In a blender, grind the garlic, oil, salt and black peppercorns to a coarse paste.
Pat the tri-tip dry with a paper towel and score the fat layer with a sharp knife, cutting through the fat, but not through the meat.
Place the meat in a sealable plastic bag, scrape in the garlic paste, press out the air and seal tightly. Massage the meat with the garlic paste until it is evenly coated. Set aside at room temperature for at least 1 hour. If you are going to marinate more than 2 hours, refrigerate the meat but remove it 1 hour before cooking to allow it to come to room temperature.
About 1 hour before serving, start a fire on the grill using 1 chimney full of charcoal briquettes, about 50. Put one-fourth pound of oak or hickory chips in a bowl and cover them with water. Place an inverted plate on top of the chips to keep them submerged. When the flames have subsided and the coals are covered with white ash, dump the chimney into a mound on one side of the grill. Drain the wood chips and scatter them across the top of the coals.
Sear the fat side of the tri-tip, cooking directly over the flames with the grill lid off. This will only take 3 or 4 minutes. Don't worry if there is a little char; that is almost necessary in order to get a good crust. When the fat side is seared, turn the tri-tip and sear the lean side directly over the coals. This will take another 3 or 4 minutes; again, don't worry about a little char.
When the lean side is seared, move the tri-tip to the cool side of the grill and replace the lid, with the vents open. Cook to the desired doneness, checking the temperature of the meat every 4 or 5 minutes. It will take 20 to 25 minutes for 125 degrees, which is on the rare side of medium-rare, 25 to 30 minutes for 135 degrees (on the medium side). Cooking times will vary according to the type of grill and temperature of the fire.
Remove the roast to a platter and set aside for 10 minutes to finish cooking and for the juices to settle. Carve the tri-tip fairly thinly (at most one-fourth-inch thick), against the grain and with the knife held at an angle to give wide slices. Spoon the carving juices over the meat.