This is just my personal experience. It is recommended to use 16hp engine to run a 28gpm two stage pump. Those recommendations are based on the Low flow high pressure output rating of the pump. Not sure the low flow rating on the 28gpm pump, dont feel like doing the math or looking it up, but I think its 9gpm and the pressure is 3000psi. Pressure only builds when the ram meets resistance. If the wood you wish to split requires 3000psi to start the split, then you will have a lot of engine stalling if you go with to small of a motor. I very seldom see 3000psi on my machine and I suspect neither do most other folks. Sure I get a forked round or a big knot every now and then that would cause a 3000psi pressure spike, but most wood will split at much less pressures. for this reason, IF, I already had the pump and engine that you mentioned, I would probably use it before I spent money on a larger engine. IF, I was going to have to buy a new engine pump combo. I would buy the 16hp engine to before trying to save a few bucks buying a lesser engine. Another thing to consider is that most likely, your 28gpm advertised flow of your pump is probably going to be less because of the inefficentcy of the pump, so your 13hp engine might be closer to being big enough than the recommended size engine.
