You can do it in 6 hours if you kick up the temp. You'll probably need to run at 325 or so. I have no idea about timing on the ribs at that temp, however.
When I cook in my WSM I bury 4 or 6 fist sized chunks at various points in the charcoal in he ring and I put one in the chimney to start.
"Ron Rico, Boss. You can call me Captain Ron..."
The Naked Fatty!
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