I doubt adding a larger diameter pipe will do much of anything, MB diesels don't suffer from low horsepower output or excessive back pressure. All you would get is more noise, not something I personally desire.

A NA diesel moves the max volume of air at about 1500 rpm. For a turbo, airflow is only limited by compressor speed and intake crossectional area, the intake manifold can pretty much be ignored (and MB makes very high flow efficient manifolds anyway).

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!