Turbo MAP & MAT interaction

[Jeff M]

>Malcolm Robb, LC 0112G Wrote:

> I have been monitoring the ALDL data on my car for over a week now, and
> have noticed that the Manifold Air Pressure (MAP) sensor reading is
> varying from about 1 bar with the engine off, to 0.25 bar at idle &
> overrun to about 1.7 bar at full whack. This seems correct from the
> documentation I have. However the Manifold Air Temp (MAT) sensor output
> hardly changes, only rising by a few degrees in a few minutes, and that
> appears to be more to do with the engine warming up than anything else.
> There is no short term, MAP related temperature change.

You are inquiring about monitoring MAT temp changes as far as the direct
effects of air temp increase related to boost pressure increase? OK.  But
your MAT does elevate in temps as the engine warms to normal temps right?
Assuming these then yes (and you have an intercooler I assume?), you would
expect the temp to rise accordingly as boost pressure was raised but not as
much as we can expect with these sensors and the layout of your system.
This is not uncommon and efforts (I say efforts, not absolute resolution)
are addressed in the ECM by code working with tables to give a more
meaningful and usable strategy to provide good values for the SD equation.
In the chip there are calc variables such as tables that provide simulated
linearized MAT that take the raw A/D MAT reading using a 1K ohm pull-up
resistor to provide more true values for those MAT sensors.  Then there are
also offsets that compare the CTS to the MAT and scale this out to
represent a more true reading.  Then you can consider that a fully warmed
engine will already have hot intake runners warming the incoming air, and
now these calcs make more sense.  There is some more massaging but I think I
explained it enough here for it to not look so off to you.   Now, I would
hesitate to relocate the MAT to try to avoid the heat soaked effected MAT
temp as you would then need to completely rework the above mentioned calcs
and, would no longer have proper readings of the air temps as they ARE
heated by the engine, and this would then make for a cold (perceived) MAT
reading making your calc offset towards the rich side of fueling, and then
your oxygen sensor would wonder why the mix is so rich and would be working
to try to resolve this in closed loop.  There are so many details worked out
in the chip that one would spend a lot of time trying to come up with a much
better approach, but for those who want the challenge, and to what
gains???? go for it.  With this type of setup as it is, I have yet to see
one that is THAT far off to warrant a total revamp of the system, though as
with this list, we know we can always tune it a little better and look for
those challenges ;-).  Now if someone really wanted to do this, there are a
host of sensors that are faster acting and accurate, but all the sections in
the chip would need to be told this to work properly.

Jeff M
















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