Thanks for the reply.
1. Ok. Although I'd understood the main effect of RAM was to increase critical alt of that supercharger stage. The other Merlin engine power curves I've looked at typically show power for both a min and a max RAM on the same line until crit alt at the min RAM is reached when that curve starts to decay with alt whilst max RAM curve continue upwards. Not seen one where the min RAM power exceeded the max RAM power.
2. I'll try to restate the question. On the P-51D_15342 level speed test graph, engine power is 1410 BHP from whatever height it switches to high blower (say 16K) until 26K. There is a video that does a great job of explaining that - power is constant as the engine is constrained to 67" MP.
Now look at the P-51D_15342 climb data, specifically the data points I posted. The power is given as:1580 BHP @ 16000 ft and 1500 BHP @ 19000 ft. Yet the same constraint of 67" MP applies to both these data points.
So, on level speed we have the explanation of the constraint that does not appear to apply to the climb power. That's why I'm puzzled...
3. Ref your comment "see chart for the climb test" are you referring to P-51D_15342 climb test or a DCS chart. I've not found a climb test chart for the DCS P51D.
I do have a Packard engine chart for V1650-7 (a bad quality scan) that shows high blower power for 72", no RAM. Drawing a line on that chart for 67" the no RAM power would be around 1375 BHP @ SL, rising parallel with but below the 72" curve to around 1500 BHP @ 19k, then decaying with alt. Well, the 1500 BHP at 19K agrees perfectly with the P-51D_15342 climb data at 19k. However, I don't really understand how they got 1580 BHP at 16K. (Unless MP>67".)
Be great if you could help me understand all this.