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Old 08-13-2013, 03:17 PM   #26
Scooby Specialist
Member#: 103631
Join Date: Dec 2005
Location: Mililani, Hawaii
2004 STI


Originally Posted by sidewayz View Post
Bumping this back up. Been hearing a lot of people saying the iat in the manifold is bad with ZERO proof as to why. It's always a blanket statement.

Been doing it and been watching afrs and all is well. Anyone else?

Also check this
I'll give a different perspective. I think that you are correct that there are blanket statements. It's all about context, and in this case it would be the actual engine management system and software.

Many maf based systems have temperature compensation for fuel but these tables "usually" dont have a large effect on fueling. Moving the IAT wouldnt affect fueling too much but it may affect timing. It all depends on how aggressive the table is now (so that would be specific to ecu)

On "some" speed density systems (actually most), there is some type of fueling compensation based on intake temperature. The map sensor is suspposedly supposed to compensate for the temperature changes.

In both cases, it depends on how many tables use IAT as a compensation and whether the values are severe and whether they need to be because of the way that software process pulsewidth. If the software and system for some reason requires very sharp value in fueling compensation, then mounting the IAT in a location where it can read falsely high due to heat soak may cause issues, but that's only if the temperatures are artificially high and if there are tables that apply heavy compensations.

Typically in all cases the system will remove fuel to some degree, but if it's only 1% at 130 degrees F I doubt you'll notice anything. But if that value were higher, and it needed to be high to keep fueling consistent, then that's where any of type of heat soak (that was actually a false reading) could be detrimental. So it all depends on knowing what the ECU in your car does with IAT as far as fueling is concerned.

I would say that in nearly all engine management, IAT affects timing and in most subarus it's pretty severe. If relocating the iAT to a location that reads high in traffic, it may cause very low timing values, which would make the car feel dead. That's not dangerous though. But it's as simple as changing the values yourself, so it's not a huge deal.
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