The original question seems to be about cold air instead of ram air. A general rule of thumb : For every 10 degree drop in intake charge, you will gain apx. 1% in power. This doesn't seem like much but consider this. If the underhood temp is 180 degrees, that's after the radiator and over the exhaust, with the outside temp being 80 degrees. That's a 100 degree difference, on a 300 horse engine you are gaining 30 horsepower. That my friends, you can feel.
