I flew my String with a HB 61 and a Slimline Pitts Muffler. The only holes in cowl were for glow plug and an air outlet just behind the engine on the bottom of the right hand cheek cowl. Looked great and flew at scale speed. Looked just like the full size Shoestring at Reno Races Don't forget the full size plane only goes a little over 200 or so which at 1/4 scale is between 50 and 60 mph. Saw a plane with a 120 4 stroke and the cowl was butchered to get the engine in and he had to mount the tail servos on the outside of the fuse under the stab. It flies great with a 60 and balanced with no weight added. PS I see Great planes now has a Cosmic Wind to go along with the String and Little Toni!!