I'm at Castle Rock, which is ~6200' and my cheap autometer boost gauge shows 8.5-9 but I think it's not calibrated correctly because it shows 5.5 at safety boost. Anyway, at sea level the air is at ~14.5 psi so with the stock 9.5 psi boost the pressure ratio is (14.5+9.5)/14.5 = 1.655 well below the 2.0 rule of thumb number. At 7500' the we have ~11.1 psi so if you are only boosting 4 psi the ratio is (11.1+4.0)/11.1 = 1.36... The stock turbo should be able to boost ~7 psi at 7500' just by maintaining the stock pressure ratio of 1.655. We all know that stock turbo has enough headroom to boost 14-15 psi at see level so at least for me the math doesn’t work out. I guess somebody needs to educate me at why there's such high efficiency lost that the turbo can only maintain a 1.36 pressure ratio…
