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I'm on my second 337, both normally aspirated. To me your numbers seem low. Ground speed is hard to correlate to airspeed without knowing both winds and altitude, but here are my numbers, about the same for both aircraft ('69D, '73G):
Cruise 5,500 ft, 23.5rpm/21mp, ground speed=140 kts, no wind.
Notice it's knots, so about 160 mph.
The low manifold pressure comes from "babying" my engines. At about 500 ft I go to 25/25 and I never touch the throttle again until the descent (when I get to cruise altitude I reduce rpm from 25 to 23.5). So mp is what it is. At 5,500 ft around 21, at 6,500 ft around 20, etc.
Ernie
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