# Painted or polished CAI question?



## Maxwell31 (Sep 6, 2004)

I was wondering if you bought a CAI painted would it help to insulate tubing against heat better than a polished pipe? 

I was reading on the net that it is better to get your CAI colored to ensure the air inside stays cooler longer (I'm guessing that the paint acts as insulation from motor heat). Is this true or just a bunch of crap? 

I want to get my CAI polished but was really concerned about this problem.

Any help would be great and thanks in advance!


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## rselah25 (Sep 7, 2004)

my guess, is that the paint doesnt help at insulating....the engine is constantly bringing in air, so that air doesnt stay in one section of the piping long enough to get real hot....just my guess....my intake doesnt seem to heat up...or heat up much at best


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## chimmike (Oct 17, 2002)

most intakes will heat up no matter what you do. Insulating won't make any difference.


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## black_ser95 (Mar 19, 2004)

heat wrap :thumbup:


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## Zac (Mar 10, 2004)

paint will not insulate anything. in fact, the heat itself would make most paints peel over time. T304 stainless intakes are the way to go.


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## lshadoff (Nov 26, 2002)

Let's analyze what goes on heat-wise with a CAI. 

Heat sources: 
Hot under-hood air outside the CAI 
Cool air inside the CAI 

Heat flows from the hot side to the cold side; the hot under-hood air heats the CAI pipe. The metal transfers heat to the cool air flowing through the inside of the pipe. This sets up an equilibrium of heat flowing through the metal and heating the air that touches the metal on the inside. The rate that the air can carry heat away from the metal depends on the temperature difference between the metal and the air. But the air is flowing through the tube, so cool air is entering and is carried along the tube into the engine. Since the flow through a CAI is laminar, there is no mixing of the air inside the tube, so the air which is near the metal, stays near the metal. If we look at the boundary layer of air as it travels down the pipe, it is heated by the pipe until it is nearly the same temperature as the pipe. Air does not transfer heat through itself nearly as well as the metal transfers heat to the air, so there is a thin layer of hot air travelling down the pipe next to the pipe. 

So what do we have? A cross section of the air temperature part-way down the pipe will be a layer of warm air (close to the temperature of the pipe) near the pipe and mostly cool air running down 90% of the pipe. 

When the flow of air through the tube is slow, the warm air layer thickness increases because there is more time for the air to heat nearby air, and when the air flow increases, the hot layer gets thinner. On the average, at WOT the average air temperature may rise 5-10% of the difference between outside air temperature and under-hood temperature. 

For example, if it is 75* outside and 125* under the hood, the average air temperature entering the engine would be 75 + (125-75)/20 = 77.5* to 75 + (125-75)/10 = 80* 

If the flow of air through the pipe is turbulent (constantly mixing), there is fresh air constantly contacting the pipe, and the average air temperature will be much higher. 

If you can slow down the heat transfer rate from the under-hood air to the metal tube, then the inside of the pipe will cool down and will not transfer as much heat to the air. An insulating layer on the outside (or inside) the pipe will allow cooler air to reach the engine. 

Lew


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## Maxwell31 (Sep 6, 2004)

Thank you for the help. 

I figured that it would not make much of a difference. The constant flow of air would have no time to heat up to a significant point. 

But as was said that insulation such as heat wrap for exhausts might help. 

I guess it's about preference.

Again, thanks for all the help and good luck with your rides.

P.S. 

"Ishadoff"
Thanks for taking the time to write in detail the heat transfer process. Was very informative and will use later.


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