Fire Tube Sizing for 5 - 10 hp generator

I have a 7000 watt Honda generator that I am planning to run on woodgas. I already have an 8" x about 15" fire tube inside a botched fema build. Don’t worry, I didn’t run the gas through an engine. I read about fire tube sizing in the premium section but those figures don’t seem to apply to small engines. What would be a baseline for firetube dimensions for this engine? I’m pretty sure it is less than 10hp but more than 3.5hp. I need a starting point.

Joe: I don’t know about your particular model but the 7000 watt Honda models I just looked at are all in the 12-18HP range. That said, I’d love to know the optimal sized tube for a scaled-down WK gasifier for this type of application.

I am in process of rebuilding my small gasifier, it was a fema converted to WK style. but because of the very thin wall fire tube (originally I used a stainless fire extinguisher for a burn tube and it burned a hole through) I still am keeping my 8" tube x 3/8 wall this time and 5x 1/2"nozzles and 14" length…I have a coleman 14.5 hp B&S 7500 watt generator that seemed to work good I did have 6 nozzles but the new tube I am going to 5… I also have a 4" restriction and have raised my plate to 11" below the nozzles. I plan on running an 18 hp Wisconsin occasionally off this also…That is why I went to an 8 inch tube instead of 6 or 7 inch…Plus the wood does not bridge as bad…you will need a grate shaker as with the constant pull and no bouncing or shaking there sometimes gets a plug at the restriction plate…smaller fuel would probably stop this but wanted to stay with same fuel I will be using on my truck build …

Dennis Pecynski

It looks like you are right Brian. Here are some important specs on the engine. Honda gx390: 11.7 hp, Displacement 389 cm3, Compression ratio 8.2:1, Ignition System Digital CDI with variable timing and pull start. Dennis, It sound like your dimensions: 8" tube x 3/8 wall this time and 5x 1/2"nozzles and 14" length with 4" restriction might just work. Should I consider a smaller diameter as you are? What do you think? Anyone else chime in. There is safety in many counselors.

Hi JoeT
Member search up Tomas McDaniel and look back on his “most recent posts”. I recall he is also buillding a downsized WK sysem for a similar engine/gen needs to replace his “Imbertized” Chinese gasifier. I seem to recall he was designing around these same dimensions for his downsized WK now.

I agree with DennisP - do not go smaller than a 4" (100 mm) restriction opening. I have used as small as 2 7/8" (73mm). Fuel cutting the small blocks for this small of opening will drive you to drinking and counting your fingers. You should with WK heat recycling and engine electrical load be able to keep the larger size sucked up good, hot and clean. DO NOT TRY AND BLOWER LOAD TO THIS LEVEL OF PERFORMANCE - waste of time amd effort. Most of my small engine powering experience is with modified high velocity/small oxidization/reduction sectioned Imbert types so I can say dimentioning experice with these will not work on a moderate velocity/larger slower oxidization/reduction sectioned WK “char-pot”.

Very good of you DennisP to put up your real world experiences on this with the why’s

Steve Unruh

what type and size fuel should be a determining factor for your build…I built my unit to use same fuel I would use to drive on…if you use say wood pellets…you can go smaller…

The fuel I am hoping to use for this small-scale WK Gasifier would include the woody brush I have covering 3/4 of my property (mostly wild rose brush “Bald Hip Rose” and snow berries.) I hope to run it through a chipper/shredder which should yield pieces about the size of most compressed wood pellets. I also plan on getting some of the spare wood chips and chunks from the local tree services which would probably be larger wood chunks, maybe 1"x2"x0.5". I’m hoping a mix of the 2 should work out OK.

So far I have a piece of 3.5"ID x 15"L pipe that I think is just too small, even for this. I did see a section of about 6"-8" ID pipe at the scrap yard that had maybe 14" of usable length to it. I didn’t get a chance to actually look at it up close though because I saw it out of the corner of my eye as they were kicking people out at closing time.


Steve, I did look up Thomas Mcdaniel and read the post about his WK build. What is WK? I didn’t find it in terms. He is going with an 8" x 14" fire tube with replaceable throat and 5 nozzles. However his build is for a 25 hp tractor engine. Perhaps I should go with a 6"x12" for my 11.7 hp. What do you think?

According to the FEMA document for stratified downdraft gasifier construction any fire tube smaller than 6"ID runs the risk of fuel bridging. Going with a 3.5"ID might make the unit more finicky about fuel size ie chips or chunks. Smaller fuel might run the risk of bridging causing the unit to overheat. The 6"x14" tube may work out better even if you have to fabricate removable throats of different restriction sizes so you can remove one and try another untill you get it just right. My answer is based on reading and very limited experience with a fema build.Just a thought.

WK = Wayne Keith Style of Gassifier

Joe, your 11.7 hp on your 7000 watt generator…is telling me you have a 5500 -6000 watt generator and it has a 10% surge time of 7000 watt…it takes 2 hp to produce 1kw of power…and you have a 30% in power loss on woodgas…so your 11.7 hp engine running on gasoline will only be about 8 hp on woodgas…so the maximum power you can expect would be maybe 4000 watt continuous …as my generator is 7500 continuous and 9000 surge…I can expect 5000 to 5500 watts on woodgas…woodgas has lower btu’s so less output of power…if you want the same output on woodgas you will have to change to a larger hp engine…I have already been looking for one for my setup…


I’d use the same math as Dennis… 1Kw / 2 Hp / 4lbs wood mass per hour.

Hi Joe

Sorry about the slow responce but i don’t get on site much as of now

I am building an 8" tube wk gasifier i don’t have it done yet but i do have a china gasifier i modified to several diff specs

I think Wayne gave me good advise on the size for if you go below 8" or so you have to use really small wood to feed good

i think i would go with 8" tube and start with a 4or 5" restrictor plate that you can change

I don’t know if your a premium member but if you are i think there are some pics of mine plus waynes and my posts on sizing

I will post this on DOW

good luck


Thomas, thank you for your reply. Yes I am a premium member. I have already viewed some of your pics but will watch for more.