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I’ve got a 2012 STI, only mods are GRedry BOV, Cobb AP, invidia r400 exhaust and invidia catless DP. Monday I installed the catless DP, and got a stage 2 tune from Delicious Tuning. I loaded the tune yesterday evening, and am getting lower boost numbers than I was when I had the stock downpipe and Cobb stage 1 tune, and I can feel the car is noticeably slower. I’ve heard a lot of different reasons the car could be limiting the boost levels, but wanted some opinions. On the Cobb stage 1 tune with stock DP, I was peaking around 18lbs of boost at WOT. Stage 2 with catless, I’m lucky to see 15psi-16psi. Didn’t know if there was something I’m missing here, just doesn’t make much sense to me. Thanks!!
 

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you'll want to talk to delicious tuning, you might be able to post a log here, but what is the car targeting for boost? if they decided to target 15-16 psi, then you have no issue other than you would want to talk to the tuner to get more out of it.

if you are targeting more boost, but not hitting it, you likely have a boost leak, and a simple boost leak test will show where it is coming from. fix that and move forward from there.

the above assumes you are talking about peak boost, it's important to note, that the stock turbo will NOT flow enough air to hold 18-21 psi at redline. typically the stock turbo will peak boost around 18-20psi on a ots map, and most tuners will put it in the 19-21psi range for a protune, but that will be the peak, and as you approach redline the stock turbo will fall off to anywhere between 14-17psi up top, as that is all it can flow (how much it will hold up top will vary that little bit pending multiple factors, such as target boost, wgdc, ambient temps, supporting mods, etc).

it's also worth noting that these cars were not designed to run a BOV, but a BPV (Bypass valve). i assume you mean that you have a Greddy bov, which typically makes quality products, but aside from noise, even aftermarket BOV/BPV's will not offer any performance benefits, as the stock one will hold what a stock turbo can put out all day long. the stock one can even hold a good amount of boost from aftermarket turbos before failing.
 

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Something to consider is that psi is essentially a measurement of baxkpressure, not flow (in a sense). With a more free flowing exhaust, you could be moving more air through the system with less resistance, so to reach the same output goals you would end up with a lower psi. Does that make sense?

exaggeration. But think of flowing the same volume of water, one using a garden hose and the other using a sewage drain. Same amount of water, but one is way more free flowing piping.

not as familiar with the STi, but can you look at airflow lb/min on the datalogs?
 
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