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Discussion Starter · #1 ·
I have a 2015 WRX I bought about a month ago. It was running an AEM intake and what I can only assume was the OEM tune. I don't know because I don't have any documentation of the vehicle afterwards. The Dealership did a check on the vehicle when they got it and it passed their checks (Whatever that might mean to the reader of this).

After looking at the vehicle myself on a lift I can see they put on the STP (STI) Catback. The downpipe appears to be OEM as it has the giant cat shielding on it.

I have since taken off the AEM intake and replaced it with the Cobb Big SF intake and had flashed it with the appropriate Stage 1+ BigSF 91 tune. 91 is the most common in my area so thats what I use. There is only 1 gas station within a 50 mile radius that has a 93 pump. After putting this 91 tune on I got about 1 mile before my DAM dropped to 0.6 under a moderate pull and continued to drop over time. I read on Cobb's site that it might benefit to run the LWG version of the Stage1+ tune so thats what I ran for about 1-2 weeks. The Dam still dropped but it bounced between 0.4 and 0.8 until it finally settled at 0.412 for a few days, then dropped to 0.312, then just yesterday dropped to 0.188. No CEL, the car never stuttered and didn't have any knocking at idle. I'm an idiot and unfortunately did not datalog the times where the DAM was dropping. I have since reflashed the ECU with the normal wastegate cycle for the Cobb BigSF 91 Stage1+ tune that comes on the AP. Casual driving under minimal boost (At most 4 PSI) and the DAM never dropped. It wasn't until today when curiosity struck and I did a short pull on my way back from lunch where I started logging before I did the pull. It wasn't by any means full throttle, just enough to feel the car pull.

Heres the log

Notes to consider:
  • The original bracket that Cobb says to use with the new model of the BIG SF intake, I didn't have so that part essentially just floats with the box cover and the box secured together with the Alan screw they provide.
  • I cant remember but I will check tonight if I put the silicone tube from the intake to far over the OEM inlet piping as shown in this image on Cobb's Docs for install
  • The car currently sits at 79,700ish miles. The last oil change was at 77,900ish miles. As stated earlier the Subaru dealership I bought the car from did not give me any maintenance documentation of the vehicle so I pulled the coil packs and spark plugs on the driver side and inspected them for ware. The Spark plugs had very minimal and normal ware so I assume that side was changed recently. I did not change the passenger side yet.
  • A few weeks ago when I did the first oil change, I did not prime the system and over filled the pan. After driving around a bit the oil cap blew off and I had to get a replacement. Oil sprayed all over the driver side of the engine bay and was a mess to clean up. Its still not all cleaned up but the majority of it is gone. I disconnected the battery and cleaned everything I could with soapy wooder as that was all I was comfortable using at the time.
  • This car is my Daily.

Performance related Mods:
Cobb Big SF Intake
STP (STI) Catback

Thank you in advance for any help.
 

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That's a pretty neat online tool. Has filtering and slicing inbuilt. I like that. I think I'll direct others to use that for the future.

The FA20F is far more active with DAM than the previous cars. DAM was created as a general octane-modifier to compensate for poor fuel quality. Because the FA20F runs much higher cylinder pressures and because its control loop is far faster than the previous cars, my guess is that Subaru had repurposed the DAM to act more as a real-time knock control parameter. That is speculation.

Unfortunately, no data labels, but light blue is FLKC, red is DAM, and orange is calculated load. It's clear that the ECU encountered multiple knock events at somewhat high load (>1.5)
Rectangle Slope Parallel Font Electric blue

The cause for this can't be known. It could be a couple bad combustion cycles, carbon buildup, bad gas, or even just errant background noise registered by the knock sensor. Looking at your other parameters, nothing seems out of the ordinary and the ECU didn't pull a whole lot of timing. Give the car a couple more key cycles and I'd imagine the DAM snaps back to 1.

This wouldn't concern me.
 

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Discussion Starter · #4 ·
Thank you for the reply zax. I have taken some time to let the car do its thing and learn over the last few days. I've recorded a few data logs now and have gotten some interesting results.

In this datalog I have a DAM drop from 0.5 to 0.37 pretty quickly. I dont see anything that seems to be out of the ordinary

In this one I have an increase from 0.688 to 0.750. Again nothing out of the ordinary that I can see.

The rest of my logs look like they're almost meaningless with everything looking normal data wise.

Today I had to drive around a lot, I started today at about 0.25 with no CELs, took the highway for about 30-45 minutes and managed to get it up to 0.75. Normal driving did not affect the DAM however I was getting "fake" readings of -3.5 FKL consistently at 2PSI of boost or less with 0 FKL at no throttle. The -3.5 wouldn't move the DAM and was really throwing me off. I've started noticing that from a stop, the car feels like its bogged down until it gets to about 30% throttle. It feels like it wants to stall but doesn't until it finally starts making more power.

These are a few things I've noticed throughout the week. If you have any ideas for diagnostics or what to log to help find more reason as to why my DAM keeps dropping and climbing outside of the normal range that would be fantastic.

As always, thanks for the help.
 

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Discussion Starter · #5 ·
Quick update, I saw a youtube video where this guy cleaned his MAP and MAF sensors. I decided to do the same and got a pretty significant reduction is dam dropping and FKL overall. I was averaging about 0.250-0.3 after a few days of normal driving on a reflash. I reflashed after cleaning those 2 sensors and now I've been hovering around 0.6-0.75

I've continued to drive the car to let it learn more and haven't had too many problems with the dam dropping really harshly. When it does it takes a couple pulls at like 3-5psi of boost and it will jump back up to 0.68-0.75 but it doesn't like to go further than that.
 
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