Start with the basics. Make sure your valves are adjusted, your compression is good, and your timing is correct. Hook up a boost gauge and make sure you're building 12PSI. If not, adjust your wastegate until you do. You should be able to pull all the way to 4300RPM or better before you feel the power start to drop off.

Check your ALDA line routing and make sure you don't have a split or a clog somewhere in the pathway. Leaking boost pressure or a restriction in the line to the ALDA will result in a lack of fuel and a reduction in power.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)