Subaru's Car Intake System Design

Subaru's Car Intake System Design

The following is an opinion on why different year model Subaru's react differently to modifications to their intake system. Primarily we're focusing on the late model Subarus which used either an air-flow system (up to 1999) or the speed-density system (2000+) in the North American market.

Before we discuss the technical details of the Subaru Intake System, lets first get familiar with the two ways Subaru has used recently to measure air flow. The first method discussed is MAF Sensors which were used on all late models until the 2000 model year. From 2000-on Subaru has begun using a speed-density system which primarily relies on a MAP sensor.

MAF Sensors
Mass Air Flow (MAF) Sensor is used by the engine control unit (ECU) to determine the weight of the air going into the engine. To do this, Subaru uses a cylinder shaped sensor mounted securely to the stock air cleaner box behind the passenger side headlight assembly. Inside the sensor there is a special wire or film which is kept heated to a certain temperature. As air passes through the sensor, this wire or film is cooled and the sensor determines how much electricity is required to keep the wire up to temperature. This enables the sensor to determine how much air (in weight) is passing through the sensor at any given moment. This signal is then sent to the ECU which it compares to a preprogrammed map to deliver fuel and ignition spark, taking into account other engine inputs such as engine RPM and temperature.

Pros:

• Highly accurate A/F Ratios for a wide variety of driving conditions

Cons:

• Present a restriction in air flow
• Fragile

MAP Sensors
Speed-Density systems (used on the 2000+ models) calculate the weight of air going into the engine by using a Manifold Absolute Pressure (MAP) sensor, the Intake Air Temperature (IAT) sensor and the Perfect Gas Law. To accurately determine the correct weight of fuel needed, the ECU has a Volumetric Efficiency map stored in it which it uses to calculate how much fuel must be added for a proper Air/Fuel mixture.

Pros:

• Easy and inexpensive to implement
• Very durable

Cons:

• Does not compensate very well for changes in VE
• Not very accurate A/F Ratios under transient driving conditions (ie: non-WOT street driving)

The whole reason behind all these sensors is to maintain a proper ratio of Air to Fuel. As you might have noticed with the explanation of how the sensor's work that we stressed the goal was to determine the weight of air entering the engine. This is because our Air:Fuel ratio is based on weight, not volume. For example, a 12:1 ratio could mean we're using 1 gram of Fuel in 12 grams of Air. Since the weight of air can vary depending on altitude, temperature and other factors, you can see the need for sophisticated sensors to maintain a proper A/F ratio.

Intake Design Observations
Now that we have a basic understanding of the differences between the two methods of measuring air, let's take a look at how Subaru has built their intake system. Regardless of MAF or MAP, all late model Subarus have a similarly designed intake manifold which resembles a four legged spider. This is the aluminum piping mounted directly on top of the engine. Facing rearward we have a single butterfly throttle body with a 60mm internal diameter. Attached directly to this throttle body is a easily recognizable black plastic chamber. Piping then snakes across the right hand side of the engine to the passenger side of the engine bay immediately behind the passenger side headlight assembly. All of these items combine together as a complete intake system and each serves a specific purpose.

Lets start with the intake manifold which is designed with long runners branching off of a relatively small plenum chamber in the center. The long, small diameter piping works well to provide low-mid range torque by improving cylinder filling at low RPMs. At high RPMs though, this long tube design can eventually become a restriction and would need to be addressed. Don't necessarily think you need to go out and spend your hard earned money on a new intake manifold quite yet though. The cylinder heads are still the biggest restriction in the intake system so you're better off addressing that first before swapping over to another manifold. If and when different intake manifolds become available, be sure they are part of a system which also includes a matched cylinder head and cam. Otherwise, you might end up with a intake manifold designed to work at high RPMs and a cylinder head and cam designed for low RPM torque. Unfortunately, there is no one manifold design perfect for every application and be wary of manufacturers who claim otherwise.

Next we have the throttle body which is what you control with your right foot on the accelerator pedal. As you press down on the accelerator, a throttle blade inside opens accordingly allowing more air to enter the engine. With more air comes more power. Subaru uses a relatively nice sized 60mm throttle body to feed the engine. With a sub-6500 RPM 2.5L engine using a stock cylinder head and cams the need for a larger throttle body is minimal but as you improve air flow into the engine, we feel some gains can be made by enlarging the throttle body opening slightly.

Mounted directly behind the throttle body is the mysterious black plastic plenum whose function has been the subject of much debate. This box was completely empty inside, serving only as a chamber, until mid-1999 when Subaru included a secondary air filter inside. While we haven't discussed the details of this design with the engineers at Subaru, we can make some assumptions as to the reasons of this layout based on some theories behind intake systems. One of the primary reasons for this chamber is to act as a supplemental intake plenum to improve throttle response. When you go from low throttle to wide open throttle, the engine's demand for air increases drastically and the stock intake manifold with it's small plenum lacks the necessary volume to satisfy the engine's demands. To remedy this, Subaru uses this secondary air box as a supplemental plenum to supply this sudden demand of air. This gives us the throttle response we want and keeps our engine happy.Another feature of this plenum as well as the strange tubes which hang off the intake piping are to tune out frequencies found in the intake system. As RPMs increase, there is a frequency created in the intake system causes primarily from the rush of air bouncing back every time the intake valve closes. Most frequencies don't make it out of the black intake plenum but those that do are tuned out with the lengths of pipe placed in the intake tubing. Just like a trumpet changes notes (frequency) when the length of piping is changed, same goes for this frequency seen in our intake system.Why all this concern with the frequency of air bouncing back up through the intake? Well, as can be seen when you install an aftermarket intake system (shown above) on a 1999 model which eliminates the stock piping and plenum box, the air flow meter becomes confused at higher RPMs causing it to give incorrect data to the engine's computer. The air bouncing back through the intake system eventually creates turbulence inside the MAF sensor itself. This is why some people state that the 1999's run lean when using an intake and need an additional device such as an Apex Super AFC to richen things up. In actuality, the engine would still run slightly rich (as it does on all other models) when upgrading the intake if it were given the opportunity to see the real weight of air passing through the sensor. The issue lies with an improperly designed intake system, not necessarily the MAF sensor.

MAF Sensor Failures
While this air reversion might cause erroneous readings of the air flow meter, it's not solely responsible for their failure. No one situation can be contributed to the high failure rate seen with the 1999 series MAF sensors but rather a combination that is the cause. Our opinion is that the number one cause is vibration. Many of these aftermarket intake systems and even turbo kits do not properly mount the sensor in a way to prevent them from receiving quite a bit of vibration. As you drive, the engine bounces and vibrates around which is torture on the electronics inside the sensor. We've even seen some kits which bounced the MAF sensor against the engine bay whenever the engine torqued over. Other various reasons have to do with poor intake filtering, sudden pressure changes (ie: blow off valves shooting hot air into them from the back side), and heat.

How do we solve this? Well, we have to understand we're dealing with a delicate, precise piece of electronics that needs to be treated with some care. First, properly mount the unit firmly to the body and use rubber bushings between the MAF sensor mounting bracket and body to reduce vibration transferred through the body. For those of you who want to instead mount it to the engine, make sure you don't have a problem with the MAF sensor hitting anything else in the engine bay and use the same rubber bushing trick to dampen the constant vibration the engine produces. Second, use a quality filter element. Cloth elements from K&N, A'PEX, and others work well when properly oiled. Even foam elements from Amsoil, Greddy, and others do a fair job of filtering. We recommend against using HKS filters and Weapon-R filters as we've had too many instances of foam degrading and getting into the intake tract (they also don't filter very well). The absolute worst though are the Blitz Stainless Steel filters. These might look better than just using chicken wire but they filter about the same. Lastly, if you're turbocharged make sure you run a blow off valve but don't have it discharge right into the backside of the MAF sensor. It doesn't appreciate too much when you do that.

Do these things and you should enjoy a long (longer?) MAF sensor life. For those who've tried all these methods and still have problem, we are working on an aftermarket replacement which will be much more durable. If successful, they will be make available in Fall 2000.

So which is better, MAP or MAF?
While Air-Flow systems are still favored by most manufacturers due to their accuracy, speed-density systems are slowing gaining favor once again due to advances in electronic engine management. In order for a speed-density (MAP) based system to offer the accuracy in A/F ratios to pass the stringent vehicle certification tests, advanced ECU technology must be used to cope with the speed-density's inability to optimally handle transient driving conditions (compared to an air-flow system).

Starting in 2000, Subaru went to a sophisticated speed-density system in the North American market while retaining the air-flow system for their turbocharged Impreza, Legacy and Forester applications sold overseas. We can only make assumptions as to Subaru's reason for switching to speed-density but our guess is was done to reduce cost while improving reliability. Also making guesses, we expect you'll see the air-flow system return when the WRX is introduced here in the US though the non-turbo applications will most likely retain their speed-density system as thus far it's proven very reliable. Why not use the speed-density on the WRX? Our assumption is based on the fact that Subaru has been using the air-flow system quite reliably in other countries and as there will be no major drivetrain/engine changes for the next generation WRX, they might not want to incur the added development costs involved in making a speed-density WRX pass the tough US emission certification tests. Again, these are just assumptions. We'll find out for sure next year.

The facts remain the same though, the air-flow systems are slightly more accurate at calculating proper Air/Fuel ratio under a wider variety of driving conditions. At Wide Open Throttle (WOT) though, they are both very similar. It's primarily under transient conditions (partial throttle changes/city driving) where they differ the most. Again, the differences are primarily in theory and hardly sometime you'll notice by the seat of your pants. Those of you with speed-density systems don't think your system needs to be improved by going to a air-flow setup. Keep this comforting thought in your head; nearly every aftermarket engine management system uses a speed-density system (ie: Haltech, TEC II, Motec, EFI, etc.).