Stihl 026 porting query...

[NickC]

Long story short I have some piston kits from Stihl that are now supplied from Brazil with open ports, as apposed to the original Mahle closed design. They are pretty much identical and good quality, I want to port a couple for some saws I have, but decided to port a Mahle set up that was a bit tired as a reference.

The question is a general one, to produce a torquey work saw you want to keep the exhaust port low to keep the compression high, right? The way I cut the band seems to give high compression, at 97° exhaust and 0.0216 squish I have 220psi after test running with a less than perfect bore and new oem rings. The new jugs seem even stronger.

The question is this, I actually cut from 100 - 97° to try and loose some compression as I felt it was a bit high, but only lost maybe 5 psi. So if I keep on cutting and end up with 200psi and 90° (guessing) am I going to get the best of both worlds? I high revving saw from the port height / size with good torque from high compression, or is the compression the factor that effectively limits the revs?

I'd imagine leave it where is, but I'm curious....

 

[Vintage Engine Repairs]

If you need porting advice, especially for this saw, @drf255 is your man!

 

[merc_man]

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[NSEric]

Imo you wont gain any more useable revs with the exhaust at 90 degrees. It will piss rev crazy high but drop down to the same or lower rpm while cutting than your getting now.
The other ports and carb size limit you to 15-16000rpm so there's no point in crazy high exhaust ports to try to get more rpm.
If you want less compression, don't cut as much off the base and squish band.

 

[pioneerguy600]

Cutting the band lowers the combustion chamber volume of captured gases.

 

[huskihl]

I’d keep the exhaust down as low as possible. 230-240 psi doesn’t hurt. It’s a light weight rotating assembly with decent size bearings

 

[pioneerguy600]

Imo you wont gain any more useable revs with the exhaust at 90 degrees. It will piss rev crazy high but drop down to the same or lower rpm while cutting than your getting now.
The other ports and carb size limit you to 15-16000rpm so there's no point in crazy high exhaust ports to try to get more rpm.
If you want less compression, don't cut as much off the base and squish band.

Agreed, that is not where one finds power in these saw cylinders.

 

[NickC]

Thanks guys, confirmed what I suspected. This is my first stab at it, I'm pleased overall as it is noticeably stronger in a heavy cut but it doesn't seem to free rev much higher than stock. Not worried about that, just a bit surprised.

I was a bit conservative with the upper transfers, I think I'll go in again to lengthen and perhaps slightly raise them a bit from 120.

 

[NickC]

Another one.... I've read lots of threads, people sometimes speak about "aggressive" timing numbers or blowdown with no explanation. Am I right in thinking an aggressive blow down would be a short blow down duration with the transfers opening relatively early?

 

[drf255]

Too little blowdown causes peaky power and loss of low end torque.

So, for race saws where one has an intended rpm in wood with a certain chain, one can tune in what they want.

When dealing with a worksaw, a broader power curve is preferable, as one won’t be cutting the same type/shape/hardness of wood like a racer will.

PioneerGuy has ported a lot of 026’s, he is your man on the saw.

I generally cut the band flat on 026 and cut around .045 flat off the piston dome. That allows me to get nearly 105 on the exhaust roof. I use around 15* of blowdown and it works.

 

[drf255]

Here is the kinda torque a 105 exhaust can make in an 026

 

[bwalker]

Here is the kinda torque a 105 exhaust can make in an 026

All things considered thats a pretty decent 026.
I've had several ported 026's and they tended to be screamers that required q delicate touch with very sharp chain. This was with 16 or 18" bars with 3/8" chain.

 

[pioneerguy600]

All things considered thats a pretty decent 026.
I've had several ported 026's and they tended to be screamers that required q delicate touch with very sharp chain. This was with 16 or 18" bars with 3/8" chain.

In stock form they have too low exhaust numbers and the transfers need a good bit of tuning to feed the top end..

 

[NickC]

Thanks for the help and photos.

Kinda wish I'd had left it at 102° then but I'm not broken hearted, it's still got plenty of comp and maybe I'll advance it up a bit too.

Part of the reason I cut the ex roof was to see what happens to the comp, it's not the best cylinder so I'm having a bit of a trial - I've learned a few things.....

I expect the transfers are holding it back, I've raised them a bit now so we'll see.

15° blow down is the lowest I've seen quoted, you don't appear to have lengthened the uppers? In the photo there it looks like you've widened the lowers out under the exhaust, or is that an illusion..

 

[drf255]

I did move the lowers towards the exhaust. Good eye.

 

[Yotaismygame]

What was wrong with the original thread?

https://www.arboristsite.com/commun...sfer-ports-on-stihl-026-260-oem.336762/page-4

 

[NickC]

What was wrong with the original thread?

https://www.arboristsite.com/commun...sfer-ports-on-stihl-026-260-oem.336762/page-4

Nothing, just seemed to be moving away from the focus of the thread into a more general porting question. I did mention my concern about the potentially excessive compression before I recut the exhaust but nobody piped up so I thought it must have fizzled out. I will revisit it when I've got the open port done similar to this one.

 

[bwalker]

In stock form they have too low exhaust numbers and the transfers need a good bit of tuning to feed the top end..

Yes the exhaust port is too high and the transfer duct area is too small. Honestly if you want a hot 50cc saw there are much better motors to mess with than a Ms260. Kinda like putting polish on a dog turd. And I do like Ms260's.

 

[pioneerguy600]

Yes the exhaust port is too high and the transfer duct area is too small. Honestly if you want a hot 50cc saw there are much better motors to mess with than a Ms260. Kinda like putting polish on a dog turd. And I do like Ms260's.

I do like these saws for their toughness, size and handling but for a hot saw out of the box the 261 is much better, the Husky 346 is the alternative, just not for me personally. I have found ways to make my own 026`s perform up to my standards but not interested in making race saws out of them, drf255 AKA Dr. Al , has taken them to a different level beyond what I want in an everyday work saw. I have literally ground a hundred of these cylinders in successive steps to find gains and have reached a dead end on just what can be done without extensive machining and swap in parts. The raising of the squish band a few degrees to gain some compression and machining the piston crown off flat is about as far as I personally wanted to go with these saws, the transfers can only be worked over so far, I have a few that the outer walls are no longer there, been replaced with some extra aluminum and epoxy..

 

[bwalker]

I do like these saws for their toughness, size and handling but for a hot saw out of the box the 261 is much better, the Husky 346 is the alternative, just not for me personally. I have found ways to make my own 026`s perform up to my standards but not interested in making race saws out of them, drf255 AKA Dr. Al , has taken them to a different level beyond what I want in an everyday work saw. I have literally ground a hundred of these cylinders in successive steps to find gains and have reached a dead end on just what can be done without extensive machining and swap in parts. The raising of the squish band a few degrees to gain some compression and machining the piston crown off flat is about as far as I personally wanted to go with these saws, the transfers can only be worked over so far, I have a few that the outer walls are no longer there, been replaced with some extra aluminum and epoxy..

I still own a MS260. The only things I have done to it is delete the base gasket, trim the chole thinner, advance the timing and and do a muffler mod. It works OK, but is no race saw. However, it is light, compact and stone reliable.
In the past I had MS 260's by several builders along with a pair of EHP 346's. The 346's would run circles around the 260's with ease.