STIHL MS390 cylinder and piston work

[wcorey]

You also took out most of the cylinder head support from top to bottom.

Cylinder head support???
Took out most of "it"???

 

[dhhusqihl3]

So I'm not going to pretend that I know alot about these little two strokes because I don't and I will admit that. But I do want to explain my thought process and I also appreciate everyone's comments and thoughts. Someone mentioned that there isn't ever negative pressure in a cylinder at any point. I think of it on the intake stroke as causing a vacuum in the bottom end and to me that is negative pressure. Maybe a better way to think of it is a pressure differential and the raw unburned gas that everyone talks about in the exhaust may also be best thought about as a pressure differential but again I think vacuum. My thoughts are, that high performance race engines have no intake vacuum, so we ran big valves, high lift and long duration camshafts to provide as much dwell time as possible to get as much fuel and air into the cylinder as possible and always ran them rich. (Methanol =really rich. So rich that you have to change oil more often because so much fuel gets to the oil pan past the rings) Lean = hp but they DO NOT last sometimes even 25 laps. The removal of the bottom skirt on the piston and the dividers were both done to increase "case" capacity and also allow the mixture an easier path to get on top of the piston. Again going back to my race engine experience, the air velocity probably does matter but it didn't in those engines so we'll see if it does in this one. I am of the opinion that the engine will get what it needs, if what it needs is available to be consumed. Also more air taken in coupled with the long duration of case compression should increase the amount of air traveling through the transfers and that cooler more dense burst of fresh mixture should help expel the old less dense and hotter burnt mixture already on top of the piston which in theory should help the saw run cooler and if it doesn't distort then the thinner sidewalls of the cylinder MIGHT dissipate more heat also. As far as supporting the "cylinder head", I suppose you mean the top of the cylinder/jug? I hope there is enough alloy still there to keep it from self destructing but if not then let'er blow . In a way I'm really thankful that most of these comments came after I did the work, instead of before, when I actually asked for thoughts, otherwise I might have gotten a little worried and not have done the work to even find out.

 

[wcorey]

A word of advice from someone who's been there...
I know it's difficult to accept (and there are many who never will) but...

Take pretty much anything you know about tuning four strokes...
And forget it, throw it out the window.
Not only does it rarely apply to saw motors, it's more often than not detrimental.

And pretty much forget about piped two strokes, though more of that applies, it's still a very different animal.

I have a theory that the more someone knows about tuning 'other' motors, the longer it takes to 'get it' when it comes to saws, lol...

I notice you don't mention much theory about what happens on the top side of the piston when the mix in the transfers hits the combustion chamber, i.e. loop scavenging. It's not just about how much gets in there but also important is the presentation and how that can keep it from going out the exhaust unburnt.
The dividers played a role there...
And with your numbers, don't go thinking like you've got an excess of case compression to play with.

 

[dhhusqihl3]

@wcorey I appreciate your words of wisdom. I believe that by leaving the two directional parts on both sides of both transfers that are on the outer edges maybe it will still do some of the same directing of the mix and by the top of those transfers not being flat then maybe that will also aid with the velocity and a sudden surge of mix hopefully into the cylinder as soon as the transfer cracks open, then with that little pop-up providing round turbulence to maybe get the juices flowing in the right direction,ie loop scavenging(maybe). Not having valves, cam, lifters or rockers are what make these things so interesting. Who would've ever thought this up. Really intelligent fellow or group of. They really are interesting and this thing may suck but it's been fun thinking, looking, measuring and grinding. It may be a little while before I get to run it because my wife is due to have our first son any day/minute but I will post a video or two (even if it fails) when I do.

 

[sledneck22]

Good luck with the baby! We just had our 3rd a month ago. Still getting back into the "baby grind".
And I give you credit for going out of your comfort zone and just sending it... Will it hold together? time will tell. But you'll be able to find another jug fairly easily and the best learning experience is the one you experience yourself.

 

[dhhusqihl3]

Thanks

Good luck with the baby! We just had our 3rd a month ago. Still getting back into the "baby grind".
And I give you credit for going out of your comfort zone and just sending it... Will it hold together? time will tell. But you'll be able to find another jug fairly easily and the best learning experience is the one you experience yourself.

Thanks man and congrats to you and yours! We go in at 9 tonight for her to be induced. Little booger is being stubborn and not wanting to come out so he's getting served an eviction notice! Lol

 

[sledneck22]

Thanks
Thanks man and congrats to you and yours! We go in at 9 tonight for her to be induced. Little booger is being stubborn and not wanting to come out so he's getting served an eviction notice! Lol

Two of ours were inductions... get ready to hurry up and wait. bring a sweatshirt, always cold in those rooms.

 

[Lightning Performance]

Thanks
Thanks man and congrats to you and yours! We go in at 9 tonight for her to be induced. Little booger is being stubborn and not wanting to come out so he's getting served an eviction notice! Lol

Good Luck to you and yours.

Enjoy the rugrat. Before you realize they are walking/running, off to school and then moving out.

 

[isaaccarlson]

Did anyone ever hear how this turned out?