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Weekend experiment w/the SRT 10 Hood

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:) For some time now I have been wondering the net affect of the SRT-10 hood and what, if anything, it brought to the party relative to air temps. Well my local diesel shop loaned me a Wavetek monitor which has two temp probes that we set up under the hood. The one monitor we hooked to the dip stick and overhung it by a couple inches. The other monitor we put inside the air intake system after the filter and just in front of the torq tube on the AFE air intake system. The ambient temp for the test was 54. 7*. My underhood probe averaged 22* to 24* at 75mph for 50 miles. The air intake temps averaged 62* to 64* and those temps were measured simultaneously while the underhood temps were taken. I did this test with the opening in the hood both blocked and open and all under the same test parameters. The net effect of cooling provided by the hood was negligible (+/- 5*).



I'll admit that the affect of the hood on engine and engine compartment cooling was next to nothing,... that was a bit disappointing, but at least it answered the question.



The testing, at least in my mind opened another question however. I wonder how many others are surprised by the fact that the air intake temps are (averaged under the terms of my testing procedures as stated above) approximately 35* to 40* hotter than underhood temps!! And, as previously stated, the air intake probe was just a few inches on the other side of the filter. It appears obvious that while air is entering then heat is exiting???. A somewhat strange occurence to me, and could only mean that to some extent fresh air coming through the intake is impeded by heat exiting.



Anyone care to take this one on in an effort to explain this situation. A bit baffling.



Thoughts??? :rolleyes:
 
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The IAT sensor visible via OBD is located in the manifold and is part of the Manifold Absolute Pressure sending unit. There is another IAT sensor at the air filter, but you can't read that one with OBD; you have to use DRB III.



The reason I believe this is the case is that when I disconnect the sensor sensor at the air filter, I get corresponding engine codes indicating MAP/IAT circuit high, but yet the IAT and MAP information reported by OBD is still active and produces good number. So this tells me that the MAP/IAT information reported by OBD is in fact inside the intake manifold, not at the intake filter.



Furthermore, what I have found is that the IAT sensor (reported by OBD) is very slow -- constantly integrating (averaging) temps as they vary. This cannot possibly reflect actual air temps, since air temperatures at the turbocharger compressor output can easily jump 200 degrees or more within a short couple of seconds (under acceleration). The intercooler probably cools by 100 degrees, but I would think (I have not measured it) the wide temperature excursion would still be reflected at the intake manifold (say, 300 degrees compressor output, cooled to 200 degress by the intercooler)



Watch IAT via OBD some time on a hot day in city traffic -- it will slowly climb at a stop light due to heat soak, and then slowly cool back down with vehicle movement.



OT: your data are interesting . Unfortunately, there could be many variables that could confound the results, such as temp probe speed, proximity to metal, etc. to interpret, we'll need to know some additional information:



1. metal AFE torque tube? home made home depot type?

2. is the temperature probe a metal tip or is it an exposed thermocouple bead type thermoouple? how big is it and how fast does it respond?

3. did you put a temp probe behind the radiator fan or near (but outside) the filter element?



measuring a higher temp inside the torque tube suggests some kind of heat soak. no, there is no hot air exiting the intake system as cool air enters -- molecules of air are flowing into the filter media and into the turbocharger, not out of it. They slow down inside the filter itself (large cross sectional area), and then accelerate again in the torque tube. if the torque tube is metal, it could transfer heat from the engine compartment to the air (or the temperature probe, depending on how it was installed).



the best way to measure these temps is to use a minimally protected type K bead type thermocouple located in the center of the torque tube cross section (avoiding torque tube wall proximity). such a probe looks like two wires twisted together with no visible "probe" on the end. These guys are VERY fast and will respond instantly. I used such a probe to further study the 03-04 turbocharger. results in TDR issue 48. The problem with general purpose metal probes is that they are just plain slow and don't give the real dynamic picture.



OT if your temp probes looked like two wires twisted together (with no "end" on them) then you probably got very accurate numbers. These can be slipped inside the AFE rubber coupler and into the torque tube and would probably be accurate, unless of course the tip itself is close to a torque tube wall. if your probes had a metal tip they are probably slow and subject to measurement error due to heat soak.



The other thing to consider is that the air flow under the hood is very complex. In order for cool air to come in via the SRT-10 hood opening and do any good, it has to have a place to go. otherwise, very litle air will actually enter that opening. My point is that the radiator fan may produce pressurized air over the engine which might tend to resist the flow of air trying to enter the engine compartment from the hood. thats what I suspect, anyway, as an explanation for your numbers.



If you have time, I'd suggest you go back to a dealer and inspect the SRT-10 truck itself (engine compartment), to see if you can tell what role the radiator fan might play there as well. maybe the whole hood thing is cosmetic on the SRT-10 as well.



it does surprise me that intake air temps measured close to the AFE filter would be hotter than under hood ambient. that result begs for more investigation.
 
I believe you that there's a thermocouple integral with the MAP sensor, but I'm not so sure that's the one OBD reads (although you're much smarter than me, so chances are you're right). I guess I just expect the manifold air temperature to be much higher than what I read (always 10-20 degrees above what the overhead reads). I run my OBD-II monitor almost all the time.



Of course, that doesn't explain why you still get good OBD readings if you unplug the sensor atop the airbox.



-Ryan :confused:
 
yea, manifold air temp should indeed be higher then ambient, due to heat from the turbocharger. That intake air temp inside the air filter should be higher than under hood temp, as OT reported? thats got me puzzled.



here's an experiment you can try



1. set up your OBD scanner to read IAT and MAP



2. dis-connect the plug at the air filter. note engine codes you get, and that fact that you can continue to read good IAT and MAP numbers. The truck is still drivable and that all the MAP/IAT numbers are believable. Its not possible to get good numbers with the sensor disconnected.



3. re-connect the plug at the filter and re-set the engine codes



4. disconnect the real MAP sensor plug. start the engine if you want (it will be in limp mode; I don't remember if it will run or not... ). note the engine codes you get. try to read MAP/IAT.



I haven't actually done step 4 yet. I stopped at step 3, convinced that IAT via OBD was measured in the manifold :D.



So why would intake air temp measured just prior to the torque tube be 35 degrees higher than under hood ambient temps? I suspect heat soak from the metal AFE filter box heat soaking into the pre-torque tube filter connector -- that short tube that connects filter to torque tube. I also suspect that the temperature probe was located close to a metal wall. something is fishy here.



I also don't understand why ambient temp was 54. 7 but under hood temps were 24 degrees at 70 mph. thats some pretty serious wind chill :D just kidding.
 
I have been wondering if a SRT-10 hood with the air induction scoop could be used to improve air intake and generally cool engine compartment on a diesel. At first it would seem like a no brainer, force more cool air into the engine compartment and it has gotta be better. But maybe not. I do understand somewhat the peculiarities of air flow.



Given the information above, BTW great posts guys, would it be better to actually route the air from the hood scoop directly into a somewhat enclosed filter box? I am sure that this has its own air flow/routing/vacuum issues as well. I dont watch enought Science channel to be able to do more that make suggestions and ask probably dumb questions. A little different question and issue is refreshing for a change, Like A Breath Of Fresh Air.
 
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Dleno

DLENO,



I'll try to answer your questions you posed above.



1) Yes, it is the metal AFE torq tube and I placed the probe just at the leading edge of that tube and inbetween the tube and the rubber seal that comes in that kit. I let the probe dangle into the opening. The probe itself is the high quality unit you spoke of with a couple wires appearing that they've been twisted together-type of appearance. The outside of the probe wire is metallic feeling though (perhaps copper coated,... dun no for sure) and maybe that touching the metal torq tube is accounting for some of that temp gain.



2) The temp probe is exposed thermocouple type.



I really appreciate yours and Ryan's input on this. This just needed a little discussion.



Pat
 
ok cool. I don't understand some of the results, but I might try some things myself after my Fluke 189 arrives :D.



sounds like you've taken every pre-caution. the temperature sensing point is at the very tip of the wires. it is two dissimilar metals tack welded together. if the very tip of the twisted wires is located inside the tube, away from any walls, then thats as good as you can get. heat soak from the metal torque tube could be part of it. did you notice rapid changes in air temp during accereration (fast moving air versus slow moving air)? If not, then heat soak is proably not happening.



when I put one of those things inside my turbocharger compressor hosing (aluminum), I found I could measure very fast temperature excursions from 100 degrees to 250 degrees in less than 5 seconds.



you might consider moving the tip further inside the filter (if you could do it again) -- even laying on the inside surface of the pleated media. as long as it won't slam around in there. I dunno man, it sounds like your experiment was good; I just don't understand the results. maybe the fan blows air across the engine block which heats up the air before entering the filter. no wonder scotty has success with his ram air product.



in that case, jvolpe's idea might give some benefit. -Doug
 
I've generally been assuming that the higher IAT indication is because the stock airbox draws stagnant air from the fender. I never really thought about "why" that would necessarily be at least 10 degrees hotter than ambient.



The whole thing's a bit odd. DLeno, I'm reluctant to unplug the MAP sensor... don't want to hear my baby limping.



OT - did I miss something? Your underhood temps were 22-24 degrees?



Just to be sure, are we certain that the OBD temperature is measuring farenheit, not celsius?
 
Well, if the air is actually drawn from the fender (and it is), then it can't be stagnant and must come from the outside. Indeed, this is one of the major "problems" with the stock airbox -- that it sucks air right off of the rt front tire. sort of.



yea, odd is right rbattelle. my dealer actually disagrees with me too -- he says IAT reported by OBD is at the filter. but he uses DRB and not OBD, and he's never tried to read meaningful data from a disconnected sensor :D



good point on the temperature units! I think they depend on the OBD scanner itself, though I'm not certain about that. mine reports in degrees F.
 
rbattelle

rbattelle said:
I've generally been assuming that the higher IAT indication is because the stock airbox draws stagnant air from the fender. I never really thought about "why" that would necessarily be at least 10 degrees hotter than ambient.



The whole thing's a bit odd. DLeno, I'm reluctant to unplug the MAP sensor... don't want to hear my baby limping.



OT - did I miss something? Your underhood temps were 22-24 degrees?



Just to be sure, are we certain that the OBD temperature is measuring farenheit, not celsius?



Right,... farenheit! I was a bit surprised by that too. What is mostly surprising to me is the higher temps in the tube behind the filter. I had to give the unit back to the guy, but if I can get it again I will insulate on both sides of the probe tip this time to be sure it isn't picking up heat from the metal tube.
 
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Something's got to be wrong. There's no way a 54* ambient temperature can result in an underhood temperature of 24* unless there's a refrigerator under the hood. Am I missing something?



Was the temperature reader properly set up for the thermocouple type (there's J and K types)?



Now, my OBD says the units are farenheit, but it could simply be mislabeled.



Weird, wild stuff!



-Ryan
 
Why not?

rbattelle said:
Something's got to be wrong. There's no way a 54* ambient temperature can result in an underhood temperature of 24* unless there's a refrigerator under the hood. Am I missing something?



Was the temperature reader properly set up for the thermocouple type (there's J and K types)?



Now, my OBD says the units are farenheit, but it could simply be mislabeled.



Weird, wild stuff!



-Ryan



Ryan,



I returned the gauge, but I'm not sure I agree with you. Check out your local weather,... how ofter do you have a 30* day and a NW wind of say 20mph and end up with a 12* wind chill. (Not exact numbers, but you see my point). So if you add to a 54* day 75mph + any wind in your direction and 22 to 24* seems entirely possible and that is what the gauge said. And I double checked,... it was farenheit.
 
wind chill is only appropriate when talking about energy loss from a live, warm body, and refers to the perceived external temperature, not the actual. In other words, when you have a 30* day and a NW wind of 20mph producing a 12* wind chill, the temperature of the air is still 30*, not 12*. 30* is what the thermometer says. yea, you've got something seriously wrong with the data. or maybe your AC compressor was running (defrost) and the temp probe was located near one of the lines. Either the data is wrong or there was something capable of cooling the outside air. no, it can't be the radiator or intercooler or AC heat exhanger (that little radiator in front of the intercooler) -- they give energy up to the air passing through them (they warm up the outside air).
 
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DLeno took the words right out of my fingers. I think it's a common misconception that wind chill somehow indicates the temperature outside for objects other than people. Wind chill is just a way of saying "well, it's 30 degrees out but with the wind it *feels* like 20". Metal does not experience "wind chill".



When I was a kid I thought it was called the "windshield factor". :-laf



-Ryan :)
 
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