Really? In the article I posted it describes the device as doing precisely that, moving the max throttle plate position based on altitude.
Is it possible that more than one type of BCO system was used over the Spit's history?
No, it is not possible it is absolutley CERTAIN. Once, as I believe you and I have discussed before, the interlinking between boost, throttle and mixture was achieved, the system changed and became a fully throttle limiting device with a bypass circuit. Eventually, even RPM was linked unless you overrode it. The "override tit/lever/switch " disappeared and was effectively controlled by the gate. Nobody is suggesting that pressure/air is being vented from the individual cylinders.
What some people here are forgetting is that all the time the A2A Spitfire was in development so was the A2A P-51 Mustang. All the original Operating, Installation and Maintenance manuals for the SPECIFIC types of Merlins were examined in detail. The differences between them (and incidentally the diff between RR and Packard) were analysed fully. The Spitfire I and II, (Merlin III and XII) are not the same as a Spitfire IX , Merlin 66 or a Packard 266, nor are they the same in 1938 as they were in 1945. There are generations of difference which can be seen if you follow the progression from one to the other. In fact the Merlin IIIs and XII's had a similar number of modifications as the Spitfire itself had. My Merlin II manual contains supplements for gear up to a Merlin 66 (IIRC) as bits of those systems were used to update even a Merlin II.
There seems to be, still, a basic misunderstanding as to what Accusim DOES. It is not a "clever set of XMLs that trick FSX". It is the CREATION of the actual systems. The Merlin is "built" virtually and RUNS virtually. The throttle does not GIVE rpm and horsepower. The throttle lets a amount of virtual fuel (which varies with alt, temp etc) into the virtual system which then generates APPROPRIATE virtual horsepower given current pressure height, OAT and other factors (engine condition, compression, temps etc). It does not just tell FSX that at 20,000ft, 2400rpm this Spit flys at 240mph. It is deriving that performance. So an older engine in well maintained condition will likely give less speed than a run in, newish, immaculate engine.
In addition the Merlin you will see in the P-51 is NOT the same virtual engine with the "horsepower" factors fudged upwards. It is it's own engine, developed FROM the Merlin III, as was the original, and does include a fully detailed boost limit/mixture/altitude sensitive system. The increased engine hp come FROM the structure, size, arrangment and increased efficiency, not from "increasing the table of values".
I have, before, told the story of Scott and my conversation (I sure he won't mind me telling it again) re the Watts (weybridge) airscrew not giving proper performance above 13,000ft in beta. He looked at the prop and all seemed well. I showed him my schedule climb tests and they looked to have been performed correctly and were true to current performance...the answer, in the end was tracked to the BREATHING of the supercharger at above 12,000 to 13,000ft. That breathing ratio was changed and all of a sudden, the Spit performed correctly. THAT is the level of system modelling we are talking about...not, "Scott, I need you to up the climb figure above 13,000ft please".
The P-40 is not a merlin engine tweaked to pretend to be an Allison, it is an Allison engine built as such and modelled in the virtual world as such.
A quick google of "superchargers" and some figure tweaking is not at all how these models are built!! Scott and I would have spent more than 10 or 20 hours together on Skype over the ABCO-O on the Mk I and II. And that is just
discussing the research and how to implement it. I've lost count of the hours of research done before and after that!