P-51D vs. P-38L with equal pilots, unless the P-51D is going to Boom and Zoom with a significant energy and altitude advantage, the P-38L is going to win. In a co-Energy fight, the P-38L will be able to gain the advantage by keeping the fight in the vertical, where it excels far better than the Mustang. If the P-51D tries to turn, the P-38L again has the edge.
Basically, the best chance is for the P-51D to come in fast from an altitude advantage and try and BnZ or if you're like most P-51 drivers, wait until the Lighting is engaged with another bandit and then come in for the pick.
The P-51D driver also has to be careful when he makes his high speed passes. The good P-38 drivers will know how to wear you down and bleed enough of your energy to either steal the advantage from you and match your energy state and get you as you go vertical or force you into a turn fight and get you that way.
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