something you may notice is a difference in speed. If you have a plane with a sleek cowling. Running w/o cowl will cause more drag. It will slow down quicker, and maintain a constant speed on the down-line easier.

The first time I really noticed this was with an Extra. I would bring it in on throttle then chop the throttle and it would stop in half the length of the runway. As soon as the cowl went on I was using the whole runway to get it to stop.

I've also noticed that on my planes the engine will run a little hotter with the cowl on. So I compensate by running a "click" or two richer. I don't like big holes in my cowls. If you don't mind a bigger hole, or are willing to build in air deflectors to get better air flow over the head, you may not notice any difference.

I like to dial in my engines on a test stand and then do the fine adjustments in the air. After that I put the cowl on and forget it (unless I have to readjust for bad fuel).

Happy flying!

Scott