Hi Widewing,
>That same chart shows the Fw 190D-9 with a max sea level speed of under well 340 mph ...
By the looks of the chart, it's data from Werk-Nr. 270002, flown with ETC504 bomb rack, "fixed" wheel doors producing slightly more drag than the normal ones, no engine cowling seal, Jumo 213A from first delivery that was substandard (lacking 400 m full throttle height), at take-off power (early rating, no MW50) of ca. 1.54 ata in low gear, ca. 1.68 ata in high gear.
So the curve appears to be from actual test data, but from a very early point in the development when there were still a couple of bugs to fix and the engine had not reached its specified power yet.
Regards,
Henning (HoHun)
