Hi Eganwp.

You'll get lots of advice on this one. Here's mine...Sounds like I'm running something similar to you (GP 27% Extra, DL50). The head protrudes from the cowl enough that no air dam or baffling is required. The cowl on my Extra has that scale, square-like cutout a few inches below the spinner (must be the carb intake I guess), but when you cut the cowl for the engine that opening gets cut out. What I did was just make a new scale intake opening above the original, thus preserving (partially) the scale affect and allowing more air to flow. The cowl on the GP Extra has a large cutout around the bottom for the landing gear creating a natural air exit.

The way I cut my cowl is initially without the muffler and spark plug. Cut barely enough to clear the engine until you are able to bolt the cowl into position. Then perfect the cut once the cowl is bolted on. Then, remove the cowl and mount the muffler. Make a template from thin cardstock (manilla folder) for the muffler. Remove the muffler, but leave the template taped to your fuselage. Mount the cowl. Transfer the muffler cutouts in your template to the cowl, then cut. Finally, cut for the spark plug and plug wire.

If you want to get crazy, add some aluminum duct tape inside the cowl around the engine cutout.