Kenneth
25-09-2007, 06:44 AM
As many know, I have long advocated using the standard airbox setup. I harp on and on about how pod filters don't work etc.
The following technical explanation gives some technical basis for these beliefs :). It may be a bit technical so if there is some point you wish clarified or explained further, say so.
Some of the material is easier to understand with a diagram, in which case please refer to the picture below. (Unknown origin, found on the net)
The Mitsubishi air flow meter is what is know as a Karman Vortex Air Flow Meter. (KVAFM)
The principle of its operation is fairly straight forward and goes like this
When something is placed in the path of a fluid, vortices are created in that fluid which vary in frequency proportionally with the speed of the fluid.
Mumbo Jumbo? The simplest demonstration of this principle is the wake of a boat. As the boat moves through the water a bow wave is created in the water. This bow wave moves faster the faster you go. (More accurately, the faster you go the more bow wave ripples you create per second)
Air is the same, you just cant see the evidence unless there is some form of colour added to the air stream.
These "bow waves" in the air cause fluctuations in pressure, just as a car has a low-pressure zone behind it when moving, so do the vortices.
In the KVAFM, there is a obstacle placed in the path of the moving air. This obstacle is of a certain shape so that the pressure fluctuations are directed at an area that carries that pressure to a mirror surface. (See pressure directing hole in the diagram)
The fluctuations cause the mirror to oscillate at a frequency proportional to that of the vortex frequency. That mirror reflects or cuts light to a photo transistor which in turn switches on and off a voltage as the light hits it.
The result is a voltage frequency output which is (all being well) proportional to the frequency of the vortices in the air.
As the frequency of the vortices increases with the speed of the air, this can be used to calculate how much air is moving through the sensor.
There are 3 parameters that can be used in the calculation of air flow through a pipe, Diameter of the pipe, the speed of the air, the volume of the air.
with any 2 of those parameters you can calculate the other.
The KVAFM provides the speed of the air, the ECU knows the diameter of the tube the air moves through and as a result it is able to calculate the volume of air moving through the KVAFM. (Air Density is then taken care of with the Intake Air Temp sensor (IAT) )
So far so good, to summarise:
The KVAFM uses the disruptions in the air flow to measure how fast air flows past the sensor, this is used to calculate the volume of air flow, the IAT is used to calculate the air density. Providing this information is accurate, the ECU can calculate EXACTLY how long to open the injectors per RPM to attain an Air Fuel Ratio (AFR). The desired AFR is found using a Load/RPM lookup.
To make things quicker, there is a table in the ECU that tells it the volume of air in relation to the frequency of the KVAFM output. The reason for this is because the KVAFM size is fixed and therefore for a given air speed the volume will be the same. Instead of calculating every time (costing valuable processor time) it can look up a table.
As you can (hopefully) see, the whole AFR calculation relies on the frequency generated by the KVAFM being correct. This is originally calibrated and tested using the standard intake system. This intake system consisting of the airbox, air filter and KVAFM is a known environment which is used to give accurate data to the ECU.
If you make a change to that system, by adding a pod filter, a bend in some pipe work before the MAF (Haven't you wondered why the MAF is bolted to the side of a box? It isn't the best shape for air movement) then you run the risk of upsetting the way the air hits the obstruction in the KVAFM.
Fortunately one assumes Mitsubishi made the airbox setup to provide a stable air environment for the KVAFM, thus when you do make these modifications the most likely result is that you INCREASE The rate of pressure fluctuation past the sensor.
So, you add some turbulence to the air going through the sensor what happens? Since the ECU relies so heavily on the sensor for accurate AFR calculations, any increase in turbulence is likely (unless you are unlucky in which case your engine blows up) to cause your AFRs to be richer than they should be.
In practice the results have shown this to be the case also, those who add pod filters put bends in their pipework before the KVAFM (usually trying to achieve a cold air intake) have fueling issues.
So, unless you run MAP (manifold absolute pressure) based engine management, you had best leave the airbox alone, buy an ARC box or fabricate one that behaves in the same way as the standard airbox.
Addition:
K&N do sell a pod filter which is especially designed to work with the Mitsubishi KVAFM. I didn't add this information originally because I don't like to endorse manufacturers claims without any proof of them working. However Mitsiman (RPW?) has endorsed this product and on his word I post a picture of it. Perhaps Mitsiman would would to post a product code or some such to help out those looking for one.
I have uploaded a picture of the unit, as you can see it is distinctive in shape and you should know at a glance if you have the real deal!
The following technical explanation gives some technical basis for these beliefs :). It may be a bit technical so if there is some point you wish clarified or explained further, say so.
Some of the material is easier to understand with a diagram, in which case please refer to the picture below. (Unknown origin, found on the net)
The Mitsubishi air flow meter is what is know as a Karman Vortex Air Flow Meter. (KVAFM)
The principle of its operation is fairly straight forward and goes like this
When something is placed in the path of a fluid, vortices are created in that fluid which vary in frequency proportionally with the speed of the fluid.
Mumbo Jumbo? The simplest demonstration of this principle is the wake of a boat. As the boat moves through the water a bow wave is created in the water. This bow wave moves faster the faster you go. (More accurately, the faster you go the more bow wave ripples you create per second)
Air is the same, you just cant see the evidence unless there is some form of colour added to the air stream.
These "bow waves" in the air cause fluctuations in pressure, just as a car has a low-pressure zone behind it when moving, so do the vortices.
In the KVAFM, there is a obstacle placed in the path of the moving air. This obstacle is of a certain shape so that the pressure fluctuations are directed at an area that carries that pressure to a mirror surface. (See pressure directing hole in the diagram)
The fluctuations cause the mirror to oscillate at a frequency proportional to that of the vortex frequency. That mirror reflects or cuts light to a photo transistor which in turn switches on and off a voltage as the light hits it.
The result is a voltage frequency output which is (all being well) proportional to the frequency of the vortices in the air.
As the frequency of the vortices increases with the speed of the air, this can be used to calculate how much air is moving through the sensor.
There are 3 parameters that can be used in the calculation of air flow through a pipe, Diameter of the pipe, the speed of the air, the volume of the air.
with any 2 of those parameters you can calculate the other.
The KVAFM provides the speed of the air, the ECU knows the diameter of the tube the air moves through and as a result it is able to calculate the volume of air moving through the KVAFM. (Air Density is then taken care of with the Intake Air Temp sensor (IAT) )
So far so good, to summarise:
The KVAFM uses the disruptions in the air flow to measure how fast air flows past the sensor, this is used to calculate the volume of air flow, the IAT is used to calculate the air density. Providing this information is accurate, the ECU can calculate EXACTLY how long to open the injectors per RPM to attain an Air Fuel Ratio (AFR). The desired AFR is found using a Load/RPM lookup.
To make things quicker, there is a table in the ECU that tells it the volume of air in relation to the frequency of the KVAFM output. The reason for this is because the KVAFM size is fixed and therefore for a given air speed the volume will be the same. Instead of calculating every time (costing valuable processor time) it can look up a table.
As you can (hopefully) see, the whole AFR calculation relies on the frequency generated by the KVAFM being correct. This is originally calibrated and tested using the standard intake system. This intake system consisting of the airbox, air filter and KVAFM is a known environment which is used to give accurate data to the ECU.
If you make a change to that system, by adding a pod filter, a bend in some pipe work before the MAF (Haven't you wondered why the MAF is bolted to the side of a box? It isn't the best shape for air movement) then you run the risk of upsetting the way the air hits the obstruction in the KVAFM.
Fortunately one assumes Mitsubishi made the airbox setup to provide a stable air environment for the KVAFM, thus when you do make these modifications the most likely result is that you INCREASE The rate of pressure fluctuation past the sensor.
So, you add some turbulence to the air going through the sensor what happens? Since the ECU relies so heavily on the sensor for accurate AFR calculations, any increase in turbulence is likely (unless you are unlucky in which case your engine blows up) to cause your AFRs to be richer than they should be.
In practice the results have shown this to be the case also, those who add pod filters put bends in their pipework before the KVAFM (usually trying to achieve a cold air intake) have fueling issues.
So, unless you run MAP (manifold absolute pressure) based engine management, you had best leave the airbox alone, buy an ARC box or fabricate one that behaves in the same way as the standard airbox.
Addition:
K&N do sell a pod filter which is especially designed to work with the Mitsubishi KVAFM. I didn't add this information originally because I don't like to endorse manufacturers claims without any proof of them working. However Mitsiman (RPW?) has endorsed this product and on his word I post a picture of it. Perhaps Mitsiman would would to post a product code or some such to help out those looking for one.
I have uploaded a picture of the unit, as you can see it is distinctive in shape and you should know at a glance if you have the real deal!