Sorry Christopher, I'll have to disagree with Albert on this one.
For one simple reason - at a dyno day a few months back we tested a car with and without a heat shield installed, and with or without the hood open on cars with open filters. Without a shield or the hood open, the dyno results were markedly lower.
The air may be warmed up *slightly* passing through the runners - but if 2.5 duratecs are moving more than 500cfm at redline as has been mentioned before, that's not much time for air to pick up any real amount of heat before it gets "blowed up".
dan
