Radiator/cowling flaps

[WTE_Galway]

The other point is to be careful of documented top speeds in online sites or mass produced coffee table books.

Even if they are not just wrong, they can often be based on what that aircraft can do at optimal altitude, air pressure, temp etc with no ammo or munitions, sometimes no guns or armor and minimal fuel running a new engine and high octane fuel (the Griffon for example needs 21psi boost and 150 octane to achieve its max rated performance) not always readily available in a wartime situation, especially to the VVS or LW.

In other words the "book" speeds are more like what you would expect from a plane prepared for the equivalent of Reno air racing rather than a typical military sorty during wartime.