I am curious if anyone can explain the water pump pulley size difference in 300D and 300SD non-turbo and turbo.

Generally speaking we would all agree that the smaller diameter pulley spins the water pump faster.

I cannot say for sure how different the radiator capacity is between
non-turbo and turbo, or between turbo 123 and turbo 126.

What I have learned (searching the junkyard and comparing my own two cars)
is that the 300SD which all appear to be turbo have a smaller water pump pulley. I do not think I saw any with larger water pump pulley.

My 1980 non turbo 300D has a larger pulley and a turbo 300D at the junk yard
also had a larger pulley.

A smaller pulley that spins the pump faster should move more water,
and faster water may not have time to cool if the radiator is smaller.

So I was thinking that 300D's in general have smaller radiators that need more time to cool the water, so slower water using a larger pulley.

But since I have a choice, and if a smaller faster pulley might keep the engine cooler then I would like to try that.

Looking for input from either someone who knows the answer to the different
size pulley , or anyone that can explain the theory/science of running cooler.

From my observation (not measuring) it appears that the crankshaft pulley
on all cars was the same diameter for the ring that runs the water pump.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

The major benefit is that the fan turns faster. The coolant cycle time does not materially affect the overall heat transfer capacity. Decreasing the cycle time just reduces the input/output temp differential, which is generally a positive thing.