whats the diff between hot ,meduim ,and cold glow plus ? which is best ?

the temprature of the plug, for the vx.18 and most small blocks, .18 and lower, hot is usually best, like an os a3, usually the lower the number, the hotter the plug. a .28 would usually use an a5 or a #8 if your running 30% nitro.

a3 = hot
a5 = med/hot
#8 = cold