With a 4" cylinder most of my wood splits well under 1000 psi, 800 to 900 about average for the ash I'm in. Some of it 20" dia.
I put a 6" round in it cross grain and still only got the pressure up to 1300. It would have sheared it right off if the wedge didn't stop 1" short of the flat plate that holds the block because the cylinder ran out of stroke.
Seems to me a 3" cylinder would handle most of what I split.
Ken
Ash has to be one of the easiest woods to split. It's one of the few I really enjoy still splitting by hand. For a real test of a splitter, you need some gnarly pieces of other species: elm, oak with knots or crotches, etc.
Having said that, I agree that a 3 inch cylinder is probably fine for a lot of individual users. I have a 3.5" cylinder on my 16 ton splitter. It works just fine in all kinds of hardwoods - oak, maple, elm, ash. I'm generally splitting from 10" to 18 or 20" diameter, have gone bigger. Are there times when I wish I had a bit more power? Sure, but it's a very rare piece that I can't split -- on those, I may have to back out and go at it again, but it gets done. IMO, the 30+ ton splitters a lot of places are selling are way overkill for even a heavy home user. Yeah, I know there may be exceptions, but the average homeowner/landowner just doesn't need that much machine. For somebody trying to make a business out of firewood, it's a different story.
On a side note: I had lots of fun setting up my city-slicker 220 lb former marine friend with a bunch of Elm to hand split. He was cussing and griping and generally not getting much done. Meanwhile, I went at a stack of Ash. One swing, and it popped right in two. I gave him lots of bogus "pointers and tips" to try to help him get the hang of it. Even traded splitting mauls so he could have the "good one". Made sure our wives were watching the spectacle. Waited till he was about ready to blow a gasket before I slipped an ash into his pile... "Hey, now you're getting it". Finally, as we were enjoying a beer afterward, I let him in on it. I almost ended up wearing his beer, but he eventually started laughing about it. Unfortunately for me, payback's a b*%ch... he'll get me back one of these days.