STIHL MS390 cylinder and piston work

[dhhusqihl3]

First post on the forum so I apologize if it's in the wrong location. I have a MS290 that I've converted to a MS390 with AM cylinder and just bought one of the Cross pop up pistons for. I enjoy playing with this thing and I want to do some stuff that I haven't found any info on anywhere else on the forum. It looks like the bearings really block alot of space for air/fuel to get into the transfers and the little bottom lip on the sides of the piston doesn't help either. I also measured the bottom of the lowest ring at its lowest point of travel (BDC) to be able to see how much of the divider between the transfers I could grind out before affecting the ring surface at all. So I have the idea to file the little lip off the bottom of both sides of the piston in order to make it identical to the rest of the side of the piston where the wrist pin goes through. I also want to grind roughly 1.75" of the divider in transfers out from the bottom up to allow as much air into the transfers as possible and make a subtle and gradual transition back to the two after that but not take them completely out so as to still provide ring support. I want to make the rest of the divider non directional as I believe the pop up on the piston really helps evenly disperse the mixture (similar to dome tops in high performance race engines) I also want to open the bottom of the intake port as much as possible to allow as big of a gulp of air and fuel the thing can get. Currently the timing numbers are Intake-100, Exhaust-99, transfers-123. 23 blowdown so I don't want to change those numbers any if I can stand it unless someone on here has good advice to change something. I know that this seems like alot for a clamshell saw but I enjoy tinkering with small engines since my background is in large 6 to 16 liter diesels and mid cubic inch dirt racing gas/methanol engines. What are y'alls thoughts on my thoughts and do you see anything about it that absolutely should not be done?

 

[Ronie]

I'm not a Stihl guy so I can't answer your questions but I'm sure someone will come along and give you some pointers. Welcome to the forum.

 

[Lightning Performance]

First post on the forum so I apologize if it's in the wrong location. I have a MS290 that I've converted to a MS390 with AM cylinder and just bought one of the Cross pop up pistons for. I enjoy playing with this thing and I want to do some stuff that I haven't found any info on anywhere else on the forum. It looks like the bearings really block alot of space for air/fuel to get into the transfers and the little bottom lip on the sides of the piston doesn't help either. I also measured the bottom of the lowest ring at its lowest point of travel (BDC) to be able to see how much of the divider between the transfers I could grind out before affecting the ring surface at all. So I have the idea to file the little lip off the bottom of both sides of the piston in order to make it identical to the rest of the side of the piston where the wrist pin goes through. I also want to grind roughly 1.75" of the transfers out from the bottom up to allow as much air into the transfers as possible and make a subtle and gradual transition back to the two after that but not take them completely out so as to still provide ring support. I want to make the rest of the divider non directional as I believe the pop up on the piston really helps evenly disperse the mixture (similar to dome tops in high performance race engines) I also want to open the bottom of the intake port as much as possible to allow as big of a gulp of air and fuel the thing can get. Currently the timing numbers are Intake-100, Exhaust-99, transfers-123. 23 blowdown so I don't want to change those numbers any if I can stand it unless someone on here has good advice to change something. I know that this seems like alot for a clamshell saw but I enjoy tinkering with small engines since my background is in large 6 to 16 liter diesels and mid cubic inch dirt racing gas/methanol engines. What are y'alls thoughts on my thoughts and do you see anything about it that absolutely should not be done?

Welcome
Your numbers appear to be out to lunch... just saying.
If you can not raise the transfer port roof the rest is a waste of time. Try filling half the lower transfers if you want any gains from it on gasoline. Open up the muffler and advance the ignition timing. You can widen the ports but it makes little difference if the stock air filter is on it. You won't find much bigger an area to feed volume to those transfer ports unless you start grinding on them. That will hurt it even more.

GL

 

[wcorey]

The transfer dividers direct the flow toward the intake wall to aid in loop scavenging, straightening them would typically be viewed as detrimental. Probably wouldn't hurt much to cut out the lower portion but don't think it would get you much of any gain either. Much more of the transfer flow happens nearer the opening and not much down by bdc so I wouldn't put much effort there.

I doubt that low of a popup provides much direction and if anything would create unwanted turbulence anyway.

Your intake opening number seems suspect at 100, in most applications 85 is excessive and with your transfers at 123 doesn't leave you much left for case compression.

Always keep in mind these motors are a collection of compromises and adding in one place always seems to subtract from somewhere else, balance is the key...

 

[dhhusqihl3]

The transfer dividers direct the flow toward the intake wall to aid in loop scavenging, straightening them would typically be viewed as detrimental. Probably wouldn't hurt much to cut out the lower portion but don't think it would get you much of any gain either. Much more of the transfer flow happens nearer the opening and not much down by bdc so I wouldn't put much effort there.

I doubt that low of a popup provides much direction and if anything would create unwanted turbulence anyway.

Your intake opening number seems suspect at 100, in most applications 85 is excessive and with your transfers at 123 doesn't leave you much left for case compression.

Always keep in mind these motors are a collection of compromises and adding in one place always seems to subtract from somewhere else, balance is the key...

Checked the intake number again and it seems to be opening 100 degrees after bdc. I was thinking about opening the bottom of the port up to get it to open around 85 if possible but you're saying that's a bad idea? If I were to do what I'm thinking then I believe about the time my intake is closing the exhaust will be opening and vise versa. I also thought that 23 degrees between exhaust opening and transfers opening was a good number? This thing has alot of distance in the stroke with everything closed before and after TDC which makes me think it should have alot of torque if I can get the fuel into it. Not crazy RPM but torque. I'm probably thinking totally backwards on all of this though.

 

[wcorey]

Checked the intake number again and it seems to be opening 100 degrees after bdc.

Assuming you mean bottom center by bdc, that would explain things.
The intake opening (and everything else) should be measured from top center if you want others to understand where you're at.
100 before/after tdc would be an abnormally high number, not low. Should be somewhere 70-80 range, lowering the intake floor increases it. The only way a saw would typically get that high a number is shortening the piston skirt or grinding a lot off the intake floor.
It doesn't create a problem with blowdown, but rather case compression. That's when the piston is on the down stroke and occurs between when the intake closes and transfers open.
You should be looking for around 40 degrees duration there, much less will start compromising your transfer velocity.

 

[dhhusqihl3]

Being 100 degrees after bottom dead center (bdc) that is the equivalent to 80 before top dead center, correct?

 

[wcorey]

Yup.
80-99-123 aren't bad numbers, the only thing I might change there is to raise the transfers a few degrees.

 

[dhhusqihl3]

Yup.
80-99-123 aren't bad numbers, the only thing I might change there is to raise the transfers a few degrees.

Ok cool, thank you!

 

[dhhusqihl3]

Well here are pics of my number notes, the intake work, the dividers ground out of the transfers, and the side skirts removed from the piston. Also put an approximate 12 degree timing advance in it. Hit on the 3rd pull choked, cranked on the 4th. Slight carb adjusting but very minimum run time this afternoon and no cutting at all. I've gotta remount the bar and chain but I also ground the stop bump off of the oiler while I had everything apart and adjusted it another half round just to see if in fact it will increase by doing that. The saw already had a muffle mod done previously. Now just to see how it performs and to find out if it will stay together.

 

[Maintenance Chief]

This is interesting now.

 

[Maintenance Chief]

I did an 039 Hyway pop up and muffler mod and it was a completely different saw after that , yours should really be impressive.

 

[dhhusqihl3]

I did an 039 Hyway pop up and muffler mod and it was a completely different saw after that , yours should really be impressive.

Nice and Thanks! Really hope it stays together and no big issues. I really just enjoy doing this and for less than 100 bucks it's been fun to tinker with. Really glad to hear yours turned out well!

 

[Nutball]

View attachment 302205 View attachment 302206 View attachment 302207 View attachment 302208
Well here are pics of my number notes, the intake work, the dividers ground out of the transfers, and the side skirts removed from the piston. Also put an approximate 12 degree timing advance in it. Hit on the 3rd pull choked, cranked on the 4th. Slight carb adjusting but very minimum run time this afternoon and no cutting at all. I've gotta remount the bar and chain but I also ground the stop bump off of the oiler while I had everything apart and adjusted it another half round just to see if in fact it will increase by doing that. The saw already had a muffle mod done previously. Now just to see how it performs and to find out if it will stay together.

Just to clarify, It did fire up with your current modifications?

I'm not sure it will stay together for very long, maybe for a few tanks.

Removing the transfer divider leaves a very wide but more importantly flat transfer port roof, and there is one of those wide ports on each side of the cylinder. That will really stress the rings and they will likely wear fast and could eventually catch the ports.

But I could be wrong, I've never tried that, and the transfers still look narrower than the exhaust port.

 

[dhhusqihl3]

Just to clarify, It did fire up with your current modifications?

Yes sir. Fired up and ran for probably 10 minutes or so

 

[drf256]

That transfer divider was there for 2 reasons. One was to give you 2 separated transfer ports and second was ring support.

They aren’t like 4 stroke engines in many ways, and as Bill said above, everything is a compromise.

Think of the transfers as valves. There is no negative pressure in a saw cylinder at any time, the transfers work via pressure from crankcase. Multiple smaller transfers give you better flow angles and more velocity. It’s like trying to move 1 cfm of air through a 2” vs 1” pipe, the 2” would have less resistance bit the 1” would flow faster.

Hopefully the lack of the divider won’t ruin your jug. It appears as wide, if not wider, than your exhaust port.

 

[dhhusqihl3]

So the transfers on that jug aren't any wider than the transfers on a Husqvarna 51/55 that I put a new jug, piston, and rings in. I understand that the husky only has one ring and is a split case saw too though.. Also the top of those transfers aren't flat by a few degrees of angle. I was originally not going to grind them all the way out but instead just take them out up to a few thousandths below the lowest point of travel for the bottom ring but once I measured and found out that they weren't flat and that the rings wouldn't be meeting that edge all at one time I figured why not try it. One way to learn, right. By the way this thing sounds now vs how it has sounded in the past is quite a bit of difference. It "hits" much harder when idling and is alot more responsive. Due to the timing and the large amount of time that the case is compressing the mix in the bottom end before the transfers open I believe will cause sufficient velocity to get air through the transfers but then again it may be a turd, only the wood will tell.

 

[Maintenance Chief]

I believe that any porter worth their salt has ruined a jug or 2 through experience. These cheap saws get little attention from most builders and the fact is your much better off ruining 2 of these then one 10mm 044 jug.

 

[dhhusqihl3]

I believe that any porter worth their salt has ruined a jug or 2 through experience. These cheap saws get little attention from most builders and the fact is your much better off ruining 2 of these then one 10mm 044 jug.

My thoughts exactly! This would be one of, if not THE least expensive thing I've ever messed with!

 

[Lightning Performance]

So the transfers on that jug aren't any wider than the transfers on a Husqvarna 51/55 that I put a new jug, piston, and rings in. I understand that the husky only has one ring and is a split case saw too though.. Also the top of those transfers aren't flat by a few degrees of angle. I was originally not going to grind them all the way out but instead just take them out up to a few thousandths below the lowest point of travel for the bottom ring but once I measured and found out that they weren't flat and that the rings wouldn't be meeting that edge all at one time I figured why not try it. One way to learn, right. By the way this thing sounds now vs how it has sounded in the past is quite a bit of difference. It "hits" much harder when idling and is alot more responsive. Due to the timing and the large amount of time that the case is compressing the mix in the bottom end before the transfers open I believe will cause sufficient velocity to get air through the transfers but then again it may be a turd, only the wood will tell.

The two main drawbacks of removing the entire divider is much more case volume to build low end power but the velocity loss might kill it on the top end. You also took out most of the cylinder head support from top to bottom.

You do end up with a huge case with low restrictions to the uppers. I'd bet it runs hotter but not much better overall. Something different and repeatable to work with. It might work well or bbq the piston pretty quick. Time will tell. Tune it fat