ram air effect is fairly negligble at the speeds we are talking about, see article below. believe the biggest benefit of this intake is that it allows more air to flow through and as we all know power is a function of mass flow.
Carl
http://www.bmwe30.net/cgi-bin/datacgi/database.cgi?file=articles&report=view&ID=00004&Section=03
The Myth of "Ram Air" Induction
Air filter modifications are a typical performance upgrade for BMW engines.
The addition of a free-flowing cone filter is the most common. Considerable verbiage has been written on this subject but I feel confusion remains.
This article addresses the perceived benefit of a "rammed air" intake system.
It is common knowledge that a car at speed can provide a ramming effect to the air entering a hood scoop. Common sense would suggest that this additional pressurization of the intake air reaching the air filter would result in increased engine performance, but is this added pressure really worth much? I have looked into this question and the short answer is no.
Engine power - pure and simple - is related to how much air-fuel mixture an engine can use and how high the compression ratio. Increase mass flow through the engine or increase the compression ratio and increased power output will result. The reason supercharging or turbocharging can dramatically increase engine power is because it in affect transforms a small-displacement low-compression engine into the equivalent of a large-displacement high-compression engine (yielding high mass flow / high effective compression ratio). But couldn't a "ram-air" intake yield some benefit? A poor man's supercharger so to speak? Let's investigate.
The ram-air pressure created by the forward motion of the car is proportional to inlet air density and the square of the velocity. Using conditions at 1000-ft elevation and 70F gives the following results:
Speed "ram pressure"
77-mph ( 0.1 psi )
109-mph ( 0.2 psi )
134-mph ( 0.3 psi )
155-mph ( 0.4 psi )
173-mph ( 0.5 psi )
Hence "legally" all one can expect to gain is a 0.1 psi increase in intake pressure (note this is a far cry from the 6-psi+ pressure increase that turbo's and superchargers produce). This increase in pressure is added to the ambient pressure of 14.17 psi raising it to 14.27 psi. But this is a less than a 1% increase in inlet pressure. This trivial increase in pressure would increase mass flow through the engine by a similar amount - yielding a 1.2-hp increase to a 170-hp engine. Not much of an increase and certainly not worth chopping up your Bimmer's hood or front end to capture this "ram air" effect. Truth is the reason some air-filter mod's yield a discernible power increase is not due to intake air pressure but temperature. Intake air temperature has a much more dramatic impact on inlet air density (and hence mass flow through the engine) than small changes in inlet pressure.
Here is an example:
Let's assume the factory airbox with its inlet behind the headlights draws in 70F air. A modified car with an exposed cone filter might breathe 110F warm air under the hood. Comparing the difference in air density with these two conditions reveals the warm air is 7.5% less dense than the cooler air and would consequently reduce engine power by a similar percentage - 158-hp vs 170-hp - a 12-hp reduction! In fact if we wanted to restore the density of the 110F air to that of the 70F stream, we would have to increase inlet pressure by 1.07-psi. So looked at in a reverse fashion, cooling the air 40-degrees is the equivalent of 1-psi of boost. Or another way, the cone filter would have to have 1-psi less pressure drop than the factory filter to yield equal power. Now 1-psi is a huge pressure drop for an air filter.
I have never personally measured this but my best guess would be a value of around 1-inch-H2O ( 0.04 psi).
So the bottom line... Forget about "ram air". Use a low-pressure drop filter. But most importantly, duct or box off the filter so that it will draw in the coolest air possible. With this last point in mind, the area behind the headlights seem optimal (closer to the road surface would likely be warmer).
