What is speed density?
Speed density is nothing more than a different method to calculate the amount of air entering the engine. Normally, the Mass air flow sensor (MAF) is used to do this job but on setups that draw in large amounts of air, or setups that have things like a rotated turbo-charger or a front mounted intercoolers, using the MAF sensor to calculate air flow isn't always an ideal approach. Front mounted intercoolers introduce a large amount of additional piping and rotated turbos can introduce air turbulance. Futhermore, the MAF sensor has a limited voltage range which is calibrated from a known pipe diamter where it sits. In order to avoid exceeding the voltage limit, its becomes nessecary to use an intake with a larger MAF housing dimater. Between the extra piping, turbulance and/or pipe larger diamter, its easy to see that the MAF sensor is operating well outside the bounds of what Subaru designed. It is certainly possible to tune for these changes but even the best tuner can only do so much on some setups. Enter Speed Density (SD). The specifics of SD can be found with a simple search. What it boils down to is using the manifold pressure sensor (MAP) and intake air temperature (IAT) sensor instead of the MAF sensor to calculate the amount of air entering the engine. These sensors take their readings right as the air is actually entering the engine so they are not affected by the things that cause issues with the MAF sensor. The tuning approach differs but that is beyond what is needed to know to get your car setup for SD.
What do you need for speed density?
The 2008+ STi has a 3 bar MAP sensor so it can read up to about 28 psi and other Subarus can read up to about 24 psi. With SD we are using MAP for fueling so we can't go close to those limits. An AEM 3.5 bar or AEM 5 bar sensor is recommended on most applications unless boost will be lower than the limits by a good margin. The 3.5 bar sensor can read up to about 36 psi so if boost will be near or higher than that, the 5 bar is recommended. A MAP sensor adapter is needed to adapt the AEM sensor to the OEM style spot. Cobb offers a nice option but there are others as well. Despite the lower cost and plug & play nature, I do not recommend Omni MAP sensors for SD. They have been found to be too inconsistent at times and not worth the risk to use. AEM is proven to work the best.
Intake temp sensor
On a Subaru, there is pre-turbo inlet temperature sensor that is integrated into the MAF sensor that measures the temperature of the air as it comes through the filter and into the turbo. This is used for intake temperature based compenations. Speed density uses charge temperature as part of the caluclation of air entering the engine. This means we need to know the temperature of the air entering the throttle body after it has gone through the turbo and intercooler. So we need a temperature sensor in the charge pipe (for front mount intercooler setups) before the throttle body, or the top mount intercooler needs a bung added at the bottom, near the connection to the throttle body. The AEM intake temp sensor kit is best to use for the new sensor. It takes the place of the MAF housing intake temp sensor by cutting and connecting to those wires in the MAF harness.
AEM 3.5 bar MAP sensor: 30-2130-50
AEM 5.0 bar MAP sensor: 30-2130-75
AEM Intake temp sensor: 30-2010
Cobb MAP sensor adapter: SUB00021