Joined

·
180 Posts

~~~~~~~~~~~~~~~~~~

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.

~~~~~~~~~~~~~~~~~~~~~

**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