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Author Topic: husqvarna 3120xp hotsaw build  (Read 1417 times)

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Offline recurve60

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husqvarna 3120xp hotsaw build
« on: May 11, 2018, 01:36:15 AM »
anyone out there have experience building a 3120xp hotsaw for loggers sports racing? I am going to build one just wondering if anyone has specs for porting and using a degree wheel as well as carb drilling and cylinder base gasket removal regarding if there is enough room left in the squish.  any info would be appreciated thanks in advance.

Online mike_belben

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Re: husqvarna 3120xp hotsaw build
« Reply #1 on: May 11, 2018, 10:03:40 AM »
I dont mean to offend with this, but those are very basic questions and a very very expensive saw to learn on.  I advise you twiddle your thumbs as long as possible before putting burrs to that jug.  The hotsaw section of arboristsite has over a decade of such topics.  Offhand i cant saw what blowdown and timing numbers are best suited for a 3120.  BLsnelling and mastermind are two user names id pay attention to.  

The higher the rpm, the looser your bores and larger your piston, the more rock at tdc and thus the more squish clearance youll need to prevent contact.  Below .030" id think is starting to get close. 

Id probably mate a dirtbike carb to a 3120 for an open class.  They have a lot more precise metering in my opinion.
Revelation 3:20

Offline recurve60

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Re: husqvarna 3120xp hotsaw build
« Reply #2 on: May 11, 2018, 11:24:30 AM »
thanks for the advise, i have 2 cylinders for it and one is an experimental cylinder so not worried about messing it up, it is just to run once a year at our local logger sports show.  also i get parts at cost because i work at a dealer so again costs are much lower for me.

Offline Al_Smith

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Re: husqvarna 3120xp hotsaw build
« Reply #3 on: May 11, 2018, 10:09:25 PM »
Do you have any idea what you are getting into? You want a 3120 hotty you'll have 3 to 4 grand or more in it if you want it to be a winner .Besides that you won't find anybody who builds the winners  give you one bit of info on what they did .I'm just telling it like it is,no offense intended .Fact to get one built,if they don't know you well you won't get one built period .

Bradley David Snelling and Randy,AKA mastermind as well as myself and others can build a decent ported working saw but I doubt they would tackle an alkie burning hotty  .I know I would not .Too rich for my blood .

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Re: husqvarna 3120xp hotsaw build
« Reply #4 on: May 13, 2018, 12:56:11 AM »
If its just for fun and you can afford it, ill give you some general things to consider.  

Transfer ports need a bellmouth inlet with generous radius for air to attach and flow as laminar as possible.  All the transition edges where the jug meets the base need to also be free of edges that create a turbulent wake.  This is how i manage that.  



A 2 stroke has 2 compression ratios but we only think about one.. That is above the piston. There is also the compression ratio below the piston.  Charge inertia will tend to fill the crankcase and it is pretty laege space able to fit plenty of charge compared to the combustion space.  but how much of that mix can we then shuffle from below the piston to above the piston?  once the piston skirt closes the intake port, the shrinking crankcase volume has to force the charge up into the combustion space.  But consider this.. The topside compression ratio might be 10:1.  The bottom side might only be 4:1.  Do you see the issue there? There is often much improvement to be made in addressing this, by a few difference means, in my opinion. 


As you raise the exhaust ceiling you favor higher rpm.  But then you usually have to raise the transfer ports (and "lay them back" toward the intake side of the chamber) as well.  

Widening ports is a no brainer, just dont get into the ring ends and dont get too thin on the gasket margin.  Make all your intake ducting smooth and see what can be done to lighten your piston as well as smooth it out for air to flow around it with less turbulence.  I have drilled some good sized radiused holes in the pin boss area a few times to help air directly under the piston crown get out from under it and into the transfer ports via the shortcut i made.  Also it reduced a few grams of weight.  I have welded domes on a handful of pistons also.   

Will this be piped or muffled?
Revelation 3:20

Offline ehp

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Re: husqvarna 3120xp hotsaw build
« Reply #5 on: May 13, 2018, 05:25:08 PM »
Al hit the nail pretty much on the head , in todays world you will not get a hotsaw made for under 5 grand , stroked cranks and parts cost to much, a good stroked crank is over a grand and you still got to get a different rod, 3120 rod is junk. if it runs on gas its not a hotsaw, its just a ported worksaw

Offline Al_Smith

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Re: husqvarna 3120xp hotsaw build
« Reply #6 on: May 13, 2018, 05:59:36 PM »
 :D I was going mention the crank and throw the poor gent in shock---- but since you did  8)

Offline Al_Smith

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Re: husqvarna 3120xp hotsaw build
« Reply #7 on: May 13, 2018, 06:32:59 PM »
Seriously it's big expensive undertaking .A lot of money for a 3 second run a couple times a year .I said 3 seconds because any thing much over that you won't place well .My best ever over the blocks was 4 and the owner of the saw beat me by about a half a second .It's fun but expensive .

Offline ehp

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Re: husqvarna 3120xp hotsaw build
« Reply #8 on: May 13, 2018, 08:02:56 PM »
not many guys building these days if any

Offline Al_Smith

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Re: husqvarna 3120xp hotsaw build
« Reply #9 on: May 13, 2018, 10:28:20 PM »
Trends change,opinions change .There was a time when the rage was cookie cutters,high speed work saws that probably never cut a stick of round wood .GTG champions .Now it seems from true piped alkie burners they want that pulling power like a Clydesdale .I'm okay with that . :)

Offline Canadiana

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Re: husqvarna 3120xp hotsaw build
« Reply #10 on: May 14, 2018, 11:20:06 PM »
mike_belben: I like hearing some support. I respect that sort of thing! What else should a guy dream about when he has 2 cylinders to practice his craft
The saw is more fun than the purpose of the wood... the forest is trembling 🌳

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Re: husqvarna 3120xp hotsaw build
« Reply #11 on: May 15, 2018, 03:20:04 PM »
Im 38 now.  I tell the 22yr olds blowing all their time and money on hot rods and lifting with their backs insteada their legs the same darn things i didnt listen to when i was 22 and those jaded middle agers whod been there done that and slipped a disc doing it told me not to do.  Part of aging i guess.  


You know how many hours i put into rerouting oil drainback not to splash on a crank, and making scrapers that fit just right without touching?  Too many.  How much power did it make?  Who cares.  It kept me out of trouble.  Idle mind and all that.
Revelation 3:20

Offline recurve60

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Re: husqvarna 3120xp hotsaw build
« Reply #12 on: May 24, 2018, 11:46:15 AM »
thanks for all the info, i appreciate the help, i realize the cost of a true competitive saw as i have asked those questions to logger sports champions like karl bischoff and mitch hewitt.  they helped me and my friend expand our Smithers loggers sports show last year.  My intent is not to build a crazy hotsaw just something to have fun with in the 15 to 20 hp range.  guys at walkers saw shop says that is acheviable with mostly stock components that are modified. once again thanks for the support


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