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Discussion Starter · #1 · (Edited)
Hello all,

I recently installed the SXTH downpipe and for a week the car sounded incredibly good to me. It was loud, felt like high flowing, and the revs "screamed".

After about 300 miles with the dp, the car now sounds like a vacuum cleaner. N mode used to have a clear audible difference from the normal engine mapping with the valve open. There's now no difference.

I've had this happen before when I purchased the gfb Mach 2 recirculating bov. At certain times the car would sound extremely loud, good, angry. But just like my situation with the SXTH downpipe, it regressed to sounding like a vacuum cleaner.

I know with many bolt ons, like downpipe, intake, anything that changes the air fuel ratio is suggested to be tuned. Does the car detect air. Fuel changes and adjusts itself accordingly? I've been told here that the ECU learns of changes after one hundred miles.

I've checked my boost hoses, solenoid hoses, they're all connected. And when I floor it I still see 10+ psi.
 

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Discussion Starter · #3 ·
When you put the car in N mode and look in the passenger side exhaust pipe, is the valve actually opened?
This is something I've been meaning to check. Thanks Quirky, I'll take a look at it.

I can say when I was at the car wash I pressure washed the tips a bit, I didn't point the gun towards the hole.
 

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Discussion Starter · #4 ·
It wasn't the culprit.. I'm looking into replacing my h2os sensors on the dp, I'm looking into new spark plugs, etc. Disconnecting the battery didn't do anything.

The characteristic of the car has regressed. N mode doesn't make any audible growl, bass like it used to.

From custom mode with normal engine and sport plus exhaust setting to pressing the n mode, there used to be a distinct instantaneous noise and power difference.
 

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No the ECU only operates within its preset parameters. It will adjust to specific changes in the engine environment, outside ambient temps and altitude. It will not adapt, this is a urban legend. It does not have any AI capability, contrary to what people think.

If you’re getting only 10+ boost @ WOT there’s a problem.

If you’ve had a change in the way the exhaust sounds and lost boost along with a loss of power, I’d check the 02 sensors and see if they have gotten grease on the tips. Do you have a data logger? Do you have a piggyback tuner or tune?
 

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Discussion Starter · #6 ·
Thanks for the reply RedVN and your knowledge :) .

My car seems to be operating well now. The highest PSI I see now is 15.2 psi from the car's dashboard/tough-screen readings. When I'm flooring it, the boost reading looks like its approaching max (close to 20), but the reading on the car's touchscreen says a little bit less. The 15.2 psi came after adding forge's FMIC and Takeda's intake. Prior to the FMIC and Takeda intake, the highest PSI i've measured was 12.7 psi with the stock fmic, stock intake, and catless downpipe.

2 days ago, 1/7/2021 I just added the afe scorcher (Inspired by @Ghost feedback with his AFE Scorcher) + the HKS M45XL spark plugs. Really enjoy it :-] I still get nearly the same mileage, what the hell?! Lol. 26-28 high way cruise .

I am pleased with the Scorcher. It did something good to my car's throttle and boost control. I can cruise in 6th gear, normal mode, without the boost kicking in all the time like it did prior to the piggyback. Sometimes I wonder if I have compression issues, but my PCV catch gets like a loogie's worth of blow-by during a total of 2 oil changes and tons of 120-140 mph pulls (legally)...
 

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One last thing, we're finding that fuel octane ratings, can cause changes in boost. For the 2.0T Theta II VN engine to produce overboost properly, the car needs to be run with 91+ octane top tier fuel. If you're buying fuel from a cut rate fuel station or have gotten some old fuel in stations that aren't constantly filling there tanks, this can cause a problem with peak boost.
 
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