exact, because like you said, MAF also calculates humidity, temperature, etc. of air, but it's a generally accepted concept where I work that something is wrong if there is more than a 10% deviation from the 1:1 or 2:1 ratio depending on the engine. I don't know the technical reason why, but MAF readings on smaller engines tend to have the 2:1 ratio while larger engines above 2.5L tend to have the 1:1 ratio. Some people also say WOT MAF readings should be more than 40 times your displacement, however I've never needed to confirm that theory.

 

[Tman220]

My N/A vehicles line up pretty well with the displacement in liters with G/sec at warm idle no accessories.

The camaro when stock
an Olds Intrigue with the 3.8
2021 silverado 5.3
2005 sierra 5.3
07 Camry 2.4

with a notable exception....
2012 Regal GS 2.0 Turbo
which does indeed read more in the 3.5-4.5 range at hot idle no accessories

I suspect it has a lot to do with emissions. This engine has 270 Hp and needs an exhaust and Cat large enough to handle that flow but at idle with small displacement you will have trouble keeping a primary possibly and most certainly a secondary cat at the proper temp unless engine controls are manipulated to help with that.

I know when I fire up the regal and monitor my torque app the ignition advance is extremely retarded, cam timing to match and the whole point of that is to get the cats lit off as quickly as possible. It's not far fetched to me that a small engine with relatively large exhaust would need timing retarded, cam timing adjusted to intentionally idle less efficiently just to keep the cats lit.

As a comparison the 07 camry with the 2.4 needs driving in order to light up the secondary catalyst. The close coupled cat at the exhaust manifold lights off quick but the secondary which is basically underneath the front seat center arm rest needs to be driven to initially reach and keep 600+ degrees

 

[tessaseveride]

The camaro when stock
an Olds Intrigue with the 3.8
2021 silverado 5.3
2005 sierra 5.3
07 Camry 2.4

with a notable exception....
2012 Regal GS 2.0 Turbo
which does indeed read more in the 3.5-4.5 range at hot idle no accessories

That's pretty much in line with my experience. Nice to have confirmation. Cool theory on why the Regal reads 2:1.

 

[LazyDog]

Thanks to all for providing some inputs to my question.