By all means, make sure that you are getting every bit of the 90something horsepower you are supposed to have. First, check/replace your fuel filter/strainers (there are three...a dirty or plugged fuel filter will keep a diesel from producing full power). Second, check for timing chain elongation and correct the problem if there is any (plenty written in this forum about chain elongation). Third, confirm that the valves are adjusted and have proper clearance (an oft overlooked thing on MBZ diesels with lots of miles). And, finally (and particularly if the timing chain is changed) confirm that the IP timing is correct.

As for radical power increases, physics and design are working against you. These are short stroke, small displacement diesels. They're made to be efficient, spin lots of revs and, unfortunately, not make a lot of torque....the oomph you feel starting off from a light. Your engine is normally aspirated (no turbo). Air and fuel make diesels go; with the engine doing the sucking instead of a turbine-powered compressor doing the blowing, only so much air is ever going into that engine and only so much power (or, oomph) is going to come out. Thus, a bigger exhaust will have little effect. Finally, these cars were designed to be efficient, utilitarian vehicles, something they excel at. Face it, when was the last time you saw a running, 1975 Buick, Olds, or Cadillac that got 25+mpg and was still as solid as the day it came off the production line??

My advice, look at (or have a knowledgable indy look at) the things I've listed above and get them fixed if necessary.

The OM617.912 engine is a remarkable piece of engineering, capable of running in excess of 250,000 miles with only routine maintenance. Additionally, the W115 chassis was well ahead of it's time with 4-wheel power assisted disk brakes and fully independent suspension. Appreciate your 300D for what it is and adjust your HP expectations accordingly.

FWIW, Converting to a manual tranny improves driveability but it is a project well beyond the bolt on level of performance enhancement.

Take care.

__________________
Never a dull moment at Berry Hill Farm.