not that its a bad thing but i cant seem to find an explanation for it. driving around my ait was at 42ish. i got into boost and it immediately plummeted to about 37. how? im thinking this is something to do with physics that im not quite understanding...

i dont know. i figure someone smarter than me will have some kind of explanation. i monitor my ait on the commander from the pfc and i have a triumph fast reacting sensor.

 

i think cause your going faster and which results the air being cooled alot more from more air being rushed through the IC？　

 

I'm not a big-brain either, but it sounds like a good 'problem' to have. Where's your IAT located and what IC? Any AI? It probably is a combination of an efficient IC and ducting, favorable ambient, fast reacting sensor and speed of the car. And "plummet" might be a little overstatement for 5 degrees C.

 

that's normal. the stagnant, heatsoaked air gets replaced with air that was sucked in by the turbo as the vehicle is moving and then pushed through the intercooler.

 

Also, if you have water injection, it will really drop it fast.

Dale

 

^Yep...if it's pre-IAT (usually not recommended) or pre-IC.

 

the sensor is in the stock location and im using a blitz fmic. no ai. i use the word plummet because the drop is very sudden. it happens fast enough that the increase in air flow cant be a factor since my acceleration is in no way proportional to rate at which the temp drops.

the temp drops much faster than my speed increases. its almost instant.

how stagnant are we talking? when not in boost, i would say the air in the pipes is still moving at a pretty good rate. recently my highway cruising ait is ~41. i get into boost then it drops. i cruise some more and it goes back up to ~41..... repeat the process.

just to be clear, im not complaining or anything or implying there may be some sort of problem. i genuinely just want to know what it is that causes this. i thinks its pretty cool that it happens and whatever law of nature is governing this effect, i would love to know what it is.

 

What kind of ambient temps are we talking about?

 

88-91F

actually, its been hotter than that. i think it got up to 97 on monday. but basically those temps. high 80s to low 90s

 

One factor could be the fact that the secondary ports are closed at low throttle due to the two stage throttle, but when you accelerate they open.

 

Five degrees is not plummeting. It was probably just from going faster. If you had dropped to say, 24 degrees (I've seen mine AIT that low), that would be weird.

 

i say tha going the faster explanation isnt valid because the ait drops almost instantly when i get into boost. the difference in speed is MAYBE 5mph, probably less. idk why were getting so hung on the word plummet.... from my experience with the english language, plummet can mean sudden or rapid drop with no reference to a quantity in which that drop is being referenced. i would think my situation could be considered sudden or rapid although its only few degrees.

 

Well, if possible maybe a vid would be more illustrative. I won't argue with your definition, but one usually plummets to their death from 40 feet, not 5.

 

This is completely normal for a car with the intake filter in the engine bay without a totally sealed "cold air" intake out of the engine bay.

I logged a lot on my TII which had the intake filter behind the headlight and though it was shrouded from the engine bay and radiator, with an opening for cooler air from the fender well, it was not a sealed system.

I would regularly have freeway cruising AIT around 10deg F over ambient and when I accelerated it would quickly dip to match ambient temps and then slowly rise back up through the pull.

The reason is at cruise you are using about 15hp depending on tires/aero add ons.
I think that is around 2Lbs/Min flow on a rotary. You are sucking down hot engine bay air at a rate of 2Lbs/Min.

You open the throttle and boost and you are flowing around 15 to 46Lbs/Min (depending on how low your boost is at 3,000rpm; 100rwhp versus 300rwhp is what I used).

This additional flow is enough to empty the hot air in the engine bay very quickly, so the rate of air flow through the radiator increases lowering the rise in engine bay air temp per volume from the radiator.

Really.
At 300hp the turbo could be sucking 111CFM of air out of the (6-10cubic Ft?) engine bay a second.

 

Not sure how relevant sucking the hot air out of the engine bay is compared to forcing that air through a large, air-cooled heat exchanger. The increased flow of cooled air would drop the temperature of the AIT sensor.

Well, maybe with an FMIC or V-mount... mine is literally cool to the touch after a track session. With an SMIC... not so much.

My MR2 on the otherhand heat-soaks by the time I've pulled out of the pits. Haha.

 

If you are very unlucky, you might have a harness wiring problem causing the resistance between the AIT sensor's ground wire and the ECU's ground wire to increase. A higher resistance would register as a colder temperature for AIT and CLT sensors. If you want to perform a quick science project to rule this out, disconnect the AIT sensor from the harness and connect a 1000 ohms resistor in its place. You should be able to safely jam the resistor legs into the connector, then tape everything up so it is insulated. Personally I prefer blue painter's tape because it doesn't leave gooey gunk behind after you remove it. That 1000 ohms resistor should trick the ECU into thinking AIT is a constant temperature, probably something between 40-50 degC. If the harness is good, the indicated AIT should never change regardless of boost or RPM while the resistor is in place. If the harness is bad, the temperature will continue to drop instantly when you get into boost.

 

wow... thats pretty cool. if i wasnt pulling the motor in the car tomorrow night i would definitely give that a shot. im 1000% sure my harness is fine since its one of the things i went over the last time i had this car apart but just doing that to do it would be pretty cool. when the car is up again, i definitely will give that a go and post the results. thanks for the suggestion.

this is exactly what i was looking for... i love the explanation. do you know of any way to test this to verify?

 

I would get one of those cheap air temp gauges for your car and put the sensor in the engine bay to test this.

That is a good point.
With the HMIC on my FC it was the same story. Humidity would precipitate out of the air onto the core and then evaporate when going down the freeway and chill the IC well below ambient.

The air entering the engine at cruise would he chilled, but such a small volume it would not be cooling the intake manifold much. Increase the chilled air entering the motor with WOT and it could momentarily cool the intake tract post throttle body.



To test this theory one could put the temp probe on the IC core.

 

here is how i check mine
in air vent gauges [ the vent still works with a/c and heat ]
top right = out side of air filter
top left = pre intercooler
bottom left = post intercooler
bottom right = after t/b

air filter temp can be 30c get on boost pre intercooler 45c 55c post front mount intercooler
20c 25c then post t/b 26c 27c

 

How much higher do you see with ac going at a stop? I see my temps around 105f and it goes to 150+ with the ac.. not many others have a frt mount with ac like this.

 

IAT sensors are very sensitive to heat soak. If you take your IAT sensor out of the intake and leave it hanging outside the car, it will read hotter when sitting still than at speed. They are not really accurate outside of WOT. This is why some ECUs will allow you to estimate the actual air temperature by blending the IAT and CLT depending on RPM and MAP.