This 07 STi came into the shop down on power and knock prone. After some diagnosis we discovered the intercooler had quite a bit of oil inside of it. Finding a large amount of oil in the intercooler was a pretty good indicator that oil was most likely making its way into the combustion chambers. When oil gets on top of the pistons in an engine it will hinder combustion efficiency, causing knock, which is very damaging to the engine. The oil gets there by way of the PCV system (Positive Crankcase Ventilation System), which carries a mixture of air and oil vapors.

By installing an oil separator, or a catch can, the lines from the PCV system are run into and out of a tank or can that usually contains a baffle inside. When the air and oil mixture hits the baffle the oil is separated from the mixture. The difference between the oil separator and the catch can is that an air oil separator drains back to the oil pan. A catch can, just as the name describes, acts as a can that collects the oil, meaning it will periodically need to be emptied.

The dyno chart below illustrates how much power can be gained with the installation of a high quality air oil separator or catch can when there is an excessive amount of oil in the PCV system.

Run 12 is the car after the stage 2 tune was finished. Run 1 is the same exact tune as run 12 except the oil has been cleaned out of the intercooler and a Crawford Air Oil Separator was installed. In this case the owner picked up 26.5 hp and 8 ft. lbs torque at the wheels.