Way back when, I held the opinion that adding louvers was guaranteed to make the airplane slower, the reason being an increase in mass flow. I've since reviewed a fair amount of in-cowl pressure data, and modified the above to may make the airplane slower. Here's why.
In simple terms, cooling drag = mass x velocity loss. It's not hard to see how slowing a mass requires a force, and how the required force grows given more mass or more slowing.
In previous thinking, I was treating the cowl exit as the only system throttle. In that model, opening the exit area (adding louvers) slowed the average exit velocity, as well as adding mass flow. More velocity loss, more mass, more drag.
Here's the modified thinking; there are really two throttles. In addition to the cowl exit, we must also consider the restriction of a good, tight engine baffle. We know there is a pressure drop across any effective baffle, with really well sealed baffles increasing the drop. The result, given the exit area of a standard Van's cowl, can be very little pressure in the lower plenum...and with no significant pressure, adding or subtracting exit area makes little difference.
