In general, there are much better and more effective ways to train your abs other than performing thousands of crunches per day. You can crunch yourself silly and do very little other than cause unnecessary harm to your back. But one crunch exercise that IS very effective is the Bicycle Crunch. Due to the number of core muscle fibers that it involves, it's one of my favorites! And I'm not a huge 'crunch' person.
Much like any other abdominal-based movement it's important to keep this in mind:
When dealing with the abs, it's not about reps and most definitely NOT about the speed of reps, rather it's about time under tension. Slow and controlled movements that keep almost constant tension on your abs throughout the exercise will rip and chisel you some rock-hard abs!
Begin with a time segment of :20 and build up to where you can perform a slow set of bicyle's for 1:00.
FastFit Training & Fitness