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120 mph is about 104 knots. From the graph, the pitot pressure at 104 knots is about 0.49 inches of mercury.
One inch of mercury is 0.491 psi, so the ram pressure at 120 mph is 0.49 x 0.491 = 0.24 psi
Since atmospheric pressure is around 15 psi at low altitude, this is about a 1.6% increase from the ram air intake. A 40 hp engine might be expected to make an extra 0.64 hp.
At 200 mph, ram air starts to get important, and you get a pressure of 1.48" mercury, or 0.72 psi, so a 180 hp motorcycle engine could make an extra 8.64 hp.
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