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maffrequencylimits

Basic MAF data

Here is some basic information for the MAF sensors used in the DSM world.

Mitsubishi MAF senors

Mitsubishi MAF sensors operate by measuring pressure, temperature and volume. The ECU takes that data and multiplies it out to calculate air mass. Air mass is then used to calculate fuel mass required to achieve a 14.7:1 A/F ratio. That fuel mass is then used (along with the ECU's idea of which injectors are installed on the car) to calculate injector pulsewidth. And then THAT is finally used to schedule the injector firing events.

But it all starts with airflow measurement. Like all devices, Mitsubishi MAF sensors were designed to operate in some pre-defined range of operating conditions. One of those, of course, is the volume of air flowing through the MAF sensor. When the true volume exceeds the measuring capacity of a Mitsubishi MAF sensor, you end up with an erratic volumetric output. The erratic output feeds into the fuel calculation described above to produce an erratic A/F mixture. And that produces erratic engine operation, typically while you're running wide open throttle!

Mitsubishi MAF sensors report volumetric data using a frequency. You can log this in ECMLink as MAFRaw. You can monitor this frequency to determine how closely you're running to the known limits of that MAF.

1G DSM MAS

A stock, unhacked 1G DSM MAS will not accurately report frequencies above 2000hz. That frequency represents roughly 180 lps (380 cfm) of volumetric airflow. At standard temperature (around 77F) and sealevel pressure, that's about 210 gm/sec (28 lb/min) of mass airflow.

2G DSM MAS

A stock, unhacked 2G DSM MAS (same as the 3000GT VR4 MAS, same as an Lancer EVO1-3 MAS) was designed to operate a bit higher. You'd expect overflow on a 2G DSM MAS around 2700hz. That frequency represents roughly 320 lps (about 680 cfm) of volumetric airflow. At standard temperature (around 77F) and sealevel pressure, that's about 375 gm/sec (50 lb/min) of mass airflow.

EVO8 MAS

A stock, unhacked EVO8 MAS (effectively the same as the 3G Eclipse MAS) was designed to operate even higher! You'd expect overflow on EVO8 MAS around 3000hz (we think…I'm not sure I've actually seen an EVO8 MAS overrun). That frequency represents roughly 420 lps (about 890 cfm) of volumetric airflow. At standard temperature (around 77F) and sealevel pressure, that's about 500 gm/sec (66 lb/min) of mass airflow.

Summary

MAS Freq limit Volumetric limit Mass limit
1G DSM 2000hz180 lps (380 cfm) 210 gm/sec (28 lb/min)
2G DSM 2700hz320 lps (680 cfm) 375 gm/sec (50 lb/min)
EVO8 3000hz420 lps (890 cfm) 500 gm/sec (66 lb/min)

When running one of these MAF sensors anywhere near these limits, use ECMLink's MAFClamp function. Read the help files and forums for more information. By using this function properly, you can effectively remove the upper airflow limitation of these MAF sensors. Even so, it's highly recommended that the 1G DSM MAF not be used simply because its limit is too low to produce reliable MAFClamp operation when running substantially more airflow than the sensor was designed to meter.

GM MAF sensors

The two most common GM MAF sensors used on our DSMs are the 3“ and 3.5”, part numbers 25180303 and 25179711, respectively. These sensors operate fundamentally different than the Mitsubishi MAF sensors. The GM MAF sensors work by measuring mass airflow directly. They do not need temperature and baro data because they're not measuring volume. They are reporting air mass directly as a frequency.

The biggest problem, IMO, with these MAF sensors is that this frequency to mass mapping is non-linear. So as airflow increases, it takes less and less change in frequency to present a larger and larger change in air mass. This makes the system very sensitive to fluctuations in frequency output from the sensor.

Neither a MAF Translator nor the ECMLink V3 GM MAF cable allow you to see the raw GM MAF sensor frequency directly. The MAF Translator provides a translation to stock DSM-like frequencies, while the ECMLink V3 GM MAF cable provides a simpler translation that produces raw frequencies that are quite different (but that the ECMLink V3 ECU code knows how to handle). So it's best to avoid talking about limiting-frequencies when discussing a GM MAF sensor when used on a DSM and rather just talk about air-mass metering capability.

Also, the GM MAF sensors, like any MAF sensor, are sensitive to anything that causes the airflow through the sensor to be unevenly distributed across the entire area of the sensor opening. It's very common to see smaller diameter piping leading to or from a GM MAF sensor cause the sensor to produce a higher airflow reading than that actual mass of air flowing through the sensor. Because the metering capability of the sensor is dictated by the maximum airflow reading it can report, having it report a value that is artificially high reduces the metering capability of the system.

That said, you can expect a 3“ GM MAF to easily measure (report) 600 gm/sec (80 lb/min) well before any “limit” is reached. The 3.5” would probably measure over 700 gm/sec (92 lb/min)! The problem is that we have very little data to back any of this up. But the bigger problem, again, is that well before you reach these limits, the frequency to airflow mapping will simply become too sensitive to be usable. The result is a very choppy airflow signal to the ECU and many of the same problems mentioned above with MAF overrun on a Mitsubishi MAF. It's just for a different reason.

So with that in mind, we typically recommend that ECMLink users enable the MAF Clamp function with running a GM MAF. Enable it around, say, a MAFRaw reading of 2700-3000hz (that might vary a bit depending on whether you are using a MAF Translator or an ECMLink V3 GM MAF cable) so that the MAF signal is effectively ignored above that frequency anyway and it won't matter what's going on with the signal. This also effectively removes any metering limit that might have existed.

maffrequencylimits.txt · Last modified: 2009/07/20 08:00 by twdorris