Anyone do crunches with weight?  Put a couple 10 or a 25 pound disk weight on your chest and crunch?  how well does that work?

I do the following.

Maximum incline on sit up board, 33 crunches with weight held on crown of head, no rest immediately 33 more cruches with weight held on chest, thorw weight on floor do 33 more crunches, sit up and get off board for last (100th) rep.

I alternate left twist, right twist, straight as I do these.

When I am out of shape I start with a 10lb weight then in a few weeks go to 25 lb plate.  Usually do 35 lb plate in semi good shape.  The 45 lb plate is so big around it usually is too big and interferes in range of motion as it presses into your stomah/hips and is jammed into your throat so I have to hold the 45 lb plate over my face instead of hugging it to my chest after the over the head reps if you can picture how silly that looks.

I hate doing crunches and sit-ups on the floor, and always felt too many were needed to be practical without resistance (added weight).

I've since bought an inversion table for all my ab exercises and it's FANTASTIC.

No back strain or pressure often caused by the floor. No additional weight needed since total inversion obviosly requires much greater effort to "sit up". You can do crunches completely inverted, or in any partially inverted position. Feels good, streches you out nicely and real easy to get started.

Where as crunches and sit-up's require a fair amount of time to feel the burn, with an inversion table, the burn occures MUCH sooner since so much more effort is needed in the 1st place.

My inversion table looks like the one below, cost about $130 (Sam's Club), easily folds away to store and doesn't take up much room when it's standing. I'll never be without one.

Ofcourse, it is another muscle group, so do it with weight will increase the density in that area.

I do delinced situp's with a 45 lbs weight. for 3 sets of 10.

Hypertrophy increases the size of the muscle. If you have a weak midsection, go for it.

However, if you already have a strong set of abs, with a layer of fat over your six-pack, bulking up even further isn't going to do you any favors. For that, you need to increase your caloric deficit to get rid of the flab. For that you want high-rep low-weight.

Quoted: Hypertrophy increases the size of the muscle. If you have a weak midsection, go for it. However, if you already have a strong set of abs, with a layer of fat over your six-pack, bulking up even further isn't going to do you any favors. For that, you need to increase your caloric deficit to get rid of the flab. For that you want high-rep low-weight.

The key, like you said is to increase your caloric deficit.  But this whole low weight high rep crap is an old wives tale.  To loose fat your need to burn more calories than you take in per day.  To do that you need to either work out more, or reduce your caloric intake.  If your goal is to reduce your BF while you retaining your muscle mass you want to keep lifting heavy.  If you dont keep lifting heavy your muscles will not retain as much muscle, because you dont need it.  

+1 high reps will not reduce fat... you cannot spot reduce fat.