In my furnace there are two holes the same size, about 3" diameter, one at the bottom on one side for the burner and the other in the top a a chiminey. By controling the thru flow you can make sure that all the oxygen is burned by the torch. Very little oxidation of your metal. I use parts from old lawn mower engines and the like so the alloy is tough!
There will be some inclusions and bubbles, but if you have ever worked in industry, especially with cast iron castings, you know that this isn't unusual. As a "home brew" casting the quality of appearance won't be "factory" but then neither is the price.
I have built working metal lathes (9x24), milling machines and surface grinders. All looked at least as good as some of the home brew machines shown on this site.
Check out some of the books on this web page, especially the one on the charcoal foundery.
http://www.lindsaybks.com/dgjp/
The gasses produced are deadly so make sure you are outside with good ventilation, Stay up wind as much as you can and remember ................molten metal is HOT!!!