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I posted this a while ago for another Cold Air intake thread:
"I did an experiment where I isolated the intake with an aluminum box that didn't restrict airflow, but allowed the intake to only draw air from outside the engine compartment. Then I suspended temperature sensors on both sides, to see the gap between intake temps and engine compartment temps with the 'isolator' in place. The results seemed compelling. Like Tbitz, I saw about a 25 degree (F) gap between ambient temps and engine compartment temps.
But there's a factor I think Tbitz missed, and I think it also explains why an ambient air intake doesn't improve horsepower when you test it on a dyno. In order to get repeatable results, the guy doing the testing tries to set up a repeatable, static set of conditions. Both Tbitz and I did it the same way. You take a stretch of road with a certain ambient temperature, and you run the car at a more-or-less constant speed. Sure enough, you see a nice big gap in temps, and the ambient-temp intake idea seems like it must be helping.
In my case, I set everything up and monitored the temperatures for an 8-hour drive from Los Angeles to Thunderhill, a track in northern California. When I got to the track, I left the system intact, since it was apparently reducing intake temps by anywhere from 15 to 30 degrees.
But when I took the car out and raced it, I honestly didn't feel any difference. So later in the day, I set up the thermometer displays again, and made an effort to look at the readouts while I was actually driving on the track. Surprisingly, the ambient/engine-compartment difference disappeared almost completely (0-3 degrees) when I was racing. The difference? Well, the part that hit me was that I wasn't running the engine at a steady speed anymore. And instead of 3-4K rpm, I was running it from idle to redline repeatedly. It's probably not too big a leap to say the efficiency of the main blower goes up dramatically at higher rpms, sucking all of the warm air out of the engine compartment faster than the engine can re-heat it.
And of course, it's under these kinds of conditions that you actually want to see a horsepower increase. Cruising at 3500 rpms, horsepower isn't a big deal. Flooring the gas is when we want the power, and it's also when the 911's design guarantees loads of ambient-temp air to the intake.
Still, isn't a 0-3 degree improvement worth something? I'd say yes, so long as there aren't any other trade-offs. But I would guess that any kind of long neck tube to bring cool air to the intake is also increasing the resistance/interference/obstruction (whichever it is, I'm no engineer) along the path the air has to travel. It seems counterproductive to restrict the intake at all rpms in order to cool the air only for steady state low rpm use, doesn't it?"
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