The Echo-E dual coil measures right at 2.1 ohms, just did some checking. I just put my Vision Stone tank which measures 1.9 ohms on the Echo-E and it won't fire, but using an 510-808 adapter it fires fine on the Echo battery. I wouldn't recommend running this tank on an auto battery though, I was just curious if it would work.
Maybe my meter is off? It reads .2 when I touch the leads together so I always account for that. Hmmm...