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It generates thrust in some cases
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Great theory that does not work out in most attempts. None of them produced any thrust AFAIK but a few meredith designs did have the effect of reducing drag somewhat. It requires very specific parameters which were not present in most installations. In fact, the radiator ducting on the P-51 was headache and plagued by duct rumble. That is flow separation in the duct and a source of high drag.
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the real difference is made by engine management.
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You are correct in that the throttle makes the largest difference in any engine on making it to TBO but that applies to any engine.
The real difference in the Merlin is made by valve springs, better coolant connectors, oil feed improvements to the cam, better bearings/races and some good machine work on the heads, just to name a few.
In the case of the Merlin, there are specific upgrades to improve reliability that are highly recommended if you want the motor to last. Those upgrades overcome the shortcomings of the design.