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Old 02-24-2009, 06:50 AM   #47 (permalink)
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I thought the point of a Cold-Air Intake (CAI) was that cold air is denser, of course, so it expands more (pre-ignition density to post-ignition density change/ratio) when heated (say by the burning of fuel) and also increases the mass of the air expanding. I will admit I have no idea on the exact equation or the numbers involved, though the concept is fairly solid. For the sake of explanation, I will throw some bogus numbers out there:
Let's pretend that air expands 10% per 10 degrees C raised. Let's also pretend that fuel igniting in a cylinder burns at 500C. It's a nice mild day at exactly 10C (50F).

"John's" car has a WAI system pulling 100C air off from around his exhaust manifold, giving a 400C temperature change. If I'm thinking the math right, that gives a 44:1 expansion ratio {400 degree change = 40*(10C increases)*1.1 [AKA 10% increase] = 44} Does that sound right?

"Jane's" car has a CAI system pulling outside 10C air in, giving a 490C temperature change or a 53.9:1 expansion ratio. {490 degree change = 49*(10C increases)*1.1 [AKA 10% increase] = 53.9}.

Like I mentioned earlier, there is also the increased air mass / volume ratio of a CAI over a WAI. Doesn't a cars computer measure the volume (L or Cu. Inches) of air going past the intake not the mass (kg/L or Lbs/Cu. Inch)?

As for the atomizing/vaporization of fuel, the inside of the cylinder is hot as hell after only a very short time once the engine has started. Except for those first seconds to minutes (depending on conditions/fuel systems/engine types) after starting a cold vehicle, a WAI's benefits are lost. My dad's old carbed Toyota Pickup ('85 I believe) has a "Warm Air Return" or something along those lines which is something of a hybrid WAI/CAI system. When the engine is cold, a thermostat closes the main intake and opens the WAR/WAI section to allow the engine to warm up quicker and helps atomize the fuel in the carb. Once the engine is warm, which in turn warms up the carb, the thermostat opens the main intake/CAI system again and closes the WAR/WAI system off.
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