Practical/reliable max. speed of Ki-27?

[GuerraCivil]

During the years I as well as others have learned to be skeptical on technical details of aircraft represented in books and web sources.

One example is Ki-27 "Nate" - some sources refer that the max. speed of the plane was as much as 460 - 470 km/h at the altitude of 3500 - 6000 meters, at 2000 meters 440 km/h and at sea level 420 km/h.

I´m a bit skeptical on these numbers. From Soviet test flight results with a captured Ki-27 I have read that it did make up to 400 km/h at sea level (I do not know speed results of Soviet test flights at altitudes, but presumambly the max. speed was lower than the supposed 460 - 470 km/h at 3500 - 6000 meters).

Also from one Japanese combat report I have read that Ki-27 had difficulty to catch Blenheim Mk IV bombers over Burma (and it was not easy to escort their own Ki-21 bombers due to the speed of the latter).

As Blenheim Mk IV made it at maximum about 430 km/h at 3500 - 4000 meters and was difficult catch for "Nate", I find it difficult to believe that Ki-27 would have been as fast as 460 - 470 km/h at the same altitude. Perhaps the max. speed was at best about 420 - 430 km/h at 3000 - 5000 meters? To catch the Blenheim the Ki-27 had to dive on it.

One thing to be considered: to my understanding Japanese pilots (and Thai pilots) flew Ki-27 almost always with opened cockpit or had the sliding part of canopy removed to improve visibility. This probably had also some slowing effect to the speed of the plane?