I borrowed my friends gas powered splitter yesterday, and him for added labour. Even with its cast wedge (guess its about 2-3 inches wide) there were a few poplar that wouldn't split through. That wedge is about 6 inches tall with a neat feature, the cutting edge is angled slightly so that the top contacts first, would help hold the block down as it splits.
Free advice is worth exactly what you payed for it.
Greg
Greg