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Old 05-30-2008, 05:41 AM   #11
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It's theoretically more thermodynamically efficient to use warm air. There's less of the energy in the gasoline going into heating the charge, and more going into expanding and making cylinder pressure. Also due to more volume of warm air being required for a given power level, it should reduce pumping losses slightly.

The reason one can use cold air setups for more power, is that you can get more air molecules in there, for an increase in volumetric efficiency, but not necessarily thermodynamic efficiency. Getting mileage gains with cold air setups is more related to "putting one over on" the ECU and managing to run slightly leaner. Can make more torque also due to great mass of air per unit volume having more momentum.

Which works best on a given engine is all down to how fuel is metered and how accurate it is. Vane airflow systems and carbs probably fare better on hot air, MAP systems can go either way, depending on where and if intake temperature is measured (Some just guess it from CTS) MAF seem happier with cold air.
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Old 05-30-2008, 06:55 AM   #12
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Originally Posted by theCase View Post
I'm a big believer in "there's no free lunch", it still takes "X" BTU's of energy to move a given mass at a certain speed, you can't fool physics.
I believe the same. However, in this context, there's a lot more data to consider. Even a highly efficient car is awfully wasteful; if there's X joules of energy in a gallon of gas, and it takes X joules of energy to move a given vehicle 100 miles, it might take X*7 gallons of gas to move the vehicle that far. There's frictional losses, pumping losses, and humongous amounts of waste heat; in my example it's probably more like X*50 than X*7.

All engine mods that we do here are about recovering waste and reducing loss. A WAI is supposed to do both.

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Getting mileage gains with cold air setups is more related to "putting one over on" the ECU and managing to run slightly leaner.
Probably a CAI helps mileage by reducing pumping losses. The intake on my truck has some awful bottlenecks, and if I change my clutch fan to an E-fan I will have enough room to replace it with 4" PVC pipe.
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Old 05-30-2008, 08:35 AM   #13
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Yes that's true, typically you're replacing a restrictive factory intake and filter with a CAI. Just sticking dryer hose to the stock airbox might not do much, though it's claimed that ram intakes done like that can cancel out the filter restriction.
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