Mine has been missing since i bought it(8 years ago). At least 60% of the 150's I look at dont have one. I doubt it makes a hill of beans difference to the airflow underneath the engine. My I/A is ok without it being there.
It is my opinion - based on no real scientific evidence - that omitting this baffle is a major contribution to the O-200's reputation for carb ice.
Leaving out that baffling allows pressurized (from outside) ambient air to flow around the filter and then carburetor - lowering the temperature of the body.
Fitting this baffling seals the carburetor in the nose bowl in the warmer air from the engine operation keeping the carb warmer - reducing the chances of carb ice.
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I also wonder about what it does to the designed air differential pressure between the cowl intake and the exit around the nose gear which an important part of the air cooling.
YMMV
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Hard to imagine the cost being justified for some if not all parts from Cessna.
Both those times conditions were OPTIMUM for carb ice formation. I dont think that the airflow around the cylinders is affected at all. That air overpowers the small surface area of the filter opening sans baffling easily I would think. Might be some stuff going on turbulence wise at the gear opening, but I doubt it has other than a very minor effect on the flow.