Hey kilroy, nice to see you're doing some great explosions.

I like your first explosion test on the road. Cam and BColbourn are probably right about size of the fire flames and lack of diffuse shadows on the ground. I would also suggest that you need to work out some burned ground (and grass), almoust like totaly black ashes and dust that remains on the ground. Ground must look totaly burned.
As for the second road test, I think you made a pretty good looking smoke, only I am wondering is that shadow spreading on the ground (that is right bellow from the smoke)? If it is, I suggest you to remove it, because accourding to background picture it's a very cloudy day, and no sharp shadows should be there, only soft - diffuse shadows.
I guess you are going to do this explosion with many passes.
Keep it up mate!