You're cruising along through eBay, trying to weed your way through a myriad of adds to find that one part you need, when it finally hits you:
"What the hell is an IAT mod, and what exactly does it do to add 15, 25, or even 575 horsepower?"
Like the snake-oil treatments of old, you are curious, yet wary of the dubious nature of such a cheap item. Well, let me be the first to tell you that snake oils never disappeared. They mearly changed form and ended up on eBay.
While the myriad of IAT modifications may take several different forms, they all aim to accomplish the same results, summed up (rather poorly) in the following quote I pulled from an eBay auction on 9/20/03 (thanks to "bigtym2000" for placing this auction).
"By tricking the computer, we are able to make the reading of the computer shift to the right in the table, which will make it think it is under heavier load and increase the power. You have the proof right there!"
For starters, it's not smart to go about "tricking" (read: supply false information to) any motor's Engine Management System (EMU or ECU) unless you know what you are doing. To improve upon this bad start, our enterprising eBay auctioneer speaks of the ECU in a manner that no one unknowledgeable of fuel mapping could possibly have a clue of what is being said, and those of us that do realize he doesn't know anything about it either. To futher bolster the show of ignorance, he/she declares the mod will make the computer think the engine is seeing a heavier load. This is plain false. Proof, indeed.
So, just exactly what is an IAT, and what does it do?
To know what's going on here, you need to a basic understanding of how engines and their ignition systems operate. As most of us know, motors take air and fuel, add spark, and voila, they run. On a more technical angle, burning fuel is a chemical reaction. To keep things simple, I won't explain the chemical reaction equation of burning gas, C8H18 + O2 + N2 = CO2 + H2O + N2. I will tell you that, to completely burn any given amount of typical pump gasoline (unleaded) you need just about 15.1 parts air for each part fuel.
An engine is properly referred to as an air pump. That's what it does; it pumps air. Pulls it in, then pushes it out. Half-way through the process, it adds fuel and starts a fire, which greatly increases the temperature of the gases (air) inside the cylinder. An increase in temperature of a gas (air, not gasoline; gasoline is not a gas, but a hydrocarbon, which is a liquid) causes it to expand, which results in an increase in pressure if contained in a given space. This is what makes your motor go. The chemical reaction of air + fuel produces a lot of heat, which causes the gas (mostlt nitrogen) to expand, which pushes on the piston, turning the crank. You can correctly call an engine a gas-expander rather than a fuel-burner, as that's the actual part that produces power. With this fact in hand, it's easy to understand that generating power is a function of the amount of air you can get into the motor.
Once you've reached that nice point of 15.1 parts air for 1 part fuel, you've got enough fuel mixed with the air to allow for a chemical reaction of all the air with the fuel. This is known as the stoichiometric air-fuel ratio. To add more fuel would result in the additional fuel going to waste as there's no air to react with it. So, for more power, you need more air. Simply adding more fuel does not create more power. Adding extra fuel is known as "running rich", and ensures a maximum reaction occurs, producing maximum power, but also results in high levels of hydrocarbons emissions (not good for passing emissions tests), and washes the cylinders of oil (not good for lubrication), as well as dilutes the oil over time. Unburnt fuel will also damage catalytic converters. Too little fuel is called "running lean", and is good for efficiency and emissions testing, but can also cause detonation. How far rich or lean you run determines whether you are going to see benefits or detriments.
Back to that IAT. By now you've probably figured out that the car's ECU must figure out how much fuel to add to the air entering the cylinders for proper combustion. It uses several sensors to figure it out. The reason for this is that the amount of air entering the cylinder is dependant upon several factors.
1) Throttle plate position. This is what is moving when you depress the "go pedal". A round plate in a tube acts like a valve, restricting the flow of air the motor is trying to pull in.
2) Temperature of the air. Warm air is expandedair. The warmer the air, the more the expansion, the less molecules of gases you're pulling in.
3) Air pressure. Altitude effects air pressure, and so does weather, to a lesser extent. The higher the pressure, the more dense, and therefore more molecules of air you're gonna get in there.
4) Humidity. This factor is really small, though, and not really worth mentioning. Even the thickest fog has little impact on power production in a motor. Not even enough for anyone to bother testing the effects.
The IAT is the sensor that reports the temperature of the air going into the motor. As the temperature changes and therefore the density of air entering the motor changes, the ECU modifies the fuel maps to compensate. Well, some conniving schmuck figured out the Air Temperature Sensor's function, and figured that if you could get the ECU to add more fuel, you'd get more power, right?
Back in the early 80's, when Ford (for example) was trying desperately to comply with the EPA's CAFE requirements, there were a few cars, especially the Mustangs, which were running quite a bit lean to meet those requirements. They were busy converting from the old carbuerators to fuel injector systems, and were new to the game like most manufacturers. Some vehicles were quite obviously running lean (and had paltry power), to meet these new emissions requirements. If these cars had come with an IAT (older speed-density systems didn't utilize the IAT, IIRC), then the current IAT mods would have been useful. I remember you could do a few changes to the sensors on the mid-80's Mustangs that were good for 10-15 horsepower. Those cars needed it.
Back to today's cars, or anything 1988 or later. That's when the SAE standardized the engine control management systems, and OBD (On-Board Diagnostics) was born. OBD is the system that monitors the engine, determines it's condition, and makes changes to fuel and ignition to maximize efficiency and reduce emissions. These systems realize two major modes while driving. They are Part-Throttle and Wide-Open-Throttle (WOT). At Part-Throttle, fuel is injected into the air stream on the lean side of that stoichiometric equation to save gas and reduce emissions. At WOT, though, when you want as much power as possible, more fuel is introduced so that the air-fuel ratio is on the rich side, to ensure 100% of available engine power is available. So, with your average car, you are running at maximum efficiency until you mash the pedal, at which point the system is maximized for power.
This is where we find fault in the argument that making the engine run rich will provide more power. Your motor already runs rich when you depress the pedal for maximum acceleration. By fooling the ECU into adding even more fuel, you are simply adding more fuel for no gain. Whether you do this by changing the signal from the IAT, Mass Air Sensor, or Engine Temperature Sensor matters not. You can even do things to get more air into the motor, like adding a high-flow intake or exhaust, head porting and polishing, etc, and doing those mods will still have the same effect. Remember, the ECU senses the amount of air going into the motor. Up to a point, it can properly respond to air increases through modifications. Note that I'm talking about actually increasing the air getting into the motor here, not "fooling" or "tricking" something into "thinking" there is more air. Pretending it's there just doesn't work in physics.
So, with these IAT mods, you get no benefit, your gas mileage will suffer, and you can cause damage to your expensive catalytic converters. Yes, it can damage your cats. Fuel that is not burned in the cylinder causes the cats to run at a temperature that's higher than normal, and this will damage them over time. Anyone that has driven with bad Oxygen sensors and didn't replace them in a timely manner knows about this problem. When the O2's go bad, the ECU defaults to a slightly rich fuel mapping. Gee, where have I heard about that?
To put it plainly, these IAT and whatnot modifications are JUNK. All the explanations and renamings of electrical components (simple resistors magically become Timing Advance Resistors) in the world will not generate more power out of any motor. You HAVE to get more air in there to accomplish that.
The only exception to this is a car that is running overly lean from the factory. I've not ever heard of or seen a car that came stock running way at WOT since the mid eighties. Bad oxygen sensors could cause a motors to run lean, but that's a situation where you need to replace the O2 sensors, not try to run a mod around them. If you were to install one of these IAT mods and your car actually did generate more power, you've got a problem elsewhere that needs to be addressed. I'm talking about dyno'd power, too, not that seat-of-the-pants power. Butt-dynos are about as useful as Chia-pets.
In case you don't know, these people selling these "IAT" mods work for the same companies that sell catalytic converters. I SWEAR. Ok, I'm kidding, but it makes you think.
If you're still not convinced, save yourself $9.70 (based on average $10 eBay selling price) and do it yourself. Put simply, the air intake sensor has a particular resistance based upon the temperature of the air. Get a wiring diagram of your car and measure the resistance of your IAT at the coldest point of the day and again at the warmest point. This will show you what the resistor does in different temperatures. You could even measure in the summer Vs the winter. It doesn't really matter when you do it as there's no gain to be had. Just keep in mind that if you apply a resistance that is out of the expected range for the sensor, that annoying Check Engine light is going to stare you down every time you drive.