Along those lines, Powerflow exhausts for the O-320 and O-360 produce noticeable gains in power and efficiency, if numerous pilot owner reports can be trusted.... We must remember, the intake plumbing upstream of the throttle body, and exhaust are often made by the airframe manufacturer and include things like restrictive tiny airfilters, complex plumbing and quiet, restrictive exhausts. I like to joke about my plane being down on power, because it is. I put a tuned header on it, and the improvement was obvious. Lopresti was able to show a 7Kt gain in cruise speed with a ram-air setup.
A Mooney M20J that I rented a few times had a ram air knob in the panel. Pull it back and it opens an alternate direct air intake, bypassing the air filter. The POH said to use only in cruise flight in clean air. It didn't do much, increase in MP was barely detectable on the gauge.