And what power did this boost yield? 2000hp? I find it strange that the boost time is the same. Either they improved cooling somehow or the 18lbs boost was unnecessarily restricted and could have been used longer?
It often was used for longer. There's lots of annecdotal evidence that pilots ran wep for longer. Early Hurricanes and Spits during the BoB were limited to 12 lbs boost, for 5 mins only. I saw one pilot account of using 12 lbs for takeoff, and leaving it at that throughout the flight (deliberately)
By 10 minute cruise you mean the engine temp was nominal or below nominal? Was this procedure limited to 10 min or could the Spit cruise for longer at mil power and still use the boost?
From the wording of the test, as well as it's purpose (to determine if cooling was adequate at 25lbs boost) the 10 mins at maximum cruise was used to warm up enough to get an accurate figure from the combat power (25lbs) runs.
Don't forget the plane would have been warmed up on the ground first, and then climbed to test altitudes.
The test is a cooling report by the A&AEE to determine if Spit cooling is adequate for the new 25lbs rating. I should think they tried to replicate real life conditions, ie long cruise, period of combat etc.
Don't forget, even the cruise portion of the test was done with rad flaps shut, whereas in normal conditions the flaps would be open.
I find this strange, couldn't the boost be used indefinitely then? Or did the piston heads overheat?
I don't know what the limiting factors were. I suspect engine life was one of the main ones. I know that with extended running at 18 lbs or 25lbs, the exhaust stubs would burn out.
If so why didn't they overheat faster at 25lbs than 18lbs, as one would expect?
I should think they did, but not by that much. The Merlin used the fuel for charge cooling, and the greater fuel flow at 25lbs would have helped reduce temperate. But even then, I would expect an engine running at 25 lbs to break sooner than an engine at 18lbs.
Did they strengthen the Merlin (new model)?
No, same Merlin. The early Merlin 61 engines couldn't take 25 lbs, but the Merlin 63, 66 and 70, all used in the Spit IX from late 42/ early 43 onwards could take 25 lbs. I'm not 100% sure about the V-1650-3, though I think it could run at 25lbs, but the V-1650-7 certainly could.