Have you bypassed the switchover/over boost protection valve on the firewall (if you vehicle has one). They get clogged and restrict boost. My 1987 300TD was a 22 second 0-60 until I did that.

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000