I think factory boost was supposed to be 9.5 - 11 psi. You can go as high as 13, but beyond that you are exceeding the design horsepower of the engine and risking shortened life and/or blow head gaskets. By now, I'd not be surprised to see weak springs in the wastegate controller limiting boost.

The 6-7 psi number is about right for gasline engines -- even with intercoolers, you risk igniting the fuel at the injectors if you go any higher, and serious spark knock will occur as well. Mind you, that is LOTS of boost for a gas engine -- my sister has a Volvo 740 turbo that I repaired last summer -- couldn't figure out what my brother-in-laws complain was with the power. Felt like a road rocket to me!

The 603 is set for about 13.8 psi, still a mechanical wastegate. The TDI engines have electronic wastegate control

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!