Knock While Cruising, None While In Boost

gathermewool

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I posted this in the FA20DIT tuning thread, but figured I'd post it here as well. Info for initial WOT run: 14 Forester XT (FA20DIT direct-injection 2.0L turbo w/ high-torque CVT) Cobb Stage 1 91 Octane 93 Octane Fuel (New England) S# 2nd "gear" (CVT) WOT to "redline" Datalog: https://datazap.me/u/gathermewool/lo...2749&mark=1934 // Info for highway cruise: I just got back from a 2-hour drive with the AccessPort mounted and visible for the entire drive. At ~ -4 to 0 PSIG boost (cruising stead at 65-70 MPH with cruise control on) I was seeing upwards of -9deg feedback knock correction (FBKC = instantaneous correction) and up to -2.Xdeg of fine learned knock correction (FLKC = long-term learned correction). The dynamic advance multiplier (DAM) mostly stayed at 0.938 - 1.000, but fluctuated frequently lower, with a minimum DAM seen only once of 0.750. I'll post the log up later today or tomorrow showing these cells, but suffice to say I'm concerned. I'll be under the car sometime this week to search for something loose, like a heat shield, but would love to hear some other advice on this!
 
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Almost seems like bad gas. I've never seem my DAM drop below 1.0, ever. Have you noticed less than 1.0 before? Did you use a different station than normal or a non top-tier station?
 

gathermewool

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Originally Posted by mcwilly
Almost seems like bad gas. I've never seem my DAM drop below 1.0, ever. Have you noticed less than 1.0 before? Did you use a different station than normal or a non top-tier station?
Good point: last fillup was Mobil @ a very busy station right off of the highway. I usually stick with TT, but occasionally fillup at a busy Stop-and-shop station. HOWEVER, why only while cruising and NOT while at WOT? I saw no knock at all when > 1PSIG of boost. From what I've read, the new ECM's (FA20DIT) are a lot more dynamic than the EJ's. I never once saw < 1.0 DAM in my '08 STI and fuel trims of ~ +/- 4% were considered OK, whereas ~ +/- 8% is the new norm. I still have a lot to unlearn from my years of logging the EJ and much to learn about my FA20DIT. My logs usually looked good, except for some > 8% fuel trims, so I never bothered to extensively research fluctuations in DAM and such.
 

gathermewool

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Originally Posted by Rand
how reliable is a canned cobb tune vs a tuner working with logs?
I never had a problem with their OTS tunes over many years with my '08 STI. I never had an issue until just now with knock in the FXT. Further, I'm running the 91 octane map and using 93 octane fuel. So, there's the (historically) pig-rich Cobb map AND two points of octane margin to account for, PLUS the fact that I'm not having knock issues while under heavy load/boost.
 
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Yeah I'm not sure the differences for monitoring your engine. I was always told if the DAM ever drops below 1.0, get rid of that gas. I thought Mobil was top tier, but it still could be a bad tank. Typically when I see the knock learn it is at a pretty specific RPM, I don't always notice but when I do I will try to remain at the same spot and the ECU adds the timing back by .35 till it gets back to zero. The only thing I can think of why it would be cruising only is if you were using cruise control you might have been sitting at the certain point in the RPMs that was suffering most from the bad gas/knock. Everything scales so it might just have been a perfect storm of fuel pressure, RPM, boost, low octane... When you went into boost, you may have been adding enough pressure to mitigate the issue. I was also told that knock/knock learn of -2.8 and below were typically okay, but -9 is pretty high, I haven't ever seen anything over -5. But again we have different engines and you have a CVT so everything may not apply the same. My first guess would be the gas, try to drain it if you have a vacuum pump and get some good stuff in there. Or if you don't have a way of draining it, try adding some higher octane in, 93 or 94 to make up for the potentially low AKI gas. Other than that I might try a reflash or a different/new map, are you using a off the shelf map? Edit: reading comprehension isn't up to par. I see you're using 93 on the 91 map, which should be added protection... odd.
 
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Originally Posted by mcwilly
Almost seems like bad gas. I've never seem my DAM drop below 1.0, ever. Have you noticed less than 1.0 before? Did you use a different station than normal or a non top-tier station?
What I was wondering ?
 
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Could the mixture be richer under boost but leaned out considerably during cruise? I think this could explain the vary in octane requirement of your engine.
 
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I once owned an 07' WRX. I modded it putting a Perrin downpipe and a Cobb tune on it. I bought the Cobb AccessPort from Perrin and it included stage 1 and 2 from both Perrin and Cobb. All 93 octane maps. Perrin sent the AccessPort first and was backlogged a few weeks on the downpipe. The Cobb maps SUCKED in both stage 1 and stage 2. Even after giving it time to learn just in case, they sucked. The Perrin maps transformed the car in both stages. Stage 2 with the Perrin downpipe and Perrin map was awesome. It made the car run from a stock 14.4 at 94 MPH to a 13.65 1/4 mile at 101 MPH. I later took the car to a WRX tuner with an AWD dyno and was a Cobb tuner. He tried to get more HP over the Perrin map but couldn't. He got a little more torque but his map made the car boost too low and like it was having a heart attack in the mid range, so I flashed the Perrin stage 2 map back on. The Perrin stage 2 map made the car pull hard until redline and felt much better to drive.
 
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Originally Posted by skyactiv
I once owned an 07' WRX. I modded it putting a Perrin downpipe and a Cobb tune on it. I bought the Cobb AccessPort from Perrin and it included stage 1 and 2 from both Perrin and Cobb. All 93 octane maps. Perrin sent the AccessPort first and was backlogged a few weeks on the downpipe.
I thought I read somewhere that the Cobb DP tapers to fit the stock midpipe gasket better, whereas Perrin and a lot of others do full 3" all the way down. Unless the Perrin DP is identical to the Cobb one, it makes sense the Cobb OTS map wouldn't work as well with other branded parts. Cobb maps are made for Cobb parts. A custom map is always preferred, but OTS works for thousands as long as you have the right parts and fuel. I ran Cobb OTS for 4 years before going flex fuel protune without issues (mind you all my parts were Cobb).
 
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