London broil... In chili?

I love using London Broil (aka top round..London Broil used to refer to a preparation, not a cut of meat) for homemade beef jerky. It's extremely lean and has little to no collagen. That's why it's perfect for beef jerky. Did it ever soften up? Cooking such lean meats for a long time usually toughness it. I hope it worked out for you.

Chili with a large chunk of meat is essentially a braise. Meats that do best under toes conditions tend to be the "lesser cuts," such as short ribs (either beef or pork), or a beef chuck roast. A tri-tip would even do well. You'll get the best results, in other words, using cuts that contain lots of collagen to hold them together. Collagen beaks down through long, slow, moist cooking, lending that silken texture and meat which is oh so easily shredded as a result. Best of all, these costs are all pretty cheap. London Broil is a lovely cut, and on the pricy end, but because of its leannes and density of muscle fibers, it does best in an acidic marinade for a while, followed by grilling.

180 because this is a good temperature to break down connective tissue.

Depending on what cut it actually is, it might tenderize eventually. If you're not going to eat it anyway, no harm in putting it in a very low 180F oven for about four hours or until it softens up.