Sirloins don't require as much smoking time as other cuts, so you have more control over the final temperature. Unlike tough cuts, such as brisket, which need several hours of smoking at an internal temperature above 180 degrees Fahrenheit, sirloins have about the same tenderness at medium-rare as they do at medium-well, so cooking time depends largely on how smoky you want them to taste. You can even use pungent hardwoods like mesquite to smoke sirloin because they won't overtake the flavor like they do other meats. Whether smoking sirloin steak or roast, the cooking time per pound is the same.
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Coat the sirloin with a layer of kosher salt and let it stand in the refrigerator for at least 24 hours before smoking it. This dry brining method leaves beef with a better texture than wet brining while providing the same tenderizing benefit. Place the sirloin on a dish under a few paper towels to absorb the moisture it releases; you can leave the sirloin uncovered in the refrigerator.
Scrape the salt off the sirloin and pat it dry about 1 hour before you want to start smoking it. Let the sirloin reach room temperature.
Soak 3 mesquite or hickory wood chunks in water for each pound of sirloin you're cooking.
Light about half the amount of charcoal you normally would for your grill and pile it on one side of the charcoal tray while the sirloin sits. Attach a grill thermometer or an oven-safe meat thermometer in the center of the cooking grate a few inches from the coals.
Set a pan to catch the sirloin's drippings on the side of the barbecue opposite the coals. Close the grill cover and let the barbecue heat for about 15 minutes.
Check the temperature of the thermometer. Adjust the damper on the grill cover to raise or lower the temperature between 200 and 275 F. The ideal smoking temperature is 225 F, but factors like wind make it difficult to maintain a precise temperature.
Place 3 wood chunks on the coals and close the lid. Place the sirloin on the side of the grill opposite the coals once you see smoke exiting the grill.
Check the temperature of the barbecue every 45 minutes and turn the sirloin over Top off the charcoal when you check the temperature and add 1 or 2 more wood chunks as needed. Wood chunks take at least an hour to extinguish, so you don't have to add them as urgently as you do coal. If you see the temperature dropping below 225 F when you check, close the dampers on the lid a bit so the temperature increases.
Smoke the sirloin 45 to 1 hour per pound; smoke the sirloin closer to 45 minutes if you want medium-rare to medium, and closer to 1 hour if you want medium to well done. Medium-rare sirloin will be a bit more tender than well done, but not much, so your cook time mostly depends on how smoky you want your meat.
Take the sirloin off the grill and let it rest for 5 to 10 minutes per pound.