Thanks. A lot of good info there regarding engine choice. The Rolls Merlin powered Canadair DC-4M would certainly be faster than the standard Pratt R-2000 powered Douglas DC-4/C-54. Although the Rolls 620 series Merlin engine had fewer CI in displacement it provided a cruise power of 1,135 hp @ 2,850 rpm @ 23,500 ft in high blower. The Pratt R-2000 only made 700 hp @ 2,150 rpm @ 21,500 ft in high blower in cruise. Of course the the Douglas DC-4/C-54 was not pressurized so wouldn't go that high to spare the pax hypoxia. So the Rolls Merlin was a better choice certainly than the Pratt R-2000. Looked a lot better too. Fuel consumption in cruise for the Pratt R-2000 was 0.42 lb/hr/hp versus 0.45 for the Merlin in cruise. R-2000 was more economical per hour but mainly due to its lower power and, being slower, would have burned more in total.
The high revs and higher boost contributed to the Merlin's lower TBO.
The Bristol Hercules powers matched the Merlin but its weight (760 series) was 600 lbs more so that would have been 2,400 lbs more per aircraft. The R-2000 was the lightest at 1,595 lb, the Merlin 620 series @ 1,720 lbs and the Hercules 780 series @ 2,345 lbs.
Interestingly, Canadair did produce one Pratt R-2800 powered version. Believe it was for Canadian Government VIP transport. Would it have been a DC-6M?
Starting any piston engine at negative temperatures was interesting. The Bristol rep in New York (we were working Britannias) told us of trying to operate Bristol Freighters up North in Canada. He said you could not even pull a cold-soaked Hercules with its sleeve valves through unless it was preheated. Our South American customers used to regularly burn out starters and induction vibrators or ice the plugs trying to start engines in a much milder New York. No experience with Merlins. Were they harder to start?