Turbo MAP & MAT interaction

[Webmaster]

If this is a stupid question then I apologise in advance..

I have been monitoring the ALDL data on my car for over a week now, and 
have noticed that the Mainfold 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 minuites, 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.

Now I seem to remember from about 20 years ago at college something about 
PV=nRT. This would basically mean that if the pressure (P) were doubled by 
the turbo (from 1bar atmospheric to 2 bar(ish) full boost) then the 
manifold air temp (T in Kelvin) should also double from 20decC (293 K, 68F) 
to 310 deg C (580K,sorry don't know what that is in F). Ok the volume may 
be changing as well, so the temp may not be this high, but there should be 
some pressure related temperature change, shouldn't there ?

So my question is, does anybody know if the MAT sensor is supposed to 
measure an 'average' MAT over a few minuites (i.e. it has a high thermal 
mass), or is it supposed to give instantaneous readings ? I would go and 
buy a new sensor to compare my existing one with a new one, but like just 
about everything else on my car, it is no longer available through GM 
(Vauxhall), so its going to take some time for me to find a replacement.

Cheers,
Malcolm Robb, LC 0112G

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