IAT sensor understanding and tuning question
 

Dear turbo (and S/C) freaks.
I recently installed megasquirt (DIYPNP) in stock my 1.6 NA as I am about to put a m45 supercharger in it. However, I have not (yet) installed the IAT. I simply uninstalled the stock air flow meter (AFM), unplugged the plug, and installed my filter there instead of the AFM. I also installed wideband.
When I first got the car startet, it was running VERY rich. Therefore I had to scale down my VR table quite a bit before it was drivable, but now it runs OK with AFR 13-14 everywhere, which I guess is OK for now, as I will have a tuner look at it when I dump in the supercharger this weekend.

Anyway. In the DIYPNP instructions it states that I need to install an Intake Air Temperature sensor (IAT) when I remove my AFM. My first guess was that this is so that the MS can pull back timing a bit, if the intake air temperature becomes too hot due to boost. However, as far as I can see, there is no way I can place the IAT past the throttle body, and I believe there will be a pressure drop (and therefore temperature drop) over the throttle body when not WOT. So my next thought was that it is nessesary so that the car can differentiate between summer/winter temperatures. But then it would make just as much sense to just mount the IAT sensor right before/after my air filter (i.e. pre-turbo/supercharger). This does not make much sense to me either, as every place I read, it states I should put the AIT sensor post-turbo/SC.

Also, when installed, I guess I somehow have to tune the megasquirt to the IAT readings. How do I know how much timing to pull at different temperature levels? Unless I drive my car to the tuner 4 times a year, I can only tune the MS at a fixed (indoor) temperature.

What am I missing here? Please bear with me, it is my first tuning car.

Kind regards Søren

you need an AIT sensor AFTER the SC; an intake reading wont do anything useful for you.

the crossover tube between the SC and TB would be my first logical thought...

Edit: I just remembered, that I need to move my throttle body to pre-supercharger. Therefore, my first thought about the temperature drop past the throttle body does not matter to me (but it will for turbo setups).
Kind regards Søren

MS uses the temp from the IAT to compute how much fuel to give the engine. Without one hooked up it just reads 70degrees F constantly. So long story short is yes you need the IAT, and I would plumb it somewhere between the SC outlet and TB. Preferably right after the IC.

I dont run a IC, so guess i will just dump the IAT in the cross over tube somewhere then :)
Okay, but is this engine fuel calculation just so that the MS can tune right for summer/winter? and if so, how do i know how much fuel it should add with a given rise in temperature?

How MegaSquirt(R) EFI Controllers Work

Read through this. In my opinion, it's necessary to understand the calculations that are going on behind the scenes to really grasp tuning. Megasquirt needs to know how much air is in the cylinder to know how much fuel to inject. Your MAF is reading mass air flow. So in a very general term, you have so much air mass coming in, and Megasquirt injects the correct amount of fuel to reach your target AFR. If a speed density setup (map pressure based), your map sensor is only giving part of the equation. Intake pressure is not enough information for Megasquirt to know how much fuel to inject, because the density changes with respect to temperature.

So, the IAT is not used as a temperature correction for different climates, the IAT is part of the main fueling equation, which is why it's so critical. Megasquirt doesn't care if it's summer or winter, Megasquirt cares about the temperature going into the engine, which will vary with ambient temperature, compressor efficiency, pressure ratio, intercooler efficiency, engine bay temperature, etc. It's very dynamic.

Ooooh. So the IAT is for air density adjustment. Now this is beginning to make sense to me. Thanks a lot Braineack.

Thanks a lot to you too acedeuce802. Just the info i was looking for :)