The A sump locates the throttle body between the sump and prop. It was convenient to directly connect the air filer, putting it at the aft end of the cowl. Per other builders, air "piles up" at this location for a relatively higher intake air pressure. The plans say the air filter is to be "Fram 3647 or equivalent". That filter has a surface area of 66.9 in2 (2.5" height & 8.52" diameter) before counting the pleats. I went with a K&N filter connected to the SDS throttle body with a section of 3" reinforced hose. (I hear rotten lab tests for the K&N air filters, but I think it is the most common make used in amateur built aircraft.) This filter has a pleated surface with an average diameter of 5.44" and height of 5.0". C= pi x d = 17. A = C x H = 85 in2. This is before counting the inner cone, estimated at 40 in2 for total area ~2x the plan filter. The additional area will offset the difficulty of breathing through an oiled filter.