038 AV Wanting to change to 038 Magnum

[HolmenTree]

I found the mods I explained are not rocket science. I did the exact same mods on my brothers Jonsered 670 as my 038 and the 670 proved to be faster. The Jonsered took the mods better :D

[Al_Smith]

Well on a work saw they'll all be enhanced somewhat .Now the part nobody can explain .

I know some of the top race saw builders in the country who have forgotten more already than I will ever know .What none can explain is that on identically reworked saw for reasons unknown one will have a slight edge over the other .It's a great unsolved mystery .Remember though on a racer a slight edge is the difference between win or lose with sometimes less than 2 tenths of a second in times .

[mastermind7864]

What in the heck are y'all talkin about?  8)

[HolmenTree]

What in the heck are y'all talkin about?  8)

Red neck repair 8)

[mastermind7864]

I specialize in that....  :D

[rj]

Ok guys/advisors Thank you all for your help so far.
I've taken the plunge & ordered a mag pot & piston for the old 038 super of mine.
But i want to pump up the mag setup.
I have a lathe with a milling vice & all tooled up.
I would like to try taking an offset skim from the intake skirt for accuracy also skim the pot seat.
When the kit arrives i'll check things out & make some measurements.
So i guess i'll be anoying you guys soon.
In the meantime what are finger ports how do they work, what are the losses or gains in performance & how are they created?
What exhaust/exhaust port mods should i consider?

[Al_Smith]

Okay here is the theory behind finger ports ,boost ports ,secondaries or whatever you want to call them .

In the transfer of fuel following the power portion of the downward piston travel the hardest portion of the cylinder to purge the partially unburned gasses is right above the intake port .This is what they are for .

They aren't some new inovation invented by a crazy man in California .The concept is as old as a two cycle engine istelf just done in many different ways .

[rj]

AL_SMITH,Thank you.
What are the bad points?
Is there a suggestion as to the position of them?
What depth & shape should they be?
How many?

[rj]

Forgive my ignorance but i'm learning on the fly here.
HolmenTree talked about squish.
In regard to compression, what size squish should i look for?
I want to measure then skim for my super to mag then do the inlet outlet piston skirt compensation from calculations.
Thats if i'm understanding correctly. Also finger ports look to be an option not to ignore.

[Al_Smith]

To be able to run at sustained higher than stock RPMs you need to adjust the port sizes .There is a basic formula suggesting what is required in terms of port time/area open parameters based on cylinder bore .In essence what it says is as the RPMs increase the time decreases and as such they need more area .You can Google it .

You have to keep in mind every port needs changed at least a little bit .Also keep in mind the piston acts as a slide valve .You haveto leave enough area around the ports and piston so it doesn't leak which is why you mark the piston as a reference .

[Al_Smith]

Also remember the piston ring end gaps ride between the upper transfers and the exhaust port .You certainly don't want to have a ring gap ride over an open port which most likely would snag the ring and turn the engine into junk in about a millisecond .

As far as marking the piston everyone has their favorite method.Mine is with an indelable sharp point felt marker .( laundry marking  pen) .

Before you get to carving on that thing do some reading .You won't find much on chainsaws or go kart engines .Try RC engines ,British motorcycle sites and such .

[Al_Smith]

Whew long explaination .As far as finger ports it's just one option.Keep in mind how a two cycle engineworks or transfers fuel .It's all based on differential pressures  .You have atmospheric pressure on both the intake side and the exhaust it's what it done in between that causes increased performance .

The intake port can be assimulated to the intake valve of a 4 cycle and the transfers to the cam shaft and size of the valve .The exhaust likewise with a "cam" .

[Al_Smith]

With a 4 cycle yu have cam lobe overlap and with a 2 cycle you have port over lap .

For example the tranfer portion is opening on a rising pressure fromthe crankcase while at the same time there is a decreasing pressure from the exhaust port opening .On most Stihls this is 20 degrees of rotation after the opening of the exhaust.As RPMs increase it is often required to allow more time and as such the exhaust port is not only widened but raised a few degrees to give it more time to transfer fuel and purge the cylinder of exhaust gasses .

[Al_Smith]

The bottom transfers are widened to kind of "funnel the pressure from the crankcase in more so than stock .The uppers are modified to not only help a more easily transfer but also to direct the charge .Fingers included which is just one option .

You need to also keep in mind bigger is not always better .You must not do alterations that lead to decreased pressure because you could end up getting exhaust gasses blown clear down to the crankcase .Not good .

What you need to do is Google Gordon Jennings two stroke tuners manual .

[rj]

You have given me plenty of homework to cross reference, for that i thank you.
My mag pot & piston have arrived so i'm gonna make the nessesary measurements of the existing super pison & pot then swap it for the new mag. I will then do the homework before anything else as suggested.
I'll rework the super, try it & if all is good i'll move on to reworking the mag.
It is easy to follow someone elses work & often prefferable but there is a major benefit in understanding ones undertakings.

[rj]

So i changed piston & pot for the 038 mag kit.
The diference was quite noticable, however i can confirm that after some studying then some grinding & milling You can make a super eat a Magnum cause i swapped back after the super was lowered & ported.
My next move was skimming the crank seating so in the future the pot can be bolted on without skimming the base but this leaves me with an unuseable Super pot, oh well it'll get used sometime. I then went to town grinding shaping & polishing on the mag pot.
"Well let me see when she bogs with a 36" bar on an alaskan mill" was my thought.
This is now one oak eating monster.

 

Horse Chestnut tree on top & a nice Oak

[rj]

Thank you Al_Smith, Thank you holmentree.
Next on the list are finger ports then reworking my 090. 8)

[Al_Smith]

Why in the world would you want to rework an 0-9-0 ? They aren't the fastest thing ever made but darned few have as much torque as that big boy .That thing is like the Clydesdale of chainsawdom .

[rj]

Al_Smith
Cause i have some massive oaks that came down in the big recent UK storm, no hurry & a Clydesdale saw that i intend to repot & piston. So if that clydesdale can pull an 8' home made bar through 5.5 to 7' oak for 300' before reworking then another 300' after i'll treat her to the new piston & pot in the way that seems to work best.