IAT Resistor Mods vs Performance Modules vs Real Chips


What are those infamous IAT resistor mods and performance modules and how do they compare with real EPROM performance chips? Need a guide to try it out by your self? Find out here…

Some background

The ECM is able to do such a gigantic job by executing a software created for that purpose. This software is called and executed every time you turn the key to start your car. The software is specific for each engine and each ECM and allows to easily control the engine in a way that previous mechanical (fixed) systems could not. Modern cars no longer use carburetors. Instead, they use what we know as Electronic Fuel Injection or EFI for short. This EFI system is controlled by an onboard computer called the ECM or Engine Control Module. The EFI system replaces the carburetor functions entirely. It does the job by sensing environment variables such as atmospheric pressure, air temperature, ignition advance angle, coolant temperature, altitude, air density and some other variables that allow such a system to precisely determine the exact amount of fuel needed to be injected to keep the engine running smoothly and in optimum conditions.

The EFI software is composed of the (1) executable software which does the calculations and movement of data and the calibrations part, which stores the tables of parameters for running the specific engine. These parameters are the ones that mark the limits and operation ranges, like fuel injection amount, calculated in injector opening time in milliseconds, ignition advance, calculated in degrees, maximum speed the engine is allowed to rotate in RPMs, maximum allowed vehicle velocity in MPH or KM/h, and many other things. These are precisely the parameter that chip tuners modify to accommodate new engine hardware, like cold air intake, different cams, boosted induction, new rim size and so on, or simply to optimize the existing hardware, like lowering engine temperature by activating the engine fans at lower temperature, modifying ignition timing to make the most out of premium fuel, etc.

The modification of the engine parameters is done by modifying values in the EPROM memory that contains the engine stock program. At least that is the most effective way. But there are also the “piggybacks”. These piggyback alter such parameters indirectly. They instead, intercept the signals coming out of the different engine sensors and modify such signals before sending them back to the ECM. This way, they “coerce” the ECM on changing the way it responds to the modified sensor signal. If for example, you modify the signal coming from the vehicle speed sensor, you can change the reference used by the ECM, in a way that it “sees” the sensed speed as being lower than it really is, allowing for higher vehicle speeds as the ECM moves up the speed limiter.

In a similar fashion, people started trying to modify some signals in an attempt to “fool” the ECM into producing more power from the engine by injecting more fuel. The most used is the “IAT resistor”. This is, a resistor connected in some way to Intake Air Temperature sensor (IAT), to “fool” the ECM into “thinking” that incoming air is cold, hoping that the ECM will inject more fuel to compensate for the “colder” air. ECMs are programmed to compensate in some way when the air is colder, as colder air carries more oxygen for being denser. More oxygen may induce an unbalance in the air/fuel ratio, so it is balanced again by increasing the fuel delivery.

The thing is that this does not necessarily happens that way, or at least not with most ECMs. ECMs are smarter each time and many ECMs take into account not only the incoming air to start doing changes in fuel delivery but take also into account other readings to do so. That is why many “resistor mods” will not have any effect, but still, scammers sell it by fooling people and offering horsepower gains as high as +100hp. The fact that it does not work is why many other scammers decided to “try luck” tampering with the oxygen sensors instead. You know, there was a time some years back that Google reported that “+20hp” was the automotive term most searched for. But then, they moved to higher numbers.

This is an example of a “real chip” or EPROM chip used to store the software of the ECM:


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Example of changes made in the EPROM chip by modifying the parameter tables, called “maps”:


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IAT Modification scams


These are some examples of external IAT sensors, which are mounted separately from the MAF sensor.


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The most used IAT sensors are internal or part of the MAF sensor instead, so to modify those, you will need to tamper with the MAF sensor connector or the wires coming out of it.


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The main focus of this article is to allow you to do your own modification to the IAT in case you are curious and do not end up giving away any money to those scam artists.

The modification was at first a simple resistor with a couple of wires for you to install it in the IAT sensor with a blurry diagram. Then they started putting the resistors in fancy boxes to look like an elaborated product, but still, you would see only two wires coming out of it. Some scammers used a dummy connection and added two wires more, making it four wires. Finally, when the scammers knew that many people found out about the “resistor in a box” scam and when  people started looking inside the fancy boxes to find just a single resistor inside, they started to put some random circuit inside and the two wires were soldered to anywhere in the circuit that measured the desired resistance. It was hilarious! Not only the $0.25 resistor with two wires in a box,  or the elaborated that the box was or putting a random circuit to make it look like it is complex equipment, or how many dummy wires they added but the fact that people paid for it! Believing their claims of “+50hp” or more! From $40 to $100 for that scam! And if you opened the box to see if it is a resistor, they tell you that you voided the warranty even when they scammed you and it was a resistor in a box! LOL.


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Do It Yourself

Well, let’s go back to the experiment. The IAT sensor is nothing more than a thermistor. That is; a resistance that changes its ohmic value as temperature changes. The thermistor used in the IAT has a negative coefficient, which means that the higher the temperature, the lower the resistance in ohms.

That being said, as you may deduct so far, if you want the computer to “think” that air is colder than it really is, then a reading of a higher resistance from the IAT is what we are looking for. Many IATs go as low as 200 Ohms when hot around 100 degrees Fahrenheit and go up to 5000 – 7000 ohms at low temperatures like 30-50 degrees Feranheith. This is only an approximation, as all IAT thermistors have s slight different response, but always with a negative coefficient.

So you will need to know the response of the IAT sensor of your car, which is how many degrees directly convert to how much resistance and what temperature in degrees you want to move up the response range of the ECM.

The first one, the IAT response, while it could be in the documentation of the sensor manufacturer or the car manufacturer, the easiest way to know is by using a low-temperature source like ice or a cooling no-residue spray and directly measure the temperature and the resistance. The second one, the range to move up the response of the ECM is completely up to you for experimentation.

Example: You want to make the ECM “think” that no matter the fluctuation, the incoming air temperature is always 50 degrees colder.  So if the IAT resistance is equivalent to 80 degrees, the ECM will “see” it as 30 degrees. If it goes up to 90 degrees, the ECM will “see” it as 40 degrees. That way, if the ECM is programmed to compensate only by air temperature, you will have more fuel injected along the whole range of temperatures.

Now, you will need the resistance equivalent value of the IAT sensor for those 50 degrees that you want to subtract so you can pick a resistor of approximate value and put it in series with the IAT sensor. The easiest way this can be achieved is by putting water in a cup with a thermometer in it and the IAT sensor touching the water. Add ice until the thermometer goes down to 50 degrees and then measure the resistance of the IAT. Let’s say it measures 4600 ohms, then you may use a commonly available 4700 ohms (4.7k) resistor in series with the IAT.

IAT Series resistor

The above procedure is easy if the IAT is separate from the MAF. Otherwise, you will need to use a cooling spray as a source of low temperature for getting the needed equivalent to 50 degrees resistance value. If using the spray, make sure it leaves no residue or you may damage the MAF sensor. If using the water cup on a separated IAT, make sure you dry it completely before installing it back in the car.

Once you get the value and find a suitable resistor, just connect it or make a harness to hook it in series with the IAT. That way, every change that the IAT does in resistance due to temperature fluctuation, it will be added to the series resistor you picked. After this, you will need to do tests in a dynamometer or some sort of setup to know if it really works. Don’t just use your “feeling of it” or any qualitative observation as it could be subjective. You need quantitative measurements here. If nothing is enhanced about performance or even economy, just think that you did not have to buy the “resistor in a box” scam to find out.

A typical resistance value widely used is 4.7k (4700 ohms), but if you wish to be near to the exact value of your car model, you will need to do the experiment detailed above.

Below is an example of a 4700-ohm resistor. Color bands must be YELLOW-VIOLET-RED. The fourth ring can be GOLD for 5% tolerance.

On the other hand, if this does not work, it means that your car’s ECM does not use only the IAT sensor for increasing fuel injection. You can be sure that it does it in some way, just not with the IAT sensor alone. Have you ever wonder why your car moves better at nights, especially on cold nights? Precisely because the ECM detects more oxygen contents based on some parameters and injects more fuel. Modern cars base such calculation on temperatures, but also on air density by measuring the actual mass of the incoming air. That is what the MAF sensor does. The thing is that some older cars used to use an Air Flow Meter (AFM) instead of a MAF (Mass Air Flow sensor). Also, some Honda cars do not use air flow or air mass at all, but engine load by using a MAP sensor or “Manifold Absolute Pressure” sensor. The difference obviously is that while the AFM senses air flow, no matter the density of the air and the MAP senses engine load by vacuum or negative pressure, none of those two check the air density. The MAF senses the mass of the air, which tells the ECM how much dense is the incoming air. Measuring the mass of the air is an indirect way of measuring oxygen contents in that air. The denser the air (more mass), the more the oxygen contents should be. So that nice accelerations at cold nights or cold weather, in part are because of the low temperature and we can take care of that by fooling the IAT sensor. But the other reason is the detected air mass and there is nothing we can do about it unless you inject nitrous oxide and that, is another story.

Variable Resistor

You may also replace the IAT with a variable setup in case you wish to adjust values on the fly:


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Other Factors

There are some other factors to take into account if the resistor does not work.

  1. Too much fuel: It is true that more fuel injected may produce more power, but only to some extent. If the fuel/air mixture gets too rich, you start to lose power instead of gaining it. Make sure that your modification(s) does not cause excessive fuel injection.
  2. Engine temperature: Remember that you are trying to “fool” the ECM into “thinking” that temperature is colder, but it really is not. So if you live in a hot area, if the engine temperature goes over a certain level, it may cause the ECM to lower the ignition timing to prevent engine damage. Lowering ignition timing has an immediate power loss side effect.
  3. Altitude: Altitude matters! The higher the altitude over the sea level, the lower the oxygen. Enough oxygen is key for producing power in internal combustion engines. More than 1000 feet above sea level will produce a palpable lack of power. The closest to sea level altitude, the more the oxygen contents of the air.

Example of scams being sold all around


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