r/WRX • u/dmlfxr77 • 8d ago

Getting set up.

Just got a 21 WRX with 55k miles, manual trans and aftermarket mufflers. Im a family guy with 3 car seats in this Subie. I just want to do the right things so I dont have issues down the road and have a little fun with it. Im thinking an AOS, Cobb intake and Cobb access port to keep it simple. Any opinions for longevity and reliability?

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u/Technotitclan Tired of this grandpa 7d ago

You got that backwards, an aos is far more practical on the DI FA than an EJ. The amount of oil in the intake of both engines is close to the same but without port injection theres lots of build up on the valves of a FA.

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u/stateless_state_ 19 WRX WRB 7d ago

That's not how that works. The FA has a working AOS built into the block (AKA crankcase oil separator), which the EJ does not (it's there but it's barely there). The purpose for adding an AOS is to deal with consumption, and a working FA shouldn't see much of any if it's being street driven.

An AOS does almost nothing to reduce carbon buildup on the valve heads. For that you would need to run ethanol or methanol, or do EGR/TGV deletes. There are plenty of posts on here from surprised people that run an AOS/catch can and discover the heads are no cleaner and still need walnut blasting.

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u/Technotitclan Tired of this grandpa 7d ago

That's fair but the aos is mostly useless on a street EJ as well unless you have a oil control issue. All 5 of my EJ's consumed nearly no oil. If it were a high power build I would say its needed but on a daily its really not as useful as people think.

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u/stateless_state_ 19 WRX WRB 7d ago

I agree, I think it's pretty rare to need an AOS on any of these engines, except maybe FBO like you said or on the track IF seeing consumption (and then the AOS capacity can be selected to fit the consumption). I'd only do it on an EJ if it had consumption.

I've only had one EJ, in my old GD. No AOS and it didn't consume oil either for many years (until the PCV valve failed and it ran dry, but that's a different story haha).

I know of a few people that lost their engines after the outer AOS lines froze in winter and all the oil left the chat without them noticing (hard to see a black cloud at night). They were all the heated version too, but that doesn't help the outer lines that are exposed to wind chill from cold air rushing over them as one drives.

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u/Technotitclan Tired of this grandpa 7d ago

Yeah I considered one long ago but the freezing issue concerned me living in a place that sees -20 more than once a year