As a newbie on the forum, I posted a thread a couple of weeks ago in which I asked questions about the effect of tuning plates. I didn't get any response, but now I seem to have some answers myself.
I modified my OK Joe Longhorn to add a deflector and tuning plates. All are made from 1/4" steel. I had hte welding shop make enough tuning plates to cover the bottom with about an inch between them. After my first cook with them, in which is seemed everything cooked quicker than it should, I removed one to have more space between them. I cooked a beef brisket and some pork spare ribs last weekend, holding the grate level temperature around 200°F rather than 225°F. I watched the ribs closely as the cook went along to make sure of the pace.
It appears my guess was right: Keeping the grate level temperature lower compensates for the radiant heat given off by the tuning plates. In other words, without tuning plates the temperature in the lower parts of the cooking chamber is cooler with very little heat given off by the shell of the smoker.
Does all this sound reasonable to you folks?