The Merlin XII was also cleared for +12. Not that I disagree with you regarding top speed at S/L at +12 vs +9lbs. boost.

I wonder what klem will say after he will test the new FMs. Sounds like good news though so far.

I think it was discussed and shown that +12 combat rating was cleared for the Merlin XII only well after BoB, though there was some sort of take off rating that was pretty much useless for anything else but to shorten takeoff.

The BoB was just starting in July 1940.

The Spitfire II Pilot Notes, dated July 1940, say 12lb boost can be used to 10,500ft. A boost of 12.5lb can be used for take off to 1,000ft.

http://www.zenoswarbirdvideos.com/Im...pit2Manual.pdf

I'm afraid zeno is inaccurate here. I think I have the Spitfire II pilot notes without any amendments, at least the amendment list is there and empty. This should be the original version dated July 1940. In there, the engine limitation for all out level is 9lb boost.

It appears that the all out level increase was at latest added with amendment list 31 / L, at a date that is not known to me.

The zeno version is missing the amendment list and does not shed any light on the "when".

It doesn't matter. If you had a 109 on your backside or gave chase then you'd hammer the thing. I'm pretty sure that most 19 year olds don't drive 'according to approved ratings' but it doesn't mean it can't do it

It does matter, because it was necessary to modify the automatic boost control in order to have it deliver 12lb when in override. It would always have been possible to override it, but in the worst case you could end up with boost exceeded 30lb and destroy the engine instantly.

Indeed JtD.

And there you have it....12lbs boost up to 1000' for take off or for a maximum of 3 minutes otherwise.

I'm sure nobody is suggesting 3 minutes was used to get to 1000'

A different system, bongodriver. It was limited to low altitude and could not provide boost at higher altitudes. Simply put, it had a full throttle altitude of 1000 feet, which would give you less than 9lb at altitudes exceeding 4000 feet. It's really just a take off setting.

Not really.

Spitfire is only the same speed as the Hurricane at altitude now (320mph at 18,000 feet), it is still very slow.

At sea level they are all good though.