AOS when rebuilding engine, yay or nay?

I’ve been in talks with Surgeline about replacing my 213K EJ255 when it goes with a stock EJ257. I got a long form quote for all the work the car would need when having the new block put in, and one thing I didn’t feel super confident about buying when the time comes is an AOS, which Surgeline had on the rebuild list (Cobb unit for STis).

As I’ve understood, you only “need” an AOS if you track the car and/or have a lot of blowby. With a new motor in, I’d hope the latter wouldn’t be an issue. The former I’m not yet certain of. This car seems like it would be an expensive car to daily drive and also track with the upkeep it’s needed as a daily. Brandon told me that it was one of those things that made sense to put in as early as possible to help aid in the longevity of the motor.

My plans for power on the car are to be going to topfeed injectors with a VF52. Car makes 270 WHP and 320 TQ on a VF46. If I made 300 WHP I can’t imagine I’d be unhappy, but I’d like to stay within the limits of the 07 LGT clutch, or whatever upgraded clutch wouldn’t kill me in PDX traffic with our hills.

Thanks for the insight.

Depending on your budget, I’d say that you could forgo the AOS and just keep the stock PCV system. (As long as it’s functioning well). The AOS really comes into play when the car is getting repeatedly beat on under high G-load situations. Since our heads are horizontal, oil can pool in the heads under high G cornering and this starts to cause issues over time. For a street driven car, (that might go to the track occasionally), a proper, functioning OEM system works quite well.

I ran my stock block at stg.2 power levels for 8 years of daily driving (with 8 years of seasons at the track about 5 days per season) On the OEM PCV without any issues until I finally cracked a ring land. Was that due to not running a proper AOS? It probably was a contributing factor for sure although we will never know. My point is that it took a ton of abuse to get me to that point. If I would have known way back then that the car would eventually be a full time track car, I would have gone ahead and done one but if I was going to just chill out on the track days from the new motor on out, I would have just kept the stock system.

Always good to remember what your using the car for. When I spun a rod bearing in my Outback on Surgeline’s dyno, Brandon suggested an IAG stage 1 block to replace it. While it’s a great choice if you have the dough, an OEM shortblock from Subaru was significantly less money and based on my previous experience in the Legacy, I knew that the stock block (with a PROPER tune) would be fine at 300 ‘ish WHP under daily driver conditions, so I went with that. Shops will always suggest you overbuild for your goals and hey, nothing wrong with that, but if budget is coming into play, there are choices that can be made. A proper AOS a la Cobb, Crawford, Perrin, or IAG is a pricey bit of kit If you don’t really need it based on what the car is being for. Guess it depends on whether your wringing it out on the street on a daily basis I’m stock on my stg.2 Outback and Crawford Performance on the Legacy.

I did my first 2500 miles of track driving on oem configuration, no aos. The engine blew when I forgot to check the oil level before going out (never work double shifts before going to the track). I replaced the broken engine with a stage 3 closed deck outfront motorsport (california engine builder) engine with the original grimmspeed aos (known to hydrolock engines). I checked/changed the oil religiously. I put 29k "drive it like you stole it" miles on it. It failed with a contaminated hg. The aos turned out to be non functioning and my oil consumption was crazy bad....3qts per track day.

The AOS, unless you are racing, is mostly hype. A properly working pcv system is going to be much better.

When yer spirited driving pops you out of the driver's seat in corner after corner, you're approaching the point at which you might want to explore an improved engine air pressure breathing system. Keep in mind, serious subaru race engines are dry-sumped and get completely new builds every 1000 miles or less.

Driving a built for the track car as a daily gets old fast. My frustration with traffic translated into a lot of "license and registration" discussions.

I'd ask yerself what you are going to do with your 213k mileage body that you don't already do. The lgt as a track platform is horribly expensive. You can buy and run a Miata for years on what brakes & tires will cost in the lgt. If you didn't do it before, yer not going to do it now. Build for the car you drive now, not the one track day you'll do.

310hp with a really wide torque range is infinitely more fun than a peaky 350hp. Cause 3rd gear in traffic translates into yer own personal tetris game.

Thanks for the responses guys. The more I put time and money into this car and wonder when it will be reliable, the more I question that I want to spend money turning it more into a track car. I had plans to track it this summer, but COVID and sudden moving plans got in the way. I've wanted to buy a Porsche Boxster or Miata for a few years now (both were cars I seriously looked at before settling on a four-door car to haul people) and I'll probably full send one of those when my income supports it while keeping the wagon as my fun daily on the side.

As for the AOS, I can understand the value proposition if it's something that definitively will aid in engine longevity. I'll opt for one if the bank account allows for it when the engine is rebuilt if only so that I don't have to worry about topping off oil all the damn time.

Factory PCV system returns oil vapors to the intake. This coats the entire intake with oil residue.

The layer of oil on the walls of the intercooler reduces thermal transfer, reducing the effectiveness of the intercooler.

Oil vapor will also reduce the effective octane of your fuel which will increase the chances of knock.

If you can afford the AOS, get it.

https://www.importimageracing.com/products/grimmspeed-air-oil-separator-subaru-sti-2004-2020?gclid=EAIaIQobChMI1NXjgNPt6QIVy5yzCh0GYwtEEAQYAyABEgKIpvD_BwE

That is the same type I have on my wagon. I like the idea of not diluting the fuel mixture with oil.

Don't plan to put one on my Spec B

https://www.importimageracing.com/products/grimmspeed-air-oil-separator-subaru-sti-2004-2020?gclid=EAIaIQobChMI1NXjgNPt6QIVy5yzCh0GYwtEEAQYAyABEgKIpvD_BwE

 

That is the same type I have on my wagon. I like the idea of not diluting the fuel mixture with oil.

 

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Don't plan to put one on my Spec B

this is similar to the first gen gs system that caused problems.

This has been there since about 142,000 miles on the wagon. Last time I had the tmic off there was just a light film of oil like normal, nothing to cause me to worry.

Mine was on for 29k. I gave in to the hype and bought a crawford aos.

I must have missed the threads about issues.

I do know that during cold weather the white moisture forms up by the cap. But I do drive long distances in Winter so my oil gets pretty hot.

I went with a Morosso AOS for easy removal for smog and simple maintenance on every oil change. Probably main reason my car is still running with nearly 250k miles on it..

This was something I didn't understand. I was under the impression that smog checks can be an issue with an AOS but Brandon at Surgeline assuaged that by saying it has nothing to do with emissions. However, anything that affects the PCV system certainly affects emission, does it not?

here in California it will fail visual

it's not stock and considered tampering with the PCV system

The oil cap style like the GrimmSpeed are better than the factory system, but not by much.

We do not offer this style to our customers and request that they replace them with an IAG or equivalent AOS when Stage 2 or higher.

Reasoning behind this... they don't work.

This was something I didn't understand. I was under the impression that smog checks can be an issue with an AOS but Brandon at Surgeline assuaged that by saying it has nothing to do with emissions. However, anything that affects the PCV system certainly affects emission, does it not?

In theory, it should not effect emissions, unless you go with a vent to atmosphere version like the IAG Competition series.

This is going to be very state to state dependant, but most states (that even check this kind of thing) will not fail for an AOS.

In Colorado, we are quickly turning into California as far as emissions goes. I have never heard of a fail due to an AOS. They have plenty of other things to pick on.

for emissions checks, i used to keep a "bad" gas cap. it was an easy find for them and a quick fix for me. washington has done away with emissions checks, so gas cap got round-filed.

Funny, so I wasn't the only one who kept a good gas cap in a clean bag. I installed that on my 86 F150 before it used to get tested. When it passed, I took it off put it back in the clean bag, put the old gas cap back on. I even purged the bag with nitrogen. The truck never failed for a bad gas cap again.

In CT if it's older then 25y/o, there's no more testing.

Funny, so I wasn't the only one who kept a good gas cap in a clean bag. I installed that on my 86 F150 before it used to get tested. When it passed, I took it off put it back in the clean bag, put the old gas cap back on. I even purged the bag with nitrogen. The truck never failed for a bad gas cap again.

 

In CT if it's older then 25y/o, there's no more testing.

washington required the box if you "fixed" it yourself. The box was pretty beat up by the time it wasn't needed. I also staggered the registration by getting an 18 month period. Until the law got changed to every 2 years regardless of registration date, I could miss the testing. On a dd, it was harder, but still managed a few times.

The wagon got caught on a visual after the rebuild. Then they did the tailpipe test. It was less than oem. Penalizing the car at that point seemed wrong so it passed.