I straight piped my car, there was definitely a big increase in power for me, across the entire rpm range. I just cut out the mufflers and welded some straight sections in. Straight piping reduces back pressure caused by the mufflers, which allows the turbo to spool more quickly, making boost at lower rpm's, and therefore more power at lower rpm's.

I also have my ALDA maxxed out, and boost adjusted up to 12lbs

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1982 300D Turbodiesel, daily driver. Mods so far: Fram 8038 paper filter, 4 brake light mod, Gen II w126 (front) rotors/calipers, boost turned up to 12lbs, non-egr manifolds, water/methanol injection, 4-speed manual
1980 300SD Turbodiesel, project car, nearly ready to hit the street

1974 240D, New paint