A number of builders have used this modification—adding an air intake scoop from Van’s Aircraft.
This description was the procedure that I used, which was based on input form several builders. It has worked well in my GlaStar, which now has about 280 hours. My engine is a 150 hp, O-320-A3B Lycoming, fitted with a Vetterman exhaust system and Van’s baffle and air inlet system. I used Van’s filtered air box for the O-320 and the cowl scoop for an RV-6.
It’s been over 3 years since I did this work so the details are fuzzy, but as I recall I installed the lower cowling half with Van’s air box in position. Its nose had to be trimmed back, but I wanted to cut the hole in the lower cowling so that incoming air had a straight path to the carburetor. After I eyeballed about where the hole should be, I took a deep breath and started cutting–SLOWLY. Once cut, the cowl scoop was trimmed fairly drastically as shown in the figure. Gel coat was sanded away and the cowl scoop temporarily positioned with several light aluminum brackets attached with cleco clamps as shown. Strips of fiberglass cloth were used to make the permanent attachment between the cowl scoop and cowling, which was also reinforced on the back side with a couple layers of fiberglass cloth.
Filler was used to achieve the desired contours, followed by gel-coating. On the backside of the cowl scoop I fiberglassed on a fabricated fiberglass tube of about the same diameter as the cowl inlet that was directed to the front of the trimmed airbox. I riveted two inch wide baffle rubber to the front end of the airbox which encircles the fiberglass tube that extends back from the cowl scoop. There is about 1/4 – 3/8 inch clearance between the front of the air box and the tube, so there is no fiberglass-to-fiberglass contact. This seems to make a fairly secure seal but requires two people to install the lower cowling to the aircraft.
