What is a short ram intake?
This is an aftermarket intake that removes the stock fender-well snorkel components and draws air from the engine bay. To generalize, it is essentially a filter unit that replaces the stock air filter box. Some aftermarket short rams also replace some or all of the stock MAF piping with aftermarket piping.
What is a CAI?
This is an aftermarket intake that removes the stock fender-well snorkel components and replaces them with aftermarket piping. These systems, depending on the manufacturer, may or may not replace the MAF piping as well. These systems draw air from the fender-well.
Which type of Subaru does this information apply to? This post is generally catered towards turbocharged models. Naturally aspired Subarus actually see good, solid, and safe gains from an aftermarket intake. The STi appears to accept some intakes without as many ill effects as the WRX and little data is known about other turbocharged Subaru models. The information about aftermarket filters’ filtration capabilities is applicable to all vehicles though and is a point of consideration for some owners.
Is the stock WRX/STi system adequate? Yes. The majority of major Subaru tuners agree that the stock intake is adequate to power levels in the 400HP range. It also has the following advantages:
1. The standard air scoop is mounted high in the engine bay. This makes it resistant to ingress of water. The design of the fender snorkel helps to remove any water that is drawn in by this scoop.
2. Air drawn in is cool, since the air comes from the edge of the engine bay, and at speed, all air will be drawn from outside.
3. The standard system is quiet since the resonator in the fender helps filter out noise from the air flow.
4. The stock ECU is calibrated to work perfectly with the stock intake system.
Are there any downsides to aftermarket intakes? There are potential problems with some intake systems. Most manufacturers utilize less dense filtration media in their filters. This equates to increased particulate matter ingestion via the intake. This depends on the manufacturer’s choice of filter media and is something to be taken into consideration. CAI can be suseptible to water ingestion. This is because through their design the filter media is quite low in the fenderwell. Extreme caution should be used when driving through standing water or other areas where water is present. The major downside of intakes are problems with the air/fuel ratio.
Do intakes cause problems with the air/fuel ratio? Many seem to believe that since their XXX intake does not cause a CEL or exhibit any drivability problems, then their XXX intake must be good. Though the apparent increase in power gives them peace of mind, what they don’t take into account is their intake is actually causing unseen harm to their engine. This is generally due to two reasons:
1. The diameter is incorrect. This can cause a lean condition in the case of larger than stock diameter piping or a rich condition with decreased diameter piping.
2. The air flow is not laminar. The stock piping, through it’s apparent unusual design, is constructed to smooth out turbulent air flow. Aftermarket systems do not take this into consideration. When turbulence occurs, the MAF sensor will suddenly see more or less air then it had previously. This will cause the ECU to respond by making the engine run leaner or richer for as long as the error from turbulence occurs.
Can the intake problems be corrected through aftermarket engine management? Yes. There are a wide variety of engine management systems that have the capability of modifying the mass air flow values to correct intake piping that is non-OEM size. Some intakes have especially troublesome turbulence problems. 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 re-calibrate the engine with a relatively rich overall tune to compensate.