120 Gallon offset throat size issue

[Miles1919]

Hey, I picked up a 120g offset pit and it has a lot of back pressure. I looked it over and found that the throat of the pit is way undersize. The cooker specs are as follows:
120G tank
24" dia x30" long round fire box
6" diameter stack
the throat is an oval cut with a roughly 50 square inch area.
After looking at a lot of pit calculators the cooker needed a 112 sq in area cut.
does that size cut sound right to every one?

[hogaboomer]

I ran it through Feldon's and the one at the top of the page, they both said 108 sq. in.

[Pete Mazz]

FB is way too big at 150%. That's what's throwing the throat size off. If the FB was sized right the throat would be ~80 sq.in.

[dacolson]

That FB is pretty big too. If I were building this, I’d cut the FB back to around 22” in length and I’d make the throat about 80 inx2. 24x30 is too big. Here are the calcs:





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[Miles1919]

Thanks for all the replies. Other than consuming too much wood, what is the down side to leaving the firebox that much over sized or should i just add a 20'' pipe inside it and insulate it?
Thanks

[hogaboomer]

Pictures would be helpful. First thing I would do would be to make the throat bigger, then see what that does.

[Miles1919]

https://www.instagram.com/p/CQUv6vTsXzK ... =copy_link

I dont know how to post pics but here is a link to my IG with picture of the pit

[dacolson]

It will be hard to keep your heat down. A smaller fire in a big FB won’t heat you CC. Build it bigger and your temps will take off. The small throat is restricting airflow so you’ll get a lot of dirty smoke with a bigger fire.


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[dacolson]

https://www.instagram.com/p/CQUv6vTsXzK ... =copy_link

I dont know how to post pics but here is a link to my IG with picture of the pit

That’s a really nice looking pit. I’m guessing the throat matches the welds between the fb and the cc? That’s too bad if so. You’ll be cutting them apart to make the throat bigger.


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[Miles1919]

@dacolson Thats pretty much what i found with my last few cooks. It took a big fire and a long time to get it up to temp and the restriction was very noticeable. Yeah Im super happy with how it looks so im a little sad to cut it up and enlarge the throat but cooking performance is king..... I only have a 110v welder but a can put down some nice welds so it should look ok when done

[hogaboomer]

Did you buy that new? Take it back and make him fix it.

[Miles1919]

I did. Tragically the guy that made it passed away the day after i picked it up...

[towtruck]

I would leave it attached and open up the throat around the bottom of the joint and fill in the area with steel welded to both the FB and CC. I would also make a baffle that directs the heat upward just inside the CC. (A member here has good pictures of one) I would then line the fire box floor and up the sides a tad with fire brick and give it a whirl.....That is what I would do if I had that pit.....small amount of work first to see if it will function correctly with minimum effort.

[hogaboomer]

I did. Tragically the guy that made it passed away the day after i picked it up...

Well, dang the bad luck.

That might be hard to fix with a 110v welder.

[Miles1919]

Thankfully its hot today and the pits out side- so with a little help from a map gas torch i'll get her to stick back together

[Big T]

https://www.instagram.com/p/CQUv6vTsXzK ... =copy_link

I dont know how to post pics but here is a link to my IG with picture of the pit

That's a nice looking pit. Maybe this'll help with posting a picture on a pc https://smokerbuilder.com/forums/viewto ... =50&t=1874

[Miles1919]

@ big T thanks for the link! I ended up cutting the fire box off and increasing the throat size and fired her back up! I was happy to see a big draft improvement. I may reduce the internal volume of the FB depending on how it handles here on out. it was pretty windy today but the cooker ran great after the clean out and so i smoked some sausages on it to see how the increase affected the temps. The bottom to pics are the old joint and the throat size with a coffee mug for scale. She not as straight as when i got it but that doesnt matter and i also added a grease dam to you know hold back the goop.

[dacolson]

Man! You made quick work of that. Hope this gets it done for you. Nice work there.

[towtruck]

Nice work!

[hogaboomer]

I wish I could do anything that fast. How big is the throat, now?

[Miles1919]

I ended up going to 108 sq in-ish. I figured if i need to make the FB smaller its super easy to make a restrictor baffle on the throat than having to open it up again. My body is for sure felling it today! I was stupidly man handling the FB into position for fit and trimming each time and then finally got a 2x4 and strapped it to the main tank to act as a crane. It worked out fantastic. Having a plasma cutter at home made pretty short work of the removal and trimming. Thank you to everyone for your input!

[dacolson]

Been there. In my mind I’m 20 years old still lol

[Big T]