Pop ups and windows. Your thoughts.

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You are confused Chris.

A popup is not a dome.

This is a popup......note the obstruction on the crown?

View attachment 394913

Yes the 543XP has a domed piston, I just ported one. I still cut the squish band. It was far more complicated than a flat squish band......
Yes I understand that - then again if you go back to the first page of this thread it was really about both domed pistons and popups. The two are minor variations of the same thing, differing only in how steep the edge of the lump is. Other recent discussion has been about how transfer flow patterns are disturbed by the presence of a disruption on the piston face, and also included domed pistons. I included the comment about Zenoah engines to point out that this is not really a settled topic, and that some rather capable engine designers are still not using flat pistons.

But I don't think that's really what this thread revival is about anyway.
 
So.....aside from the whole popup thing.....

What about windowed pistons, and their effect on case compression?

And these days all the cool kids are discussing intake height. How does that enter into this conversation?
 
I agree Shawn.

I also like the added control I have over the port timing.

For instance, on a lot of saws, I can lower the exhaust. I can make some of the older saws (think 262XP and friends) run like they would never run with a popup because of that.

Stihl 660 is a classic.

You are confused Chris.

A popup is not a dome.

This is a popup......note the obstruction on the crown?

View attachment 394913

Yes the 543XP has a domed piston, I just ported one. I still cut the squish band. It was far more complicated than a flat squish band......

I usually deal with domed pistons by chucking the piston in the lathe to dial in the cross to the start angle of the face and then add 1- 2 degree and cut the squish at that angle.
 
So.....aside from the whole popup thing.....

What about windowed pistons, and their effect on case compression?

And these days all the cool kids are discussing intake height. How does that enter into this conversation?

Your intake opening has an effect on your effective crankcase compression. Hence why someone would fill in the bottom of an intake port.
 
Depends on the application Matt.

In a high RPM saw.......I would want much less than in a working saw.
 
Depends on the application Matt.

In a high RPM saw.......I would want much less than in a working saw.
Agreed... But would you say anything over 158-160 intake duration is excessive and will turn your saw into a fuel swilling, slow excellerating, pig? Or would it have more to do with the relationship between case volume/transfer length/ and overall crossectional area of your upper transfers??
 
Agreed... But would you say anything over 158-160 intake duration is excessive and will turn your saw into a fuel swilling, slow excellerating, pig? Or would it have more to do with the relationship between case volume/transfer length/ and overall crossectional area of your upper transfers??

I don't think there are any "always" situations.

The MS660 doesn't seem to mind intake durations in the 170* range at all.

Some other saws don't work well at all in those ranges. So........I don't really know. :(

I've still got so much to learn too. I feel like I'm just now getting a decent scratch in the surface.....
 
I don't think there are any "always" situations.

The MS660 doesn't seem to mind intake durations in the 170* range at all.

Some other saws don't work well at all in those ranges. So........I don't really know. :(

I've still got so much to learn too. I feel like I'm just now getting a decent scratch in the surface.....
I haven't even cut the plastic off yet...
But....
Knowing what you know, "why" can the 660 handle that kind of intake duration?
What's different about it than say, a 390?
There should be a huge difference in case volume, right?
 
The 390 has much tighter, higher velocity transfer ports. It also has a full skirt piston........
 
I don't think there are any "always" situations.

The MS660 doesn't seem to mind intake durations in the 170* range at all.

Some other saws don't work well at all in those ranges. So........I don't really know. :(

I've still got so much to learn too. I feel like I'm just now getting a decent scratch in the surface.....

The 660 is an odd duck, what somehow works in it won't in about any other saw.

If you put 170 deg ex duration in a Husky, as seen in some stock 660's, you better have a pipe on it.
 
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