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Ok, I got to thinking of whether or not a steel Cold Air Intake system like a MAC or BBK could actually increase intake temperatures by soaking heat from the engine bay. So I dug out my Heat Transfer text and decided to check for sure. If you don't feel like reading through the engineering Heat Transfer mumbo jumbo, then skip to the end, where I give my results.

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First, I assume that the steel intake is immersed in extremely hot engine bay temperatures for a long enough time such that it becomes heat soaked. This is an unlikely scenario, but worst case nontheless, so I assumed this condition.
I specified wall temperature at 150°F, figuring this was as close to an engine's operating temperature as this tube might get.

I calculated maximum mass flow rate of air into the engine by assuming 5000 rpm, 4.6L of displacement (intake stroke pulling 4.6L every two revolutions [four stroke engine]). I got 406CFM, or 814kg/sec. This, again, is a worst case scenario in which the maximum amount of air is being passed through the pipe at wide open throttle. Note that I neglected any cooling effects that the incoming air might have on the tube, and assumed wall temperatures to remain constant.

Rather than bore everybody with the calculations (I have them on an excel spreadsheet if you REALLY want to know), I calculated Max heat transfer using:
qcmax = MassFlowRate * cp (T0 - T1)
where cp is a property of air, looked up on a table, with ambient reference temperature at 70°F. T0 is wall temp, T1 is ambient temp.
I then calculated the effectiveness, E, using Reynold number, fcp, Nucp, and the heat transfer constant.

For this kind of airflow, it was interesting to see that E became 0.0188 for this case. That means that only 1.8% of the maximum heat transfer possible for this temperatre differential is occuring in this case. Intake flowrates would have to decrease to .008 CFM just to see a ten degree jump in temperature. But I'm getting ahead of myself.

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FINAL RESULTS:

Real Heat Transfer was on the order of 685kW, which translates to an outlet temperature of 71.51°F. An incredible 1.51°F degree increase.
Tube dimensions were assumed to be 3" in diameter and 24" in length (contact patch). For a larger 4" tube, that increase goes down to 1.2°F.

What does this mean? Even if the walls of your inlet tube do reach 150°F, the air is flowing far too quickly to allow heat transfer to occur.

When everyone started discrediting the steel walled Cold Air Intake systems for heating up the incoming air, I knew I was skeptical, but I had to prove it. And the bottom line is that, the plastic units can become heat saturated just as well. They might take a bit longer, though.

So why are we not seeing impressive gains with these Cold Air intakes? Because the stock setup is, in fact, a cold air setup. The only way you'll see power gains is through larger tubing, and corresponding decreases pressure drops. And even then, only if your engine demands that extra air. A 3" tube seems to do a pretty good job at supplying most of our cars with the air they need.

Sorry for such a lengthy post, but I thought these findings were rather interesting. If anyone would like to see my calculations, they are welcome to them....
http://www.mustangmods.com/publish/GulfCoastMustang//Engine Flow Analysis.xls
 

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Good tech! However, have you done any real world testing of this theory? Have you considered that the ACT sensor would read higher temps due to the metal pipe heating up the rubber grommet more than plastic piping would? I know for a fact that the stock cobra pipe is a restriction. Just look at it. It has a HUGE dent in it for clearing the strut tower brace. That can't be good for air flow.

If the ACT is warmer due to the surrounding materials, then in turn the EEC will think that the air is warmer than it is. Essentially you are tricking the EEC into adding LESS timing, thus killing HP/TQ slightly.

Personally, I still strongly believe that a plastic CAI is better than a metal one. The ACT is going to noticed the difference in temp and it is also going to physically be "warmer" when inserted into a grommet that is attached to a metal pipe.

Unfortunately, I don't think this research is going to prove or disprove heat soak on a metal intake is "non-existent."
 

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Discussion Starter #3
Undoubtedly, a plastic intake is better than a steel one. But only marginally. On the order of one or two degrees Fahrenheit. You do bring up an interesting point about the ACT sensor, but again, with upwards of 400-500 cfm of air blowing past it, I don't think the sensor will be able to see many warming affects from the hot tubing. Have you even felt what a 500cfm blower felt like? That's a lot of air.

I dont' have any empircal data to back up this theoretical calculation, but the results do seem in line with what most of us have seen. The stock tubes definitely do pose a restriction, especially the Cobra inlet you mentioned. I am not at all trying to discredit the effectiveness of a CAI, rather I think they are a great modification, if their diameter is larger than stock.

The bottom line is that you can throw away the myth that steel CAI systems heat up the incoming air. They do, but in such miniscule propotions, it can be considered negligible.
 

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I'm no mathematician so I'll assume your figures are correct and congratulate you on the research behind this. I have a PVC powerpipe that I am redesigning right now and have considered using 1/16" aluminum pipe for it's heat qualities over steel. I want to get away from the PVC pipe if possible for a couple reasons. I looked into all this once before and also determined that even if the pipe was hot, the flow of air was fast enough so there would be no great heat transfer happening. As far as the ACT sensor goes, this also makes some sense, however the sensor sends a voltage signal based on the temperature of the filament and how much voltage the filament can handle. As the intake air passes over the filament it cools the filament. Depending on the amount the filament is cooled determines the how much voltage is passed through the filament which adjusts the voltage signal send by the sensor. The cooler the filament, the more voltage it can handle. The hotter the filament the less voltage it can handle. It seems reasonable to think the filament is somewhat isolated (or insulated) to some degree from the mounintg surface. So even if the sensor did absorb some heat from the surface, chance are the filament temp will not be afffected as much as you think, however it may be slightly. Just my opinion.
 

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Discussion Starter #5
Someone caught a math error, but so far that seems to be the only one. I got all my Reynolds number, fluid analysis and heat transfer coefficients right, but screwed up on a time conversion, go figure, haha.

It changes the outcome fairly significantly, but not enough in the whole scheme of things. The temperature rise is not 1.5°F, but rather 5.4°F. Still doesn't seem enough to impact power output by the car. You'll see far more temperature increased just in flowing through the long runner, narrow diameter intake runners.
 

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GulfCoastMustang - Excellent Info!

Some of the manufacturers of the polymer CAI's taut the heat transfer aspect of their product while completely neglecting to mention the considerable flow issues (flow restriction and turbulence) caused by the multiple 90 degree bends in their product. More then one CAI that i looked at had two 90 degree bends before the MAF meter.

Heat transfer is only one factor and probably the least significant for any CAI material/design. It seems to me that the best CAI is the one that offers the shortest/least restrictive flow path (from ambient air all the way to the TB), that lets the MAF meter measure the air most accurately and that provides a cool air charge. I tried a MAC CAI, but the bend before the MAF meter, just killed metering accuracy, even with the 'specially' calibrated Pro-M meter.

What I ended up with is a custom 3.5" MAF meter to TB inlet pipe, the stock air box with K&N filter and a custom 8" diameter plastic velocity stack that extends from the air box into the fender. This set-up provides a short, low restriction flow-path, cool air from the fender, and minimal heating of the air from underhood heat.

In any case, I'm glad that you quantified the amount of air charge heating for a typical metallic CAI.

I wonder how much heating occurs from an uninsulated aluminum intake at the same 150 degrees:) Likely far more than any of the CAI's.
 

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I think my point might have been missed here. A "CAI" kit is supposed to serve 3 purposes as I see it.

1. Larger diameter piping for added air flow
2. Grab ISOLATED cool air from the fender area
3. Create less turbulance than the stock setup

Regardless of your inlet setup, you are not going to be able to avoid the air heating up once it reaches the metal intake manifold. I am sure that Ford has taken this into consideration and has programmed that into the tables for the ACT/Timing.

With that in mind, if you give the ACT sensor 5.4*F hotter air plus any added "heat" due to the warmer materials around it, then the EEC is going to compensate for that by removing some timing. So any added "flow" from a metal intake is going to be negated due to the added temps that the computer is using for calculations.

Just take a look at the 5.0 crowd for a second. The ACT sensor is a threaded metal sensor that screws into the metal lower intake. I now see why a lot of guys relocate this sensor to the inlet tube or ambient air in the fender. They plug off the hole in the intake and move the ACT sensor to a cooler area. They do this so the EEC will read the cooler air temp and add more timing.

Just food for thought.
 

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I was that "one person" GulfCoast was referring to earlier, and I have raised a few questions about the analysis he performed.

Although the idea is good, I think his spreadsheet is too simple a model for what really goes on inside the intake pipe. If the fluid of interest was water (which has physical properties that don't change much with Temp) then you could probably do a pretty good job with a system the way he described it.

Unfortunately he is only looking at a single data point (5000 RPM) and his physical constants (heat capacity, air density, Nre, etc.) are static and do not change according to system variances.


FORTUNATELY there is some outstanding modeling software available for just this type of transfer phenomena problem, AND my company holds a license for it. Monday I'll see if I can get one of our programmers to model airflow through a three inch steel pipe that is 2 feet long at 150 F. Its a pretty simple model and I can probably get a convergent solution pretty quickly.

I'll post results here and on Stangnet when I get the results.
 

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Wow! Great tech! Far over my head!! Anyways, I'm glad you put your schooling to use to help prove a point here!!! Thanks man, pretty interesting!!! :D
 

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Got to love this thread.

<Homer> Mmmmmmmm, Fluid Dynamics </Homer>:drool:
 

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If it's not too time consuming and/or PITA, try a few different models i.e., no bends before maf, 1 and 2 90 degree bends. As for the individual properties of the CAI, any CAI has got to be better than stock, not because of it's material (plastic or metal), but because of the flow. The stock pipe, in order to be flexable (in Ford's wisdom), has to have wrinkle zones built in (those little ripples in the tubing). What those do, more than providing a flex zone, is cause turbulance in the air stream. That in itself has a more adverse effect on the incoming air by not providing a smooth path. Those little turbulance wakes are like hitting a brick wall. Most if not all CAI's are made of a solid tube (no flex wrinkles) and thus produce little to no turbulance zones for the incoming air to hit. Correct me if I'm wrong (Lord knows I'm not the brightest star in the sky), but wouldn't the turbulance have a greater effect (N/A engine) than the transfer of heat?

:confused:
 

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It seems to me that the objective is two-fold. First, does a CAI bring colder (denser) air into the Throtle Body vs stock. I prefer a more experimental method. Insert a temperature probe in front of the T/B to monitor incoming air temp. The trick here is to use a probe that does not significantly affect the incoming air flow. Back to back measurements with stock pipe, then different CAI setups, making sure that you include a smooth plastic pipe, metal pipe, and ceramic coated pipes. These runs should be made in a variety of different conditions, (stop and go traffic on a summer afternoon, different cruiding speeds on the interstate, 30mph through city streets, etc.).

Second, does a CAI improve performance. Fot that, back to back 1/4 mile runs with the same pipe combinations as above.

To me, if you are going to call it a COLD Air Kit, then it's use should cause the T/B to see a lower air temp.
And if you are touting it as a performance enhancer, then it should ENHANCE performance.
IMO. :idunno:
 

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No Control said:
If it's not too time consuming and/or PITA, try a few different models i.e., no bends before maf, 1 and 2 90 degree bends. As for the individual properties of the CAI, any CAI has got to be better than stock, not because of it's material (plastic or metal), but because of the flow. The stock pipe, in order to be flexable (in Ford's wisdom), has to have wrinkle zones built in (those little ripples in the tubing). What those do, more than providing a flex zone, is cause turbulance in the air stream. That in itself has a more adverse effect on the incoming air by not providing a smooth path. Those little turbulance wakes are like hitting a brick wall. Most if not all CAI's are made of a solid tube (no flex wrinkles) and thus produce little to no turbulance zones for the incoming air to hit. Correct me if I'm wrong (Lord knows I'm not the brightest star in the sky), but wouldn't the turbulance have a greater effect (N/A engine) than the transfer of heat?

:confused:
I'll try a couple of different setups. Actually turbulence is typically a good thing in terms of airflow. It ensures even velocity profiles, good heat transfer, and good mixing. The problem with those ripples is not turbulence, but friction. An uneven surface like that is certain to have a higher friction coefficient than a smooth pipe, and that is going to cause a higher pressure drop across the inlet.

Like I said, I'll let you guys know ASAP
 

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Anyone ever think that Ford was intentionally making an after market for cheap HP? either abs plastic or metal??? Oh wait, I forget we're supposed to have all the power when we buy it, right?
Run your model, examine your data, then add in the effects of driving in a fog, rain, desert, etc. Smooth cooler air will always prevail. Now, the real question I hope you're attempting to answer is. Is the air cooler and smoother from a plastic CAI, or metal? That is the debate is it not?

Just to play devil's advocate, :evil: , what would happen if you opened up that useless hood scoop and added some cooling fins on your MAC CAI kit and drop the temp to say 120 or less??? Kinda makes ya think that this becomes a moot point.
 

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put it this way, i have a fernderwell mac CAI and a roush blower. I cut the the last 90 turn in the fenderwell off and stuck the air filter on the end, tight fit. Before the modification, I ran 5 degrees hotter at cruising speeds than I do now.
 
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