The problem is simple: an exposed filter under the hood is not a cold-air intake. Even with only about 0.7 psi of filter drop, feeding the engine 135-degree underhood air instead of 60-degree outside air crushed air density from 100% to 83%. On the test engine, that took output from 661 horsepower to 550. That is why we build the Banks Ram-Air Intake System as an enclosed cold-air intake. Intake performance is not just about pressure loss. Air temperature changes density, and density is what the engine makes power with.
The discussion begins with air density as the controlling baseline for engine output. Under the stated starting conditions, the air density is treated as 100 percent, with the air mass quantified at 76.4 pounds of air per thousand feet. Using that density and those ambient conditions, the example engine-a naturally aspirated 427-would produce 661 horsepower at 7,000 RPM.
The engine in question is described as a relatively small but capable 427 cubic-inch naturally aspirated combination. Its quoted 661 horsepower figure is not presented as an abstract estimate, but as the expected output when the engine is supplied with the baseline air density established at the start of the explanation.
The focus then shifts to a common intake layout mistake: placing the air filter under the hood, exposed to engine-compartment heat. This arrangement may appear attractive in magazine photos, and it is often assumed to be low restriction because of its open, simple layout. The example shown is essentially a filter mounted openly underhood rather than drawing cooler outside air.
The key point is that restriction is not the main problem in this case. The underhood filter setup is said to have a manifold pressure loss of only about 0.7 pound, representing the filter drop. That pressure penalty exists, but it is relatively modest. The much larger issue is the temperature of the incoming air.
Instead of ingesting 60-degree air, the engine is now breathing air at 135 degrees. That temperature increase dramatically reduces air density, and with it the engine's ability to make power. As a result, the same engine that would have produced 661 horsepower under the original conditions now falls to about 550 horsepower.
The explanation emphasizes that this is not primarily a pressure story; it is a density story. Higher inlet-air temperature reduces density, and reduced density means less oxygen mass entering the cylinders. In this example, the density drops from 100 percent to 83 percent. That loss in charge density directly explains the substantial horsepower reduction.
The conclusion is blunt: focusing only on low restriction while ignoring inlet-air temperature is a serious mistake. An intake that looks open and free-flowing can still be highly damaging to performance if it pulls hot underhood air. The top-line takeaway is that air temperature, because of its effect on density, can be more important than a relatively small pressure drop. In this example, the result is a loss of more than 100 horsepower, which is presented as an almost criminal misunderstanding of intake design.