heat soak and IAT relocation

[tequila mike]

I pulled this off a Dodge forum.
It clarified a few issues for me.
Hope it helps someon else.

IAT heat soak issue? Do the colder readings cause the ECM to add fuel?

What this modification does is improve the accuracy of the IAT reading. This results in your engine running more efficiently and ensures that you aren't loosing power due to detonation/knock and spark trim from a higher than actual temperature reading. A more accurate reading will cause the ECM to add the correct amount of fuel for the actual air temperature (ECM will calculate an air density) entering the combustion chamber.

Minimizing the effects of Heatsoak

Heatsoak Defined (http://cjbfire.com/Heatsoak.pdf)

Understand, heatsoak is not fixed because of this mod. Heatsoak is inevitable, especially once you shut off your engine, however, while your engine is operating, heatsoak is reduced through engine coolant. Measures we can take during engine operation are are using a 180 t-stat to start the cooling process sooner, reducing the fan settings, and using engine oil coolers. While at the track, you can use the 25 Cent Fan Mod to turn on the fans to cool off the engine when it's shut off.

We minimize the effects heatsoak has on engine performance (specifically the air entering the combustion chambers) by insulating the IAT sensor from engine materials that store heat and pulling air that is cooler than engine compartment air from the fender. We also can reduce the effects of heatsoak by using materials in the engine compartment that have low thermal conductivity and a high specific heat (e.g. rubber, plastic). A final way we reduce the effects of heatsoak on the IAT sensor is by relocating it away from the engine block.

These same principles listed above can be seen in the redesign of the 392 air intake tube and air box. The new manifold is a plastic composite material, the new air box is made of plastic, the sensor is insulated better than the previous and reads from the middle of the airstream, the sensor is located away from the engine block (the highest heat source), and the throttle body is thermally insulated by the new manifold that is a plastic composite material.


Effects of higher or lower sensor reading than actual air charge

With the heatsoak issue, if the sensor reads higher than the actual air going through the tube, it'll subtract fuel resulting in a less than efficient burn and also you'll be more prone to engine knock/detonation. This engine knock causes the ECM to reduce spark advance to compensate thus reducing the power output.

On the flipside, if you take the sensor and let it read ambient air temperatures (lxforum sensor relocation thread did this), this won't be representative of the actual air going through the intake tube. As a result, you'll have too much fuel added and a less efficient combustion. This is also the reason why too much fuel is added as a result of a sensor that gets dirty or oil on it.

The argument has been that the IAT sensor relocation mod tricked the ECM into adding more fuel when in reality, as MikeyChallenger has shown in his thread 20+ lbs/ft torque, allowed for the ECM to read the actual air temperature flowing through the tube rather than the heatsoaked temperatures of the materials surrounding the sensor, resulting in a more efficient burn. This also helps to explain why the transmission shifts are smoother and the acceleration lag is nearly gone because of the new sensor.

Also for reference, An air temperature charge pulled from the fender well is about 4-5 degrees above ambient and the air temperature charge pulled from inside the engine compartment is 10-15 degrees above ambient. At highway speeds, I've shown that an unshrouded short ram intake can read 1-2 degrees difference from a fender pull intake (http://www.challengertalk.com/forums/f1 ... res-32385/).