Aftermarket Air Flow Sensors and Descreening

[JennaBear]

I frequently get questions regarding aftermarket MAF sensors/descreening and what gains they will net. My answer to that question 9 times out of 10 is: NONE as the OEM unit can handle ~550HP.

The aftermarket MAFs are not calibrated the same as the stock MAF. So, for a given volume of air, the aftermarket MAF will underreport that volume, this causes the PCM to increase ignition timing slightly, which is the resulting increase in power.

When tuning for an aftermarket MAF, the PCM will be recalibrated so that the MAF reports correctly (or close to correctly), thus leaving no real benefit to the aftermarket MAF. Descreening has the same effect as an aftermarket, and has no benefit other than underreporting the incoming air.

 

[oak510_tahoe]

Good Info!

 

[wmgood64]

thanks!

 

[jmildoc]

Really? Wow. I figured that.....

 

[JennaBear]

I thought I pretty much explained how it gives more power in my first post. They "mimic" a tune to increse timing. The Granatelli's often cause vehicles to run dangerously lean
I know of someone that recently lost his transmission because the Granatelli underreported airflow, causing torque calculation to be lower and line pressures to be lower.

On average with a descreened MAF, we have to scale the MAF calibration by almost 10%.

The aftermarket/descreened MAF tricks the computer into making more power by underreporting the the airflow which causes the PCM to reference a higher ignition timing value which makes more power. At the same time, underreported airflow yields a lower torque calculation; line pressures are defined by torque output (lower torque=lower line pressure). We have seen Granatellis underreport by as much as 60%.

The reason that you will SOMETIMES see a power increase with an aftermarket or descreened (they are/do the same exact thing) is due to the relative decrease in reported airflow. Ignition timing on all Gen-III and newer trucks is defined by rpm and cylinder air mass. Cylinder air mass is derived from a combination of MAF frequency table lookup and MAP/IAT/ECT and engine RPM, with the MAF reported value comprising the bulk of the airflow calculation. When MAF flow calculation decreases, cylinder air mass decreases and the resulting ignition timing lookup yields more advance. In addition, incredibly high fuel trims are extremely common due to the underdelivered fuel from the underreported airflow, as well as extremely lean operation at WOT.

A MAF sensor by itself is a useless piece of equipment. It does not report a signal that is usable in any way unless it is paired with an accurate calibration table. The factory PCM is calibrated to work with an OEM, screened MAF. If you change that piece of equipment without changing the MAF calibration table, you're fooling the PCM. Once the calibration is brought in line with that needed for the descreened MAF, the reported flow is right in line with the OEM sensor.

Commonly seen scenarios:

Average 5.3L peak mass air flow is in the 31-33lb/min range
Average 5.3L peak mass air with a descreened MAF is in the 26-27lb/min range

2000 Trans Am with an aftermarket MAF reported a peak of 29lb/min (at 6400 rpm). Actual air flow is 43-44lb/min. That's an underreporting of over 30%. Car ran EXTREMELY lean at WOT and had considerably too much ignition timing. Customer was fortunate that no engine damage had occurred.

 

[blueflamed03]

nice post Jenna!

 

[blueflamed03]

Dang nice post Justin, some good info in that post!

 

[Man Van]

A MAF screen will also help the smoothing of the flow of air going accross the sensor. Which in the grand scheme of things, depending on the vehicle, is needed. After market MAFs at times are needed depending on modifications. For example if running a supercharger with smaller pulley and supporting mods it's possible to outflow a MAF(depending how large your factory one is and what your scans are showing)
An example of this are Pontiac GTP's, the factory MAF will not flow past 11,000HZ. A LS1 or comparable MAF is needed to properly calculate the fuel needed. Thus running in a lean condition/creating knock

Digging the site so far.
Jenna and Volant you guys have websites? Just picked up my Hoe a week ago, and itching to see what's out there.

 

[Justin]

The problematic MAF with the LS1 was not a JET. First set of runs was with the aftermarket MAF. After multiple diagnostic tests, the MAF was determined as the cause of the poor reading (17:1 AFR is dangerous at WOT). It was then swapped with a factory MAF, significantly more power was made and the car was completely safe to drive (again).

I think you're still missing the point though. A MAF's SOLE job is to meter airflow. Provided that the measurement is accurate and the MAF is not a restriction (as is the case with the stock MAF in 99.99% of applications), how will an aftermarket MAF help? The answer: it won't. Since no aftermarket MAFs are accurate with the stock calibration, all of them inherently fit into your description of having a bad reading. If actual airflow is 32lb/min (equivalent of 9000hz MAF for example) and the aftermarket MAF is reporting a frequency that matches up with a calculated value of 29lb/min (say, 8400hz), is that not a bad reading?

What is the claimed benefit of running a Granatelli if they are now selling screened units?


Bottom line, an OEM MAF should always be used with an OEM MAF calibration in the PCM.

 

[AquinoSteven]

So does this mean that a de-screened MAF could be potentialy dangerous? I just did mine and i barley saw anything but im sure theres a way to test it.