Tech Article Title Author Date
Intake Air Charge Temperature Don Pavlik 2000

Car feeling a little down on power lately? Maybe it's just too hot! After reading numerous discussions from folks who felt their car were down on power once summer rolled around I was curious what the air charge temperatures were in normal driving and when I was really hammering on the car.  With my trusty laptop I went out and captured data from my S4.  Conditions on this fine morning at 9 A.M. were 88F, 51% relative humidity with the car fully up to temperature. I ran some errands to get the car up to temp then motored out to my "testing grounds".  My drive could best be described as leisurely with shifts under 2500 rpm with very light throttle... light enough that my boost gauge had not registered anything.  The last 3 miles of the drive were steady-state 40 mph in 6th gear.  I pulled onto a parking lot to hook up the laptop. I sat and idled with the A/C running for maybe 2-3 minutes while I was getting everything set-up.  Imagine my surprise when I discovered the intake air temperature was a cool 156.2F!!  Gee, I betcha the air coming out of a hair drier is cooler than that!  What follows are the details of my morning fun.

This is leaving the parking lot then turning onto a highway and taking a short drive to see how steady-state conditions would help the temperature.  Talk about heat soak. After several miles at 60 mph the temperature stabilized at 122F... still 34F degrees above ambient temperature.
With the temps stabilized I did a quick full throttle  0-80ish mph run. As shown, the temperature when I lifted off the throttle was 145.4F.  I upshifted to 6th gear, slowed to 60 and drove to see how long the intercoolers needed to recover to the pre-run temperature--21 seconds. 

The green vertical bars represent the respective data listed for right and left cursor information.  

After the sprint run I wanted to see what the temps would look like without much time for the intercoolers to recover.  From a stop I did a full throttle run to 80 mph, jumped on the brakes down to 40 mph, then full throttle in 3rd gear back up to 80 mph several times.  Recovery time between full throttle worked out to about 5 or so seconds.  After the 5th charge to 80 mph the intake temp was 161.6F.  I got held up by a lumbering dump truck doing 30 mph so I didn't have a chance to sample the recovery time.  I had planned to repeat this run after I was done but fuel was running mighty low and I was out in the middle of nowhere so no data on recovery time for the intercoolers for this one.

I noticed while poking around that the fog lights seemed they would be quite effective at blocking airflow to the intercoolers.  I removed the fog lights and did runs with the grills in place.  I have not decided if I'm going to leave the lights out or not.

 

With a clearer airflow path I repeated the heat soak run.  Without the fog lights installed air temperature peaked at 156.2F after 6 runs.  I bobbled and let up too early while braking to 40 mph so I did a 6th charge to 80 mph.  The temps for the 5th and 6th runs were both 156.2F.  As on the runs with the fog lights in place, allowed recovery time between full throttle was about 5 seconds.  All other conditions remained the same so, as best I can tell, removing the fog lights allowed better airflow and reduced the intake charge temperature by 5.4F.   
This time I was able to watch the recovery time to cool off to 122F.  Unfortunately, I had to get back on the throttle because of traffic but it had almost cooled off... the temp reached 122F 9 seconds after I downshifted and eased into the throttle.  Note the temperature dip shortly after the 122F was reached.  I noticed that air temps briefly drop just as the boost comes up except when the intercoolers are heat soaked. I guess air quickly rushing through the system briefly drops the temperature until pressure builds and raises the temp.  
After the car sat for 4 hours in my garage I had to make a quick run to the grocery store. This presented the perfect opportunity to monitor temps in daily driving with a brief heat soak while I was shopping. Note the temp right after I started the car: 143.6F (after sitting 4 hours!).  Once underway the temp quickly dropped to 123.8F which surprised me since ambient was 8F warmer.  Ironically, the lowest temp of the drive (a bone chilling 104F) occurred when I dropped a gear and stepped into it to pass a slow moving minivan.
And now for the killer... this is probably a very common occurrence.  I did my shopping--maybe 15 minutes, then came out and hopped into a "cooled down" car.  My "cooled down" car was ingesting air mostly over 150F for quite some time and it didn't really cool off below 140F until I did a steady 40 mph for a mile or so.  At 514 seconds I arrived home and sat in the driveway to simulate being stopped at a red light; maybe your back far enough to have to sit through the light twice... well my friends after sitting stopped for 2 minutes my intake air temperatures crept up to 145.4F!

I don't have an absolute solution or suggestion how to combat these high temperatures.  It may just be a fact of life... I have no idea what intake temps look like on other turbo or normally aspirated cars.  I suppose you could insulate all the intake hoses, the airbox and all but man what a mess that would be.  The engine is pulling cool air from behind the grill so I seriously doubt any of the cone-type "cold air induction" hacks that sit in the engine compartment would be any help.  I saw on an RX-7 site where they had mounted electric cooling fans on the intercoolers to keep them from heat soaking while stopped with the engine running. Might be some merit there... dunno.  I do live in a very hot climate... there may not be a reasonable, cost-effective solution.

What really started all of this was a suggestion from a friend that we devise a way to mist water onto the intercoolers to help efficiency when running hard.  Stuff I've read suggests a 12-20% increase in intercooler efficiency when properly misted. 




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