Chainsaw pipes

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Chainsaw pipes

Post by Rx7man on Sun Jul 12, 2015 12:36 am

I have a bunch of OLD chainsaws (Husqvarna 65's), and since I have 4 of them, they are all in various states of tune.  Some mild port work and unbaffled mufflers has improved them dramatically, as well as rebored carbs, etc..

Basic info for the saw is 48mm bore, 38mm stroke, 70mm con rod

So in stock form, here are the durations
154/116/154 (exhaust, transfer, intake) or as timing it's 103/122/77

I went wild on one of them, and got the numbers to 176/132/170 duration or 92/114/85, and I quickly found out that was far too much intake duration.. I had serious carb reversion below 5000 RPM... so since I couldn't put metal back, I did the next best thing of improving transfer port flow.. It was a closed transfer  2 port design, and I'd pretty much hit a wall as to the improvements I could do to it, so I cut a lot of the cylinder skirt off and shortened the transfer runners, then put windows in the piston.  By improving the transfer efficiency and effectively reducing the residual charge in the case, I made a decent running saw, though it was still a little gutless on the bottom end.. I didn't expect any different

I took it all apart again and reduced all my durations since I had a notches on the piston for the exhaust and transfers, and a shortened skirt on the intake side, and I made a new piston.. so now my durations are *about* 166/128/164 in duration or 97/116/82.. It's not a drastic difference, but the powerband does drop off slower as the revs drop.

In stock form, the saw was factory rated at 4.5hp with a redline of 8000 RPM, the ported out saw freerevs to 12,000, seems to work really happily around 9,000, and starts to bog under 7,000... I estimate it's around 6hp now, and that's a pretty good increase already.. but never enough, right?

So of course the next step is to build a pipe for it!
Now since I don't think the bottom end of the saw is going to take much more punishment than 12,000, I'd like to improve the torque in the area it's cutting at, perhaps with a peak torque around 9,000.. I do have a bit of space constraints on a chainsaw, and will probably be taking some shortcuts on the header design (a spiral of squarish material) to save some space despite it not being ideal, and again on the diffuser and baffle cones may be a bit squared off... I'll see how it all works out.  I think I'll use a steeper than normal baffle cone since from my reading it is mostly effective above the tuned RPM and is an effective rev limiter

I'm still figuring out ECcalcs... it'll take me a while

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Re: Chainsaw pipes

Post by Admin on Sun Jul 12, 2015 2:26 pm

Actually you can "put metal back" on the intake port by using JBWeld. I've used it with success on ports. It just needs to stick to some of the vertical walls of the port as well to give it extra staying power.
Squaring off the cones is not a good idea but experimenting is fun and the best way to learn.
The steeper the baffle cone, the more of a dip in power you'll have before it comes "on the pipe". It also shortens the powerband.

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Re: Chainsaw pipes

Post by Rx7man on Sun Jul 12, 2015 8:01 pm

I'm alright with a narrow powerband, on a chainsaw you can rev it up good, then go into the cut, I want the steep baffle angle to help prevent it from grenading the bottom end if I can help it...

My current angles are 4.5/6/8 over 90/54/30mm, a 64mm diameterx52mm belly, then a 10/12* over 100/41mm... I managed to keep all my max area changes to right about 15% as well that way too.

By some dumb luck, the engine is running really strong at 9500 RPM as it is, and the pipe is going to come on just above that (from my durations, ECcalc says my ideal RPM is 9800), so I think once everything works together at that RPM it may be a very peaky power, but should really pull hard

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Re: Chainsaw pipes

Post by Admin on Sun Jul 12, 2015 11:11 pm

the RPM calculated based on the exhaust port duration is the top RPM, not the peak power RPM which is lower. A steep baffle angle will give more power and therefore be more likely to "grenade" the bottom end because the bearings are handling the pressure from the increased combustion pressure. But if they are good bearings then I wouldn't even consider them as something to be careful with. Just design for what peak power and powerband you want.

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Re: Chainsaw pipes

Post by Rx7man on Mon Jul 13, 2015 4:16 am

I'm more worried about the centrifugal forces from increasing the RPM... the engine was originally built for 8000 RPM, my peak is already 12,000, and a working RPM of 10,000 would probably be close to double the centrifugal load, though I think a more torque could be applied as long as the revs are kept in check, and one way to do that apparently is to have a steep baffle angle which makes it come 'off the pipe' quickly... I have a few saws I can 'donate' to the cause Razz

One thing I'm quite surprised about is that Blair, nor anyone else mentions much about oil!  the chainsaw forum I'm on has HUNDREDS of pages debating how much to use, and I found forum.dirtrider.com/discussion/7169620/spooge-101-/p1 to be the best explanation of it yet.. the short version of it is the more oil you have, the better, oil seals the rings, which makes more power, the only caveat is you need to rejet richer as you add oil in higher ratios.. the only problem to adding more oil is that you have keep cylinder temps up to burn it off, and unless you're on the throttle all the time, you're going to start fouling plugs.  In a piped chainsaw, you'll probably not be idling it much

Are my bottom end bearings good? well.. they're nearly 40 years old, so they may be of better quality than what you buy new now Razz

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Re: Chainsaw pipes

Post by Morehp on Sun Oct 18, 2015 12:57 pm

As you alluded to you should work on ways to improve efficiency, not rev to the moon. That's old school way of doing it.

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Re: Chainsaw pipes

Post by Rx7man on Sun Oct 18, 2015 1:21 pm

I'm glad you brought this thread back up.. I built my pipe last weekend AND IT'S AWESOME. The tuned length was a little long, and the port timings weren't designed with a pipe in mind.

So the saw free-revs to about 12,300 RPM and stops dead in it's tracks at that point, it makes really good power from 9500 to 11,000, which is what I was looking for. My port timings are 97/121/80 (exhaust, transfer, intake).

I guess a video is the best way to show it off.. The saw is a Husky 61 with a 272 top end (52mm bore instead of 48) and a Husky L65 bottom end (36mm stroke instead of 34mm) making it a 77cc saw. A BIG fir tree had fallen and my neighbor had a 32" bar I borrowed to put onto the saw, I put a new chain on it and I was very impressed with how the saw pulled it. I may go back to a Husky 61 top end and port it more aggressively and go for a 14,000 RPM free-rev speed.

What do you think?
https://www.youtube.com/watch?v=FBnPjkSUAiM

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Re: Chainsaw pipes

Post by Rx7man on Sun Oct 18, 2015 1:25 pm

I had to do a lot of machining work to the L65 crankshaft to make it fit in the casing of the 61, the flyweights were about 3mm too big, so I ground them down on a lathe... I was expecting it to become a little off-balance from it, but as it turns out it feels perfect

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Re: Chainsaw pipes

Post by Morehp on Sun Oct 18, 2015 1:44 pm

It's a ripper. It needs a little more power when it gets to the thick parts. Again I wouldn't go for revs, I'd work on the details to improve effiency.

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Re: Chainsaw pipes

Post by Rx7man on Sun Oct 18, 2015 2:25 pm

It really should have a 7 tooth sprocket on it instead of the 8, but it would cost me money to do that, and this is a budget build.. The top end was $34 USD with all the gaskets straight from China, and that's the whole sum I have invested into this saw other than about 100 hours of time! (OK, the chain was $40, but I can put that on any saw)

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Re: Chainsaw pipes

Post by Admin on Sun Oct 18, 2015 5:51 pm

nice job. don't look likes it lacks any power.

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Re: Chainsaw pipes

Post by Rx7man on Sun Oct 18, 2015 9:58 pm

It originally had a 80mm belly that I cut down to 55mm, and I cut about 30mm off the header pipe as well.. It sounded like it wasn't working before it hit the 'rev limit' so shortening it brought it up to where I wanted it. Header is about 210mm long, but a little hard to measure going around the bends

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