Air Intake FAQ

It is through the air intake that the engine breathes. Just like the cars exhaust, constriction in the car’s intake system can be robbing you of added performance. As was stated before, if we can increase the amount of air and fuel that is supplied to the engine, we can create more power.

The Subaru intake in its stock form is composed of the Ram-Air Scoop (1), the Silencer (2), the Airfilter Box(3), and the MAF Sensor(4). Other components that are contained within the Subaru intake system include the Turbo Inlet Hose (5), the Intercooler and IC Hoses (7), and the Blow Off Valve (8).

Air travels in through the Ram-Air Scoop (1) which is located at the front of the car. This ensures that the air entering the intake is initially at the outside temperature.

The air then travels down into the Silencer (2) which is located in the fender. The Silencer’s job is to muffle the noise of the engine as well as the noise created by the blow off valve releasing under load.

After exiting the Silencer, the air travels into the Airfilter Box (3). Here it makes it way through a panel filter that removes any dirt or dust from the air.

Next, the air travels through the Mass Air Flow Sensor (MAF) (4). The cars ECU uses this sensor to calculate how much air is entering the engine.

After the air passes through the MAF, it travels through the intake charge pipe and into the Turbo Inlet Pipe (5). From here the air travels through the turbo (6) into and through the intercooler hoses into the Intercooler (7).

Finally, the air exits the intercooler into the intake manifold where it is mixed with fuel.

When throttle is released, the excess air charge that is left in the system is released though the By Pass Valve (BPV) (8) which in its stock form, re-circulates the air back into the Turbo Inlet Hose.
 
Panel Filter

The most simple of intake mods is to replace the factory paper filter with a high flow panel filter.

A high flow panel filter will not only allow air to enter the engine a bit faster, but it’ll also save you a few bucks in the long run as your can take it out and clean it.

Short Ram Intake

The short ram intake replaces all of the parts of your Subaru’s intake system with a simple pod filter and MAF sensor housing. By removing any bends in the system, air is allowed to flow freely into the engine.

It is important to keep in mind the size of the MAF sensor housing. I can’t stress this point enough. If you replace it with a larger diameter housing your engine could be running dangerously lean. The ECU is making its calculations based on the size of the stock diameter, so it will think that less air is flowing through the intake. It is always important to properly tune your engine when using a larger MAF sensor housing.

An added side benefit of the short ram design is that you can hear the stock blow off valve since you removed the stock silencer. This is due to that fact that Subaru designed its stock blow off valve to blow back into the intake.

Cold Air Intake

The cold air intake has much in common with its smaller short ram brother. The only difference is that the pod filter has been extended down into the fender to ensure a colder charge of air. Why is colder better? Well the colder the air is, the more densely its packed. Denser air means that you can force more of it into the engine. (why else would the car have an intercooler?)

Again, like the short ram, it is super important to have the proper engine tuning to compliment the size of the MAF sensor housing.
 
Turbo Inlet Hose & Intercooler Hoses

This one is short and simple, the stock intercooler hoses and turbo inlet hose are made from a hard plastic that contains some accordion pieces to decrease the likely hood of cracking. Replacing any of these hoses with an aftermarket set will basically remove these accordion pieces, thus removing any turbulence that they were causing. Less turbulence = faster spool.

Silicone and cast alloy aftermarket hoses will also help in keeping the heat outside, thus cooling down the intake charge flowing into the turbo and intercooler.

Intercooler

So what does the intercooler do for the engine? Well the temperature of the air charge entering the engine increases as it is rushed through the very hot turbo. Before entering the intake manifold the air runs through the intercooler which utilizes a setup just like a radiator to cool the air down. A cooler air charge allows you to turn up the boost creating more power and decreases the chance of detonation within the cylinders.
 
Top Mount Intercooler (TMIC)

Less restrictions on a turbo car is always better. A larger TMIC allows you to put all the pressure that your turbo is making and put it to the engine.

Your stock IC has a 2-3 psi pressure drop across it. This means in stock form, 13.5 lbs of pressure at the manifold means your turbo must make 15.5-16.5 lbs of pressure at the turbo. A larger volume and air to air surface area will allow you to keep those low temperatures and put the boost where it belongs – in the engine.

A large, aftermarket intercooler requires modifying the hoodscoop splitter and rubber skirt for proper flow, but ensures that your turbo is only making .2-.5 more lbs of boost than your engine sees. This can take you in two directions. 15 lbs of boost in your engine, while actually making the turbo work less–or 17 lbs of boost in your engine while keeping near-stock pressure levels at the turbo.

If you choose to up the boost with EM, make sure your fuel injectors and fuel pump are up to the task, the stock 2.0 and 2.5 fuel injectors start to lose their steam as you raise the boost, even on the stock turbos.

“I run my aftermarket IC to decrease the stress on my turbo and to free up the restrictions that make the 2.0 a tad laggy and also cause the TD04 to fall off at high RPMs; my car boosts quicker, puts down more power, and maintains relatively low boost levels, all of which will make the Saabaru a blast to drive for many years.” – Jesse

Front Mount Intercooler (FMIC)

A front mount intercooler (FMIC) takes the lessons learned from the TMIC to the next level. By using an even larger core than a TMIC and placing the IC out into the bumper, a FMIC allows even larger levels of boost to be achieved through even more efficient cooling.

One important thing to take note of is that as you add piping and a larger core to the intercooler system, the turbo tends to experience more lag. A FMIC should really be reserved for larger turbo systems.

Some things to keep in mind when choosing a new FMIC:

Does it come with a new bumper beam? Many times you have to replace the stock bumper beam to utilize a FMIC.
Does it come with a BPV flange? Some FMICs do not have a BPV flange which means that one must be welded into the system.
Do I have to remove the stock fog lights? The large piping many times requires the removal of the stock fog lights.
Do I need to utilize a short ram intake? Most, if not all kits utilize the hole that is occupied by the stock intake silencer (see picture). The removal of the silencer is necessary in order to route the FMIC tubing down into the bumper.

By Pass Valve (BPV) or Blow Off Valve (BOV)

When you take your foot off of the throttle, the excess charge of air that is in the intake system needs to be released — this is where the By Pass Valve (BPV) comes into play. In its stock form, the BPV recirculates the air back into the turbo inlet hose. A whooshing sound can be heard if you have removed the stock silencer box in the wheel well. The stock BOV is capable of handling up to ~20psi of boost.

If you have a setup where you will be boosting your vehicle higher than 20psi, it is recommended that you replace you BOV with an aftermarket one. Many aftermarket BPV’s recirculate the charge back into the turbo inlet as the stock one does, but others know as Blow Off Valves (BOVs) vent the excess air directly out into the atmosphere creating a spectacular whistling sound as you would hear on a WRC car.

There is a downside to utilizing an atmospheric BOV though. In its stock form, the air is recirculated back into the turbo inlet hose because it has already been accounted for by the MAF sensor. When the air is released into the atmosphere through a BOV, the ECU will be utilizing incorrect calculations and will inject too much fuel into the engine for a brief moment causing the engine to run too rich. This rich condition only last for moments of a second and engine hesitation can be experienced. Many feel that the sweet noise being emitted from under the hood far out ways the inconvenience of the rich condition.