Exhaust exit; Trim? or not?

Working with the idea that its always better to leave a little extra vs. cutting off too much, I left an "extension" on my lower cowl when I was cutting it to fit on the firewall. Should I cut this flush with the rest of the cowling; leave it long, or scallop it like 787 engine nacelles to reduce noise?

That's interesting, I'd say leave it for now and trim as needed for cooling (as a last resort). I like the scallop idea. You might need to trim the tops of the extension just enough so it doesn't rub on the bottom of the fuse. On my -9A I ended up trimming it forward of the fuse, at an angle, to improve cooling but I think it cost a couple knots. So I think from a speed standpoint; long is good.

I bet that scalloping the exit will do nothing favorable for you. The cooling air exhaust isn't going fast enough for there to be any noticeable noise reduction.

(But as a tip, my shop vac made a horrible screeching noise with the reduced-diameter adapter on without a nozzle. I filed notches in it like you're referring to, and it got a lot quieter.)

Keeping the exit long might help straighten the cooling air that's and help cooling and speed simultaneously.

Dave

49clipper

Leave it on until you are sure you need more cooling. I am in the process of adding something similiar but even longer to try and warm up the oil. O-320 runs way too cool.
RV-6