IAG SS AOS for 04-07 STI. Keeping your oil where it belongs, in the engine crankcase and not the intake tract!
Hey!... And welcome to another #modmonday! Today we will look into my STi's engine bay and the AOS (Air Oil Separator) that keeps tabs on the engine. I upgraded it to the IAG SS AOS last year and have been quite happy with it.
Why a catch can or AoS? With any performance type car you will see elevated crankcase pressures and with those loftier crankcase pressures comes the ability of your engine to consume it's own oil supply. The factory installs breather hoses that allow the crank case pressure to flow into the intake, thus causing your engine to involuntarily ingest it. It keeps the roads and your driveway cleaner but can have an adverse effect on your engines performance.
In come the AOS's and catch cans. I decided on the IAG Aos due to it's looks and built in pcv. Due to my auto-x classing I must utilize a Pcv and this is one of the few ones that are setup as so. Routed into the AoS can are coolant lines to keep the temperatures similar to help stave off oil emulsification.
It is a very complete kit with various hose sizes, fittings, clamps and a whole mess of zip ties. The factory installation pdf is very easy to follow through and will help anyone complete the task. The toughest part being a slight shift in metal brakes lines near the passenger strut tower, otherwise it is well thought out and put together.
I spent awhile without a AOS and noticed that with my forged piston motor I was amassing more than your typical oil in the intercooler. After having driven with it for a few months I am very pleased with it's effectiveness and would highly recommend it. Helps keep the oil out of my engine allowing the timing to not be scaled back and the full octane of my fuel to be utilized.
I did have a little trouble with the need to move my alarm horn and bracket but it was easily sorted. Which may just be an 05 Impreza issue though. I really enjoy the placement IAG choose, it looks very at home in the empty space between the tmic and strut tower. My only other qualm has been that with my short drive the coolant temps in the can don’t get warm enough to keep the condensation out. I really have an extremely short drive, so if you drive less than a mile to work this may also be an issue for you.
These are quite easy to modify and install on SG Forester’s from my understanding. Not much has to be changed which makes you wonder why they don’t just offer them for the application. Anyways, I may be picking up one in the near future for the Forester as well!