Luis,
A true ram air intake can increase the pressure entering the carburetor by no more than the the dynamic pressure, q, of the external air flow, which can be calculated by the equation q = 0.5*(rho/gc)*V^2, where rho = the density of the air (0.0765 lbmass/cubic feet at sea level), gc = 32.2 (lbforce/lbmass)ft/sec^2, and V = velocity in ft/sec. Thus, at 100 mph, V = 147 ft/sec, and at sea level the dynamic pressure will be about 25 lbforce/ft^2, or about 0.18 psi, or about 5 inches of water. This could increase the power by 1.2%, which is line with what Mike G reported in his post. At 50 mph, the theoretical gain would only be 0.3%, but at 200 mph it would be about 5%.
The "first-order' correction for the effects of changing the intake temperature is to assume that the power varies inversely (added by HWW after reading the post) with the square root of the intake temperature. Thus, if the intake temperature drops from 100F to 60F with a good cold air box, the power could increase by about 3.7%. As Mike G pointed out, the ducting would need to properly routed or insulated to keep from picking up heat on its way to the carburetor
