I noticed a drop in maximum boost too when I threw a glass pack on my car. Only mine was about 1/2 a psi. My theory is since the waste gate valve has less pressure behind it is easier for the exhaust gasses to push it open.

If you know how to adjust your boost on your turbo that fixes it.

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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons )
white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

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