I am trying to track down this issue. I have a 2018 mustang with a 2200sl..3.6 pulley. I usually was hitting around 10psi and 1.8 ish air load in similar temps. Now I am only getting 5 or 6 psi according to my boost gauge and air load seems to not go over 1.5 or so.. it seems to flatline at a 5000 rpm or so. I changed my paxton supplied belt with a green belt and tightened it as far as it could go on the tensioner holes..also pressure tested the pipes and connections. Checked my vacuum lines and bypass valve as well. Log below (not in hp tuner format) from a quick 3rd to 4th gear on the freeway...
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