I am here to follow this post with some sources that proves the F6F-5 can do 340+mph at sea level.
The source F6F-5E ACP proves that F6F-5 can do 343mph at sea level with bomb and radar racks, and 347/389mph at sea level/ACA for clean condition.
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In July 1944 F6F-5N ACP chart, it showed 340mph/391mph at sea level and ACA respectively.
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In FAA manual, the Hellcat MK.II showed 340mph at 1500ft. Considering the neutral stage of the Hellcat's supercharger, the brake horsepower would only increase to 2250hp for altitude under 1500ft, i.e. the sea level for Hellcat MK.II would be at least over 340mph, and it also fits the F6F5E ACP chart.
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In ww2aircraft forum, there's a whole post about the performance calculation.
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However, compare them to the F6F5 performance in the game, we see a huge gap between the game and the historical data:
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The Hellcat's performance is clearly underpowered, with WEP it can barely do over 500kph, and with military power, only 440kph at sea level. Its speed at low blower stage is obviously too slow, and the chart also shows an unrealistic gap between military power and combat power. In realistic, the military power generates 2000hp at sea level, above ACA, the military power would also generate a top speed around 380mph.
Thus I would like to see the performance of F6F-3 and -5 be remodelled in the future.