Aircraft peformances, is this joke ?

[winny]

Originally Posted by jf1981

In my opinion, we should have the right performance without use of bost cutout, up to my understanding. Octane grade 100 was probably also not available in 1938 for the Mk I prototype. The prototype itself fit with a wooden fixed pitch propeller reached 349 mph TAS at 17'000 ft.

I'll try to get more accurate datas from one of the big books I have related to Spitfire, a very good and complete one indeed. Those are interesting lecture.

Edit
I find no information supporting that the max speed of Mk I & II were measured with more than the rated power at 6 psi of boost.
Apparently the Mk I had 1030 bhp at altitude (either 16,250 or 17,000 ft ?) and Mk II 1150 bhp at 14'500 ft, source "The story of the spitfire" book.

I can add some more to this, the figure that is usually quoted for a MK I Spitfire is 362mph at 18,500. This was referring to K9787, the first production Spitfire, in 1938.
Between then and the BoB there were some substantial modifications that lowered the top speed. A 73 lb armoured plate, bullet proof windscreen, 3mm of light alloy covering for the top of the upper fuel tank, installation of the IFF etc..

The AUW of K9787 was 5,819 lb, the AUW of a BoB period Mk I was around 6,115 lb.

There were also aerodynamic penalties, the IFF aerial was reckoned to cost 2 mph, the windscreen cost upto 6 mph. The top speed of a fully equipped Spitfire in Battle of Britian trim was closer to 350 mph at the same altitude.

Sources - Spitfire The History and Alfred Prices Spitfire in Combat