The use of Aftermarket Intakes on 2002+ Subaru factory-turbocharged vehicles
by Trey Cobb
As long as people have been modifying their vehicles for improved performance, one basic upgrade philosophy has always remained constant regardless of the model -- replace the restrictive factory intake air cleaner system (aka air box). This simple modification has been a proven way to easily and safely improve the performance and sound of your vehicle's engine. As modern vehicles get more complex, however, this once infallible upgrade path needs to be reevaluated.
In this technical article we are going to discuss the type of intake air cleaner system used on the 2002+ factory turbocharged models from Subaru, the potential problems that might arise from modifying it, and potential solutions.
Please note that the content of this technical article is specific to 2002-newer Subaru factory turbocharged vehicles including the WRX, STi, Forester XT, Baja Turbo, and Legacy GT Turbo/Outback Turbo. The systems used on other year models and non-turbo versions do differ in design and the information included here does not apply.
Intro to the Intake Air Cleaner System (aka Air Box)
The factory intake air cleaner system is designed to work as a complete system to provide a steady supply of filtered, cool air to the engine. A large emphasis is also placed on maintaining an acceptable induction noise level that enters the passenger cabin. Believe it or not, not everyone wants to hear the roar of the engine under acceleration or the whine of the turbo. The goals of the engineers at Subaru include, among other things, proper power generation with a low induction noise and a consistent air intake charge and temperature.
The components that make up the factory intake air cleaner system are:
- Air Intake Duct
- Air Resonator Box (in fender)
- Upper and Lower Air Cleaner Cases
- Paper Style Air Cleaner Element
All these items are what are typically replaced when changing the system out for an aftermarket unit. The most common configuration is to replace all these components with a metal pipe that has a conical style filter element attached at the end. Filter placement is either located inside the engine bay or in the inner fender well where the factory Air Resonator Box normally resides. Systems with filters located in the engine bay are normally marketed as Ram Pod or Short Ram intake systems. As underhood air temperatures are normally quite higher than ambient temperatures, we will herein refer to this style of intake as a hot air intake. The systems with the filter located in the inner fender well are normally marketed as Cold Air intakes, as they reside in the cooler wheel well.
How it works
One critical design feature of this system, which differs from those used on most other vehicles, is the installation of the Mass Air Flow (MAF) sensor into the Upper Air Cleaner Case - instead of its own housing which has been traditionally the case with MAF equiped vehicles. It is primarily due to this design feature that problems may arise from modifying the factory air cleaner system.
When changing the factory air cleaner system out for an aftermarket system, you are required to relocate this MAF sensor into the new system. Due to this relocation, we have now opened up the potential for problems to occur.
The Mass Air Flow (MAF) sensor is a critical sensor used by the engine management system. The job of the MAF sensor is to monitor the Mass of air that is entering across the intake system. This is important because all of the fuel, timing and other critical engine management decisions are based primarily on what the MAF is reading. If this reading is off, then so will the fuel, timing, etc which has the potential to cause poor driving conditions or worse, engine damage.
Potential Problems caused by Aftermarket Intakes
MAF Sensor Housing Size
The MAF sensor only samples a small portion of the air coming into the system. From that sample measurement, the engine management system can calculate how much air is actually entering the system because it knows the inner diameter of the MAF sensor housing it is sampling from.
If the MAF sensor housing inner diameter changes in any way, the calculation performed by the engine management system will be incorrect. This means that the amount of air actually entering the system will be different from what the computer thinks is entering the system. This is critical because the computer determines how much fuel and timing to run based on this value of air. When it's wrong, it can cause the engine to run richer, or leaner, than it should.
With aftermarket intakes, it's extraordinarily common for the piping used to have a different inner diameter than the MAF sensor housing incorporated in the factory air box. This is due to the fact that the exact inner diameter of the factory MAF sensor housing is different than what's commonly available in the standard metal piping used for aftermarket intake systems.
It is true that some aftermarket intakes show an increase in power when installed on a stock vehicle. This power increase is due to the fact that the aftermarket intakes have a larger inner diameter for the MAF sensor housing to mount into which in turn leans out the Air/Fuel mixture because of this error in calculating how much air is actually entering the system.
Please note this is NOT due to the fact that the aftermarket intake is so much less restrictive than the stock air box and therefore more air is getting in but rather that the computer does not know it's now sucking from a larger tube and therefore more air is getting in.
So to recap, whenever the MAF sensor housing size is increased compared to stock, more air will enter the system than the engine management has been calibrated for and you will run leaner. If the MAF sensor housing size is decreased compared to stock, less air will enter and you will run richer.
Fortunately, there are corrections that can be made for changing the size of the MAF sensor housing to allow the engine to run properly which we will discuss later on in this article.
In order to accurately measure the amount of air coming into the system, it is important for the air to flow smoothly across the MAF sensor. Any turbulence in this air flow will create errors in the amount of air measured versus what is actually entering the system.
In the factory air box, the Subaru engineers utilizes a smooth velocity stack style inlet and internal ribbing on the inner sections of the cases to dampen pressure waves and promote a laminar air flow into the MAF sensor housing. The MAF sensor housing itself is straight and smooth internally and positions the sensing portion of the MAF sensor properly in the air flow stream (ie: not offset to any angle left/right or up/down).
For aftermarket intake systems, turbulence across the MAF sensor is an all too common occurrence. Often times the MAF sensor is placed in close proximity to a bend, pipe joint, or weld. In the case of some hot air intakes (ie: Short Ram), the MAF sensor is mounted directly after a conical intake -- well before the air has a chance to smooth out.
When this turbulence occurs, the MAF sensor will suddenly see more or less air then it had previously. This will cause the engine management system to respond by making the engine run leaner or richer for as long as the error from turbulence occurs. The most noticeable driving characteristic associated with this would be a hesitation or flat-spot in the engine's power deliver. A worse case scenario would be the engine suddenly leaning out while under full throttle that resulted in engine damage.
Unlike the errors created by the different size of piping, there are no corrections one can make for an intake design that allows turbulence across the MAF sensor.
The other issues to consider with upgraded intakes are the filtration capabilities of the element used, the chance for water ingestion or damage from road debris.
Location of the MAF sensor in terms of its distance to the inlet of the turbocharger (or some other point of reference) is not critical so long as the system is free from leaks. Having it too close to the turbocharger inlet can of course create errors from turbulence as can having the plumb-back line from the compressor bypass valve (aka blow-off valve) too close.
Now that most of the common problems associated with aftermarket intakes have been outlined, let's discuss what possible solutions there may be for those that run one.
When dealing with the issue of a different MAF sensor housing inner diameter, the best solution we've found is to simply recalibrate the factory engine computer for the new air flow. The factory engine management system has a rather large table of data that basically tells the computer how much air is coming in per sample read. By recalibrating this information, the engine will run correctly -- so long as there isn't also a problem with turbulence.
For those intakes that generate turbulence across the MAF sensor, there unfortunately is no easy solution. Since this turbulence can occur at different points in the power band, based on both engine RPM and engine load, all that can be done to make the engine run safely is to recalibrate the engine with a relatively rich overall tune. While this isn't the best for power, it will help protect the engine.
Overall, the factory air cleaner system doesn't fall into the traditional "first item to replace" like we've all be accustom to. In fact, it may even be suggested that it be one of the last thing you do in your quest for power.
From our testing, we've found the stock air box to work well even at 350-375 HP. From that point, packaging requirements for other necessary modifications such as front mounted intercooler may require the use of an aftermarket intake.
The best solution for an aftermarket intake will be either one that has been designed to use the stock inner diameter for the MAF sensor housing or one that has a tuning program behind it to support the change in MAF sensor housing size. If there is any turbulence in the design, however, this will negate any usefulness of the intake.
For those still looking for something a little more in-depth.
The MAF sensor generates 0-5V based on measured air flow. The engine management system then determines the actual air flow based on this sensor's voltage. This mass air flow value can be represented as g/sec or lb/min, based on your preference. The voltage vs mass air flow value is not linear but rather more exponential in form.
To correct for a change in size of the MAF sensor housing diameter, we must modify the mass air flow values held within the ECU for each given voltage. These values are defined approximately every 0.1V of the MAF sensor range.
There is also a limit to the amount of air that can be read across the factory MAF sensor when using the factory MAF Sensor Housing diameter. This limit is reached at approximately 38.64 lb/min (293 g/sec). Those with the ability to manipulate the ECU tables will be able to maximize the MAF's sensor capabilities up to 39.6 lb/min (300 g/sec) but that's the ceiling of the factory ECU's programming. Using our Accesstuner technology, we actually have the ability to rewrite the ECU software for a limitless MAF sensor calibration.
The MAF sensor itself does not need to be considered a limitation, even in large turbo applications. All one has to do is simply run the appropriate size intake diameter to the turbocharger and recalibrate the MAF sensor for that intake's inner diameter. If someone cannot properly recalibrate the MAF, then the limitation is in the tuner…not the hardware.