How can I do more sit-ups in a minute, as I can already do crunches with 160 pounds?

What I mean by the weights versus situps is that you are comparing apples and oranges. Case in point, at the GNC show a few years back I got third in the pull-up competition. When I was banging them out, a bodybuilder was laughing because he was up next. This guy was huge, made me look like a tiny man to say the least - muscles rippling all over, lat spread the size of one of the great lakes. However, when he got up to do the pull-ups, he only was able to do about 8.

Now, if you loaded a 200 lb plate around his waist, he could have probably done a pull-up but it would have ripped my arms off.

Why?

Apples and oranges.

Doing a 160 lb crunch simply means you have explosive strenght in your abs - you can generate a major force in a short period of time.

Doing more situps in a minute, however, doesn't require strength, it requires local muscle endurance. These are two different factors entirely.

For example, when you train for endurance, your muscle responds by building more organelles - mitochondria, myoglobin, etc - to facilitate recovery and endurance. When you train for strength, your muscle becomes more efficient and you have a central nervous system response.

So if your goal is to do more situps, train with volume rather than weight - create the endurance to complement the explosive power.

Just like if you want to bang out more pull-ups, you don't do so by adding weight. You do it by doing more pull-ups.