I would not want any heavier,
and I can not really for see driving any thing but the T posts,
now this time I did not even have the hand pounder on the truck, but when I got it I kind of did a test and it seemed like that the "normal" hit with the hand would drive the t post about the same as with the air powered pounder, so any way, the difference was the air unit keep pounding even if the progress was slow,
but it was such I could take hands off and just let it work. after starting the post.
the ground has been very hard do the lack of nearly any rain here for the last year, we got a little rain in mid September and the top few inches are some what moist, but down about a foot or so, it is very hard,
I did not try this, but my guess is if I would have taken the tractor and loader, 1970 JD 4020 about 9,000 pounds and use the loader on the post I do not think I would have been able to push them in more than about 6 inches and it would have stopped the post and just lifted the front wheels off the ground, and most likely bending up the post not driving it. an any more.
so it is some hard soils,
I would rebuy what I have, I am reasonably sure of, the only reason I may consider the next larger size would be for the extra capacity in size of post, but I guess if I get between a rock and hard place I can use the wood post pounder I made, so I see no reason to consider the larger unit, (my guess the larger unit would hit some harder, which I would think would be a benefit in hard soils, but since It has passed my tests, I am satisfied,
(just an example of how tight the soil is, I drive in one wood post with the tractor mounted driver, and then changed the idea some and went to pull the post, my tractor I know can lift over 4000 pounds on the forks, it could not pull the post or budge it. until then I had not ran in to a post I could not pull)
I would think the 4.5 cfm would run it nicely, for the small unit the 98E Basic, one thing it runs at 75 psi, so the compressor can keep up with it easer, (unlike many tools that run at 90 to 100, the compressor starts before your below the operation pressures,
Runs off of any small air compressor that delivers at least 2.5 CFM at 90 PSI. Runs off air set at 70-75 PSI.
I see the larger units tank more air,
small trick to help smaller compressors, is to hook in an air bubble tank, in to the line, and it will give you more reserve air,
last year I was using a large air hammer, called
chipper, with a home made rod driver, and it was about 20 cfm, and had a small compressor but it would drive the post, and the compressor could recover as we moved up to the next post location, now under normal conditions the time that the 98E would drive the post is reasonable quick, (now I will admit these posts I just drive did take a few mins to drive)
one more thing is this is not a large AIR HAMMER,
it a air powered driver,
there is a sleeve that clamps in to post, and the air powers a small piston that pushes up the driver up that is weighed just like a manual pounder,
and when it is raised up the piston reverses and drives the outer sleeve down with the weight , hitting the top of the inter sleeve clamped to the post, driving the post in,
thus it is not a constant pounding of a air piston. like in a air hammer, but a powered driver that work similar to a hand driver, up and down, up and down, up and down. thus the more efficient use of the air source,