Cold Air Intakes
Have you purchased a cold air intake kit for your vehicle yet? If so, you are among the many thousands who (for various reasons) have bought into this technology. But does it really do what you thought it would?
The most common reasons people give for installing a Cold Air Intake are:
Better Fuel Efficiency
More Horsepower! Now this one is true, but might be a little misleading. Cold air intakes can result in actual gains in power, but not necessarily in the way most people are expecting them. The key that is typically missing in the mind of the average consumer is that the only time power is increased is when the engine is at Wide Open Throttle (WOT). For those who don't know what that means, WOT is basically when the gas pedal is pressed all the way to the floor.
Some are now thinking "Wait a minute! The cold air intake lets cooler, more dense air into the engine. It also has less restriction than the stock setup. This lets it 'breathe' better, and therefore is more efficient." What can I say? You're right...at wide open throttle. What many people don't realize and what many others simply forget to consider is that there is a throttle valve connected to your gas pedal that is constantly restricting the air flow into the engine. The only time it is not restricting the airflow is when it is wide open...hence the term WOT. Some will now say "But it will still breathe more efficiently even at part throttle." You must then remember why there is a throttle valve in the intake to begin with. The throttle valve determines how much air enters the engine, and the engine computer (for fuel injected engines) determines how much fuel to mix with the air. Several factors influence how the computer determines this, but the result is basically the same: a fixed Air/Fuel Ratio will be supplied to the cylinders. This should be obvious since all modern vehicles have to comply with EPA emissions standards.
That means that more air = more fuel = more power = more speed. Now that's good if you're at WOT, but how many of you drive at WOT even 1% of the time? The fact is that you almost always drive in part throttle...because you're usually limiting the vehicle's acceleration. That is the key. Any time you limit vehicle's speed/acceleration with the gas pedal, you are actually limiting (restricting) the amount of air entering the engine. This defeats the purpose of the cold air intake, high-flow intake, high-flow filter, etc. It is only effective when you're at WOT.
One thing should be clarified here...when I refer to WOT, that can also be a little misleading. The condition where these devices increase power is not always exactly at WOT. The throttle valve on a car's intake system is typically a butterfly valve. The more it is closed, the more restrictive it becomes. The least restrictive condition is when it is wide open. Keep in mind that the intake system is sized to operate the engine through a full range of speeds and load conditions. If you get full power from your engine at 5,000 rpm, the throttle valve is sized allow air to flow through it at that speed. Now how much air do you suppose is flowing when you're only turning the engine at 2,500 rpm? This is the situation you have when your're pulling a heavy load (trailer, etc.) in a high gear. This is the situation I find myself in with my pickup when pulling a travel trailer. In 3rd gear at 60 mph, the intake manifold pressure and mass airflow don't change at all from about 70% open throttle to 100%. Why? The throttle valve is effectively at WOT for that speed. There is no measurable resistance being offered by the valve when at 70% open. Does that mean that a cold air intake would help when towing heavy loads? Probably not. The reason is that when you have the condition described above (you have more pedal, but it doesn't change anything), the intake system (filter & piping) is providing very little air resistance. You know this because the primary form of resistance--the throttle valve--isn't even doing anything from ~70% and up. That tells you that there is not as much air flowing through the intake as there would be at high rpm (like if you down-shifted), and therefore your air filter creates a very small amount of resistance by comparison. NOTE: This is assuming that the air filter is clean. If it is clogged up, that's a different story.
In summary, the cold air intakes and high-performance filters are good for improving full-pedal engine power. Practically that means that you are buying a modification that helps you pass (at full-pedal) or race. Don't expect fuel economy gains...sorry.