Crawford's AOS uses a large canister swirl pot with mutlple chambers and baffles to catch oil vapor in the PCV system before it reaches the intake. These kits have vehicle specific mounting brackets as well as all the necessary fittings, lines (pre-cut to length), and hose clamps.
The V3 AOS offers improved internal baffles, a new mounting system, and removes the heater circuit to reduce complexity and cost.
How a PCV system and AOS work:
The PCV, or positive crankcase ventilation system, serves a few important purposes. It improves emissions by collecting and recirculating gasses from the crankcase back into the intake, and also improves engine power and efficiency by drawing a slight vacuum, which reduces windage losses and improves ring seal. In racing applications a vacuum pump for the crankcase can result in a measurable power increase.
The downside to the PCV is that the recirculated gasses previously mentioned contain oil vapor, which is burned by the engine. Engines generally do not like to burn oil, as it reduces power and increases knock potential. Avoiding knock, especially on a turbo Subaru, is a critical part of keeping an engine running well and healthy. We suspect that additional knock from oil being consumed by the pcv system plays a part in the EJ25 ringland issue (along with brittle pistons and 91 octane fuel). Direct injection engines also suffer from a sludgy buildup on intake valves due to the PCV.
An Air-Oil Separator (or AOS), acts as sort of a filter for the PCV system. It collects oil vapor in a canister between the block vents and intake system, which reduces oil consumption and improves power and reliability. Some systems will return the collected oil back to the crank case automatically, while others need to be drained (although we generally refer to that type as a catch can and not "air-oil separator"). These retain a functioning PCV and emissions system while reducing oil consumption. This means more power and less knock. We highly recommend an AOS system for most turbo Subarus to increase power, protect the stock pistons, and prevent oil buildup on intake valves for DIT engines.
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Manufacturer Crawford Part # S0699 Made in United States MSRP $374.95