We have just purchased a 1620 and are wondering the same thing. The 20 series has a 20" rotor while the others have 24'. The grain tank holds 125 bushels. We plan on running a 4 row and when it is muddy rent or borrow a 6 row head.
Not sure about the 6 row, sounds pretty big for 1620. 125 bus tank. 124hp It depends on your situation maybe its all you'll need. My dad always told me "The day you buy is the day you sell".
i bought a 1620 2 years ago. it replaced a 715. very happy with it. nice not having those straw walkers. i would not put on a 6 row head. it will fill up the grain tank faster. you might need a 3rd lift cylinder and weight on the back end. it really is a 4 row machine. what model corn head do you have. i'm looking for a 1043 corn head. good luck! Duane
We use to own one with 4 row 36". Neighbour has one use to run 4 row wide now he runs a 6 row 30" seems to get along just fine with it.Have heard of guys running 20' grain head never seen it but should be okay if the hills are not to steep good luck!