Not to hijack your thread, but here's info on CAI's from Ricksfreeautorepair advice - Let me know if you'd me to remove this info and start a new thread....
You see cold air intake systems advertised all over the net. The manufacturers make wild claims about improved horsepower. How can that be? Well, let’s take a look at the theory behind a cold air intake.
An internal combustion engine works like a doctor’s syringe. Pull the piston back and it sucks in air (or medicine). If the syringe is a 25cc syringe, it will suck in 25cc’s of air. But if the air is cold, the volume may be 25cc, but there will actually be more molecules packed into that same 25cc’s. That’s because cold air has a higher density than warm air. (think in terms of more air molecules per gallon, as an example). We can assume that the extra load of air molecules also includes more oxygen atoms.
So, here’s where we are: If you suck cold air into your cylinders, you’ll get more oxygen atoms than if you suck in warm air. Great. Or is it? Modern vehicles have Mass Airflow Sensors (MAF) that are specifically designed to determine the mass of the air being sucked into the engine. The MAF measures the temperature of the incoming air and determines its mass through an algorithm based on the wind chill across a heated wire. The MAF knows the temperature of the wire and the temperature of the incoming air. By watching how quickly the heated wire cools off, it can determine velocity of the air. Get those two variable and it now knows the density of the air.
The Computer Knows All
Why am I belaboring this point? Because even though you’re sucking in dense cold air, the MAF and the powertrain control module (PCM) KNOW ABOUT IT. In pre-computer days, cold air tricked the carburetor, because the carburetor metered gas solely by air velocity. Not so with computerized vehicles.
Since the computer knows the temperature and density of the air, it also knows that
COLD AIR makes it harder to vaporize gasoline. Raw gas won’t burn—it has to vaporize first. That’s why car makers suck heated underhood air into the intake manifold—to assist with fuel vaporization. Install a cold air intake and you forfeit that advantage.
Now let’s talk about what is and is not a true cold air intake.
If the system you’re considering is pulling in air from behind the radiator and inside the engine compartment, that is NOT a COLD AIR INTAKE system. It may be a shiny air intake, but it’s not a cold air intake. To be a true cold air intake, it must draw outside air from IN FRONT of the radiator, under the front bumper, or from inside the fender/wheel well.
More Horsepower is Really Horsepucky
Let’s look at the power ratio curve supplied by one manufacturer. I will not identify the manufacturer, but their actual dyno test speaks volumes about the true effectiveness of their system. This power curve is for a 1998 Chevrolet Corvette. The sales literature brags about an 18 horsepower increase. But if you look at the graph, you’ll see that they don’t achieve that increase until the engine is screaming at 5,500 RPM.
Pardon me for asking, but how often do you drive at 5,500 RPM? Let’s get real here, look at the graph at 4,000 RPM. See much of an increase in HP? It’s miniscule. In other words, unless you’re planning on spending your days at the racetrack, you will NEVER SEE that increase in horsepower.
But wait, didn’t I already debunk the benefits of a cold air intake by saying that the MAF knows all and compensates for it? Yeah, I did. So how DO they achieve an increase of 18 HP? The literature says that the larger air filter reduces air restriction through the air filter. It also says that they tune the intake tube to reduce internal air resistance. Sounds good, huh? Both of those claims are true, as far as they go. To reduce the resistance of air going through a filter, you either have to use larger holes, or more of them. So these manufacturers opt to add more holes. You really do need that extra air flow at 5,500 RPM. You will NEVER need that extra airflow at normal driving speeds.
Which leaves us with tuning the intake tube. Car makers do a horrible job of tuning the intake air duct from the air filter box to the throttle plate. In fact, they often use pleated rubber duct—that’s a horrible obstruction to air flow. So why do they do it that way? Because it DOESN”T MATTER what happens to the air BEFORE it reaches the throttle plate. They make the pleated duct larger diameter than necessary to compensate for the increased resistance. What DOES matter, however, is the tuning between the throttle plate and the intake valves. In that area, car makers have made tremendous advances. In fact, that’s precisely why many upper intake manifold are now made from plastic—to reduce air resistance and tune the airflow.
Finally, let’s talk about fuel economy. You cannot get more horsepower from a cold air intake system and still get better fuel economy. No stock engine is designed for maximum fuel economy at 5,500 RPM, period! Remember, if the MAF sees denser air, it will add more fuel. There is simply NO FREE LUNCH with a cold air intake.
