General IAT vs MAT

[cornboy]

I had a bit of a one off question.

Why do speed density setups use intake air temperature instead of manifold air temperature?

Is it because it's just easier to install and use a IAT sensor than it is to place it in the manifold, or perhaps some engineering issue with early MAT setups that made it unfeasible at the time, and by now it's tradition to just have an IAT sensor.

It seems like a MAT sensor would be better because you can get a lot closer to determining the density of the air going into the engine than a IAT would provide, since you don't have to account for intercooler efficiency.

 

[Mech Addict]

The temperature should be measured in the same place as the airflow is measured (MAF), since that properly represents the total mass of air going into the intake system (and presumably the cylinders). The ECU can then correlate that amount of air with a proper A/F ratio and deliver that metered amount of fuel for desired combustion characteristics. Now a manifold air temp could yield useful information for tuning purposes to see how effectively the intercooler is working, especially when tracked against boost pressure (which will always raise the air temp in proportion to the amount of compression). So the density could be measured in the manifold using temperature and pressure, but the the volume of air cannot easily be determined there, and that is critical in determining fueling.

 

[cornboy]

But wouldn't a MAT sensor account for that anyway. It's not like a manifold is going to leak air out, so if you know the temperature and the pressure of the air directly before it goes in, and you know how quickly the valves are going to open, then you know how much air is going into the engine.

 

[Mello]

The temperature should be measured in the same place as the airflow is measured (MAF), since that properly represents the total mass of air going into the intake system (and presumably the cylinders). The ECU can then correlate that amount of air with a proper A/F ratio and deliver that metered amount of fuel for desired combustion characteristics. Now a manifold air temp could yield useful information for tuning purposes to see how effectively the intercooler is working, especially when tracked against boost pressure (which will always raise the air temp in proportion to the amount of compression). So the density could be measured in the manifold using temperature and pressure, but the the volume of air cannot easily be determined there, and that is critical in determining fueling.

This is only true on flow based systems but is incorrect for pressure based, speed density. MAT & IAT are supposedly one in the same. MAT would be the important measure. When an IAT is installed in a proper location the difference should be negligible. MAT improperly installed can heat soak under some situation like restart after a short shutdown. Many OEM speed density setups have the temp sensor in the intake plenum (e.g. 90's Honda). I usually see IAT in the per throttle body on MAS/MAF system converted to speed density for accessibility & ease of install.

 

[cornboy]

Okay thank you man, just trying to learn more.

 

[Mech Addict]

Now I'm curious. I'll have to do some research to better understand the operation principals of speed-density. I'm always glad to learn, and appreciate the correction.