Cut/paste from my CarDomain thing:
I’ve seen the air temperature argument so many times and thought to myself, this was something I could easily validate. I went to the hardware store and looked for indoor/outdoor thermometers. I found this digital thermometer with an outdoor sensor on a 10’ wire for $10. Perfect! I bought two so I could take temperature readings in the engine bay and in the fender at the same time. I hurried home and opened up the hood. I attached one sensor along the front brace, just behind the passenger side headlight. The sensor was about two inches in front of the K&N filter in the engine bay. I attached the second sensor to some cable inside the fender. It was about 4 inches into the cavity for the air silencer. I ran both wires into the car and attached the two thermometers to an old CD. I rechecked all my connections and I was ready to start my data collection. The geek inside me was happy and I’m sure I got a few weird looks from the neighbors. Time to drive around.
The data tells me that once I am moving above 15 mph, the temperature difference averages around 3.5 degrees. With the a/c running, the engine bay was around 4-5 degrees warmer than the fender area. At speeds less than 15 mph, the difference goes above 10 degrees. The engine bay is definitely hotter but only by a few degrees when the car is moving.
Does the air temperature matter? I can’t answer that. I’ve seen some claims on the Internet about losing 1 hp per 10 degrees increase in air temperature. If we assume that is true, then having the filter in the fender gives you 1/4 to 1/2 hp over the guy with the filter in the engine bay.
Do I miss that fraction of a horse because I got the K&N FIPK? No. If we assume the 1 hp per 10 degrees is accurate, then my car will produce 2-3 hp more in the early morning than my drive home at 5:30 pm only because of the weather. The 1/4 to 1/2 hp is negligible.
Does the extra half hp result in better mileage or performance? In a laboratory or on paper, Yes. But in the real world, a slight incline in the road, or a light wind, or having a passenger, or a little traffic will have so much more impact on mileage and performance that the 1/4 to 1/2 hp is again neglible.
My recommendation is to choose a CAI that has the larger filter for more surface area, and one that has the shortest path to the throttle body.