Replaced Heat Exchanger

Forgive me if this is a re-hash, but I didn't see the subject in recent threads. After investigating the usual suspects for overheating at RPM above 2,200, I investigated Torrensen Marine for a replacement heat exchanger as a back-up plan to cleaning out the original one on my 1986 32'. There I learned that a 3" upgrade was recommended over the original 2". I replaced the old with the 3" new, which fit snuggly under the exhaust riser elbow - but it did fit. With generally the same environmental conditions and RPM, my temp dropped from 175 to 160 F. In fact, pushing the hull to 7.2K and 2,500 RPM, the temp never rose above 160.
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Whispering Swan, Quantico VA

My temp gauge always reads about 140 degrees (not sure of actual teperature since these gauges can be off by 16 percent according to manufacturer), which has been a concern to me because diesels like to run at something like 180 degrees.