I'm glad someone started this topic. I thought someone might, but of course got caught up in the holiday time crunch, and am just here now. Interesting, and reading through... I'm not sure I know any more than I did before, but people sure are worried/mystified/opinionated about their rib roasts.
We had 22 people over and I bought a 24 lb. bone on 7 rib roast. $180, which is a massive amount of money for us, so I've been worrying about it for the last 2 weeks. Have made a couple rib roasts before, but nothing this size. I have used the slow and low method, but in my experience it really does vary as far as cooking time. Have done it with the ribs and without the ribs attached and that makes a difference, the size/weight makes a difference, the crowd's anticipation makes a difference, the stress level of the cook, etc. This year, with 22 people, all with varying schedules, I had to make sure it was on the table by 2:00PM.
According to my CI recipe, they calculate 30 min. a pound @ 200 degrees. So that meant starting my room temp roast 12 hours ahead of time, @2 AM. But the last time I followed CI, it took much longer than the recipe. I've seen other slow and low versions that say more like 45 min./pound is more realistic. That meant putting it in @8:00PM the night before. So somewhere between 12 and 18 hours for a 24 lb. roast. I wanted to err on it being done early, b/c I figured I could always keep it warm for a few hours if necessary, but didn't want to go into a holding pattern with 22 guests if it wasn't ready.
I ended up putting it in @8:30 @200 degrees. I lowered it to 175 when I went to bed @midnight because I got worried and thought I could always blast it with heat if it wasn't anywhere near done by mid morning, and left a note for my early-waking, but non-cooking wife, to check the temp and pull it out if it was anywhere near 125 degrees when she got up @6:00. Though with the lower temperature, I figured I would be safe until 8:00 or so and could just keep it warm until dinner.
Well, I slept horribly, wondering about the roast, wondering if I left clear enough directions for my wife, wondering if we were throwing $180 worth of beef into the garbage. Finally, I got up @4:30 AM to rewrite my note. Checked the temp, and it was @133 in the the small end and 128 in the thickest part of the roast. Basically, perfectly done, except for the fact it was 4AM.
Pulled it out, tented it, rewrote the note for my wife and we kept it warm by throwing it in and out of a super low oven all day until dinner. The upshot is that everyone loved it, but I am revising any advice on the lb. per min. formula. This 24 lb. roast was done in exactly 8 hours cooking it for 4 hours @200 degrees, and then lowering it to 175 for the last 4 hours. That's way early by any recipe I've read. Of course, it makes sense that at some point an oblong shaped piece of meat will heat through within a given time, even if it's bigger or denser than usual, but I haven't seen an accurate gauge for large roasts. I would say, that beyond some point (where exactly, I don't know) you don't have to continue calculating longer cooking times for large roasts, even though they weigh more. If anyone knows where that change in the formula kicks in, I'd love to hear about it.
Edit: Dang, this was supposed to go in the Rib Roast Successes and Disasters thread, but I screwed it up and made a separate post. Don't know how to move it, so...