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'Ram air' style scoops can help, though if sized incorrectly large amounts of drag can be manifest; negating the benefits of the scoop. Furthermore 'speed stacks' implicitly accelerate air into the combustion chamber for additional pressure effect. If you're mathematically inclined you can figure out the 'ideal' size of the 'ram air' scoop as well as installation of the 'speed stacks' (though technically speaking they're nozzles).
Use Bernoulli's equation for the incompressible flow regime (basically anything less than 0.7 Mach) which gives you pressure at a specific speed.
P=1/2*rho*v^2
If you're hoping for increased operating pressure you can then add atmospheric pressure to the dynamic pressure calculated above and get the stagnation pressure (or the point where air's velocity becomes 0 in the scoop). The proper sizing of the scoop should ensure that all nozzles get the same pressure of air--no sense in getting increased pressure on one nozzle while the other is still stuck in ambient pressure. Doing this will also help you understand at what speed the 'ram air' scoop will become most effective. If you have the means drag force profiles can be plotted and the ideal scoop criterion can be selected from both Bernoulli's equation (will give you the operating speed for effective pressure) and the drag profile (at what point is the scoop a hindrance as opposed to a benefit).
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