Well, if you guys were interested in an estimate on how much of a difference intake air temperature can theoretically make it can be estimated with ideal gas laws. Since we always want about the same ratio of air to fuel we can focus on just the air and assume it's bringing proportional fuel and power. Say 70 outside temps and 120 under the hood, that's about 529 and 579 in Rankine. Gasses expand proportionally with temperature so density goes in the inverse of temperature. So 579-529=50, 50/579=8.6% decrease in volume or it's at 91.4% of what it was. Density = m/v, so 1/.914 = 1.094. So by dropping 50 F from 120 you should pick up 9.4% hp, not to mention you should have a cooler running engine and improved efficiency. Now if the intake tube is restricting flow this would of course offset the power gains, but I think that if you're moving air is actively being crammed into the intake tube, so I bet you're actually getting more flow with the tube than with a flipped lid once you're past 25 mph or so. If that's the case it might be in the ballpark of 15%.
