now for part 2 the advanced mods:
first is easy: go to a muffler shop and buy a length of new 2.5-inch exhaust pipe. the length you want is probably 9.5 inches. then, have a 45-degree angle cut at one end. finally, have your muffler guy expand two inches of the end of the pipe to fit the base of your chimney (be sure to bring your chimney base - mine was somewhere between 2.5 and three inches), thie chimney extension will now fit very snugly in the base and extend 7.5 inches down into the smoking chamber. this forces the air, heat and smoke to draw across the bottom of the smoking chamber, rather than skirt straight up diagonally from the firebox to the chimney at the far end.
over the last few weeks, i have taken the final steps in maximising the potential of the outstanding properties of this pit. the first is the manifold. otherwise known as a baffle, tuning plate etc., the purpose of the manifold is to both deflect direct heat from the food and act as a heat retention device in order to maintain steady temperatures across a pit. we can argue semantics and the science of heat transfer of all day, but that's a good layman's description. the thick, heavy manifold takes a while to heat up to temperature, but once it does, it is like you are on autopilot and you can maintain temsperatures for a very long time with only a very small addition of fuel. the improvement in fuel efficiency is nothing short of miraculous, which is important any time, but even moreso in winter - and that factor is multiplied exponentially where i live.
this manifold is modeled on RIVET's design, with a few differences that reflect both my preferences and also the necessities imposed by geography. The manifold is constructed of quarter-inch steel plate. rivet has a spec sheet drawing with specific dimensions etc., but basically it is wide enough to sit in the smoking chamber under the grates, and long enough to reach to the midpoint of the smoking chamber. the "western" end of the manifold is canted at a 45-degree angle and butts up against the opening between the firebox and the smoking chamber perfectly. there are a series of vents blown through that start small on the canted portion and gradually get larger toward the eastern end. there is a handle for conveniently lifting the manifold in order to set it on or take it off the pit, and also for moving it east or west, if desired for temperature and smoke control. the space between the canted portion and the edge of the handle will accomodate a loaf pan, should a water pan be desired.
there is also a secondary manifold that is the same width as the primary, but just long enough to reach the western edge of the chimney. this secondary manifold is specifically designed for this north montana latitude and climate, where temperatures during the winter are so low that they don't get talked about on the news. because of our special tempeerature conditions, i wanted another level of heat retention, and the secondary manifold provides just that.
the "great northern" moniker is a reflection of the history in north-central montana and is a nod to james j. hill's great northern railroad, which blazed the trail into this area. in the winter, temperatures can be very, very far below zero; last week, i woke up to 38 below zero, no windchill. in a climate like that, you need all the thermal mass you can get!
here's a series of pictures showing the design features of the manifold.
due to the fabrication process of the manifold, it is necessary to do a "pre-burn" in order to burn off any residues or oils. we prepared for this by filling the trusty charcoal basket (another outstanding rivet-designed mod) with cottonwood and some chunks of scrap 2x4. note: the 2x4 sections (and pine, for that matter) would NOT have been used if we were going to cook, of course, but for quick, hot heat, it works very well.
i would have used charcoal to get things going, but there were no opened baggs of it around, so this was an all-wood burn.
we set the primary manifold in place; butting it up right against the western wall of the smoking chamber:
here's a good view of how the canted portion works to deflect direct heat away while allowing smoke and ambient heat to waft into the chamber:
we then set the secondary manifold in place, off to the east of the primary, below the chimney:
here's what we had when both manifolds were in place:
as you can see, the secondary can be moved around if necessary in order to regulate airflow, smoke infiltration etc.:
we wadded up some newspaper and tossed it under the charcoal basket, then lit it up. before long, we had a good, crackling fire, but because much of the wood was wet on the outside, there was a lot of smoke. all the better to see the vent pattersn perform exactly as advertised! soon, we were getting some good warmth on a chilly day.
temperatures took a little while to rise while the firebox lid was open (we were letting the wood catch well before closing the lid). half an hour or so later when the wood was burning well, we closed the lid, and the temperatures rose quickly and dramatically. the outside of the smoker got good and hot, too hot to touch, and i took a look in at the oven thermometer i had set in the smoking chamber at the far end (so i would know the temperatures farthest from the fire). the temps were already past 300 degrees. the manifold was doing it's job perfectly, keeping direct heat away while also evening out temperatures. it wasn't that much later that we were at 400, then 450.
i had a ridiculously easy time holding the temps at 450 for a half hour, which i figured would give enough time to burn off any residues. the manifold of course blackened as it seasoned, but it's all good, that's what it's supposed to do.
i had to leave for a while to head out to my parents' place and help my dad with something. i got back maybe an hour later and temperatures were still above 350. not bad, consideirng i was using fast-burning cottonwood by now, and it was a cold day. i put a few small logs of cottonwood on and brought the temperatures up quickly up above 450 again, and then since then have let it burn down.
all-in-all, a very successful pre-burn. i am very much looking forward to using this at regualr cooking temperatures for cooking barbecue. i was going to toss a chicken on today to cook and then de-bone/chop for a soup tomorrow, but in the end decided not to until i can clean out all ash etc. from the treated 2x4s i was burning. i will be sure to report on my first smoke with this outstanding modification package!
a million thanks to RIVET for all of his assitance with the great northern manifold project. without him, it wouldn't have been accomplished and i am grateful!
once the manifold was accomplished, i turned my attention to the firebox door:
the reason for this mod is because the original (on the right) is made of very thin metal. after a season or so of real barbecue, the thin metal will warp and the door's ability to regulate airflow is compromised. the solution? build an exact copy, except in 1/8" steel plate.
as you can see, this metal isn't going to warp any time soon, if ever. the difference between the two is clear. there is an extra advantage in that retained heat is kept in the firebox rather than being lost through the thin skin of the firebox door. we're currently looking into the possibility of a "sheath" that will insert into the bottom of the firebox to perform a similar heat-retention duty, and also to protect the bottom of the firebox from deterioration due to heat and moisture.
the new firebox door is an exact copy, using the old one for a template. the butterfly "shutter" regulates the airflow and can be adjusted accordingly. here's a view of the back side:
the shutter turns easily from open to closed: