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 yourself? Find out here…

Some background

The ECM can do such a gigantic job by executing software created for that purpose, and this software is called and executed every time you turn the key to start your car. The software is specific to each engine and ECM and allows for easy control of 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 (1) executable software, which does the calculations and movement of data, and (2) the calibration 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 speed in MPH or KM/h, and many other things. These are precisely the parameters that chip tuners modify to accommodate new engine hardware, like cold air intake hardware, different cams, boosted induction, new rim size, and so on. This can also be done simply to optimize the existing hardware, like lowering engine temperature by activating the engine fans at a 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 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 of directly modifying data values, they 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 into 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.

Similarly, 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 the 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, which is denser, containing more oxygen by volume. Since “more oxygen” may induce an unbalance in the air/fuel ratio, the ECM increases the injection volume by means of opening the injectors more, increasing the fuel delivery to compensate for the “colder air” it is “seeing” through the modified sensor.

The thing is that this does not always work that way, or at least not with most ECMs. ECMs are smarter each time, and many now take into account not only the incoming air temperature but also other readings. That is why many “resistor mods” will not have any effect, but still, scammers sell them, offering horsepower gains as high as +100hp. Since people are getting conscious about this, scammers decided to “try their luck” now by tampering with the oxygen sensor instead. The term “+20hp” used to be the automotive term most searched for on search engines in past years.

This is an example of an EPROM {“real”) chip, used in your ECM:

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Example of changes made in the memory of 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 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. Then they started putting the resistors in fancy boxes to look like an elaborate or complex product, but still, you would see only two wires coming out of it. Some scammers used a dummy connection and added two more wires, making it four wires. In the end, when many people found out about the “resistor in a box” scam by looking inside the fancy boxes, scammers started to put some random circuit inside the box, and the two wires were soldered to anywhere in the circuit that measured the desired resistance. It was hilarious! Not only the few-cents resistor with two wires in a box, or enclosed in a box, 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 “+20hp” or more, paying from $40 to $100 for that scam should be treated as a fraud.

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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 have deduced 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 Fahrenheit. This is only an approximation, as all IAT thermistors have s slightly 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 are directly converted to ohms and what temperature in degrees you want to move up the response range of the ECM.

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 measuring the temperature and the resistance. The desired 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 when the resistor is added in series. If it goes up to 90 degrees, the ECM will “see” it as 40 degrees and so on. That way, if in theory, the ECM is programmed to compensate by incoming air temperature alone, 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 to subtract 50 degrees from whatever the real incoming air temperature actually is.

IAT Series resistor

The above procedure is easy if the IAT is separate from the MAF sensor. Otherwise, you will need to use a cooling spray as a source of low temperature to get the needed equivalent resistance value of the temperature that the spray produces. If using the spray, make sure it leaves no residue, or you may damage the MAF sensor, and if using a spray, bear in mind the temperature will mostly be lower than 50 degrees F. If using the water cup on a separate 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, 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 be happy that you did not have to pay for the 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 only use 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 wondered why your car moves better at night, especially on cold nights? Precisely because the ECM detects more oxygen content based on some parameters and injects more fuel to compensate. Modern cars base such calculations on temperatures, but also on air density by measuring the actual mass of the incoming air. That is what the MAF sensor does. Some older cars 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, neither of those two checks the air density. The MAF senses the mass of the air, which tells the ECM how dense the incoming air is. Measuring the mass of the air is an indirect way of measuring the oxygen content in that air. The denser the air (more mass), the more the oxygen content should be, at least in normal conditions. So those nice accelerations on cold nights or cold weather are because of the low temperature of the incoming air.

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: More fuel injected may indeed 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) do not cause excessive fuel injection.
2. Engine temperature: Remember that you are trying to “fool” the ECM into “thinking” that the 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 above sea level, the lower the oxygen content by volume of air. Enough oxygen is key to producing power in internal combustion engines. More than 1000 feet above sea level will produce a palpable lack of power. The closer to sea level altitude, the more the oxygen content of the air.

Example of scams being sold all around

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