Carl Chitwood wrote:
If your engine is already operating within the proper range, it is possible to make it run too cool by improving the cooling too much, particularly in cooler climates.
The lip and larger plenum inlet holes are required by the Thompson STC; However, the STC was conceived with an improperly timed O-200 that was running hot. The dill rods (the previous owners of the STC, not Randy,) hacked the cowling up before they figured out that their engine was out of time.
Cessna 120/140 cowls and baffles are well designed, and do not need to be modified in order to properly cool a few more inches of engine displacement. I installed the Thompson STC and did not add the lip or hog out the plenum holes, and the only problem I ever encounter is getting the oil temp to hit 180 at OAT's less than 70F.