Prime Rib cook time isn't determined by weight, but rather, by the thickness of the loin. A 10LB 4bone hunk of PR that is 5 inches thick will take the same amount of time to cook as an 18LB 7bone whole PR that is also 5 inches thick. Picture grilling a 1.5in thick 1lb NY Strip. Cutting off the top 1/3 of the Strip (and reducing it's weight by 1/3'rd) won't change the cook time as the piece you are left with is still 1.5 inches thick. Same thing works with an oven/enclosed cook chamber.
As for chamber temp, as others have noted, higher chamber temps around 300 and above will give you a bullseye effect where the outer part of each slice will be well done, then mid well, then medium... as you work towards the center.
If you want a more consistent edge to edge color, go with a lower chamber temp like 250 or even less. I usually try to stay under 225 if I can help it.
@250 chamber temp, a avg PR about 5 inches thick should take around 4 hours or so. Monitor the IT with a remote therm. If it looks like it will finish too early, reduce your chamber temp. If it's going to slow, increase a bit (up to 275) to get it done. If it does finish too early, pull it, let it vent, then stick it in 170 degree oven, or in a towel lined cooler to hold.
As for chamber temp, as others have noted, higher chamber temps around 300 and above will give you a bullseye effect where the outer part of each slice will be well done, then mid well, then medium... as you work towards the center.
If you want a more consistent edge to edge color, go with a lower chamber temp like 250 or even less. I usually try to stay under 225 if I can help it.
@250 chamber temp, a avg PR about 5 inches thick should take around 4 hours or so. Monitor the IT with a remote therm. If it looks like it will finish too early, reduce your chamber temp. If it's going to slow, increase a bit (up to 275) to get it done. If it does finish too early, pull it, let it vent, then stick it in 170 degree oven, or in a towel lined cooler to hold.
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