From Jon Steiger:
There are a couple of different "ram air" systems out there; one style
uses a "ram air" hood which has a scoop that is sealed off to the air filter
underneath. This is the most efficient type. The other style involves
a cover that goes over your air cleaner which has a one or two openings on
it (usually two) which allow a couple of hoses to be connected to it,
which are routed down to the front of the truck, either behind the grill
or under/in the air dam. The purpose of both systems is to feed the engine
with cool, outside air rather than hot under-hood air. The hood scoop
ram-air systems are more efficient because the air has a more direct
path to the throttle body, and the engine doesn't have to suck it in
from as long a distance. Also, with the "tube style" ram-air systems,
the air is heated as it travels through the hot tubes (which are generally
located right over the hot exhaust manifolds or headers). As the air is
heated, it expands, so for a given volume of air, hot air will have less
oxygen molecules in it than cold air; the more oxygen you can cram into
the engine, the more power it will make, that's why colder is better.
One qualification I should probably make is that neither of these systems are truly a "ram-air" system (on a street car, anyway); more like a "cold air" system. A true ram-air system will actually ram the air into the engine at a pressure higher than ambient. (i.e. its sort've like a mini-supercharger effect) However, you need a big scoop and you need to be going *very* fast. As a point of reference, a "standard" supercharger will generally make between 6psi and 9psi of boost. A salt flat racer going 300-400mph with an average sized scoop will generate about .25psi of boost from "ram-air" effect. So, the amount of "ram air" produced by a street car going 60-100mph is basically negligible. However, these "ram-air" systems are still good for street cars because, although they don't build any real pressure, they *do* allow nice cold air to get to the engine, so they will make power based on that alone. In my experience, adding a "ram-air hood" style system to a 318ci Dodge Dakota will drop your 1/4 mile times by about 3 tenths. A "tube style" will drop it 1 to 2 tenths.
From Bob Tom:
I am using a Ram Air Inlet system with two 4" ducts, one running to the left of the rad on the passenger side and the other running to fender hole by the battery (soon to be to the right of the radiator).
In recording sensor inputs during 1/4-mile runs, I found that this setup has a significant effect on barometric pressure readouts.
At the Mopar Nats (National Trailway Dragway is about 950' above sea level), I got a barometer reading of 29.1" while waiting at the tree. When I hit 25 mph, the barometer reading had risen to 29.4" and, at 39 mph, reached 30.3" (all in 1st gear). The barometric pressure then oscillated from 30.3" to 30.5" for the remainder of the 1/4 mile.
So, while air flow (cfm) may not be significantly increased with a ram air setup until speeds of 80-100 mph or so, a ram air setup does have a pretty big effect on the barometric pressure that the PCM sees.