Thanks Freon, it makes sense in what you say, run the furnace with the blower door off, removes any air restrictions on the return side, more air in then we would expect temp rise to be lower. I will try this tonight when I get home and yes easy to tape off the switch.
If that is the case above, the temp rise is less, I am not sure where I could add the extra returns, I think if too close to the furnace, which I really only have access to, would that not limit the return air from the other returns that are much farther from the furnace. I mean the air would take the least path of resistance and perhaps not heat the upstairs as much as it did before? or is that why you suggest filtered as it would provide more air but be slightly restrictive.
For the entire bungalow I found 5 return metal grills, 2 sets are from a within the common wall, these grills are back to back and common to the same inside wall space for the return. The other does not have back to back grills. So when you think about it the returns occupy a 3.5" stud depth by 16" wide * 56 sq in., which I think is about 289 CFM, times 3 * 867 CFM for the entire house. So the total area of space for the return air is 56 sq in x 3 * 168 sq in how can that be enough space for the air to return, if the furnace is pulling to provide 1200 CFM. I guess I will find out tonight.