Merlins...

The -3 and -7 Merlins were both built by Packard and were used in the P-51B/C and D models respectively. The -9 was also Packard-built, but was used only in the P-51H, few of which were actually built. The only difference between the -3 and -7 Merlins is the gear ratio in the dual-stage blower; the -3 being better suited for high altitude performance, and the -7 better for overall performance. The differences between the -3/-7 and -9 are many. The -9 sees a return to the same blower gearing of the -3 Merlin, but the blower itself is vastly different, with more efficient fueling due to a cleaner flow path. The oiling is also much more efficient in the -9, which makes many of the -9 pieces and parts desireable for racing. If I'm not mistaken, I believe the flow of coolant in the heads is also different than the -3/-7. The nosecase of the -9 is also desireable for air racing due to the 420:1 "slow" ratio versus the 479:1 ratio in the -3/-7. So in summary, the -9 is a later model, more advanced version of the Packard Merlin, and thus has many characteristics which make it a good choice for racing. However...

You asked about "transport" Merlins. The "transport" Merlins were manufactured by Rolls-Royce primarily for use in the Canadair Northstar version of the DC-7, I believe. Due to the rigors of such flying, the 500, 600, and 700 series transport Merlins were built with extra rigidity and strength, particularly in the case, heads/banks and nosecase, which also happens to have the "slow" 420:1 gear ratio. Has the light gone on yet? :-) The only problem with the transport motors from a racing standpoint are that they were fuel injected, and when high manifold pressure is the ultimate goal, carburetors are the best way to go. Sooooo...

Today's most powerful racing Merlins are really a combination of all the best qualities from the -7, -9 and transport Merlins. I'll let you figure out what goes where. :-) Hope this helps...

TPS