Windowed pistons, when and why?

[wigglesworth]

Anybody ever try to stick an upside down glass in a sink of water?

 

[tlandrum]

yes i have and you dont get any water into the glass so the air thats in the glass is not freshly soaked with water,so you have a stagnent charge of air in the glass:hmm3grin2orange:

 

[wigglesworth]

yes i have and you dont get any water into the glass so the air thats in the glass is not freshly soaked with water,so you have a stagnent charge of air in the glass:hmm3grin2orange:

I'd rather have the water run around the glass and up the sides muhself.

 

[Naked Arborist]

I think all of those mentioned are valid points.

The thing Dennis mentioned about ports does have it's merits IMHO. I think the ported piston vs windowed has more of a tunnel ram effect that puts a higher pressure charge up the transfers which leads to better filling of the chamber. I have seen that a little bit off the bottom corner of the piston below the second ring seems to help direct the incoming charge up the transfers. A short skirt section under the wrist pin would in theory seem to help this also. I might try that on the next quad transfer cylinder on a 250 290 390 build.

Another thing to consider is the piston window allows a fresh cold charge to pass under the center dome and through the piston walls near the wrist pin pillars. Hence, more intake flow with less restriction. The con rod and wrist pin also get a bit more cooling. All this together will cool the hot spot at the center of the piston. This will allow for more advanced ignition timing, leaner A/F mix and a cooler top end all around by moving more heat out to the cylinder side walls and case via incoming fuel/air charge. The effect will be greater on larger engines. In essence you are recycling the heat back to the next incoming charge there by boosting the output power. Kind of like a mini inter-cooler. The heat is minimal and added to the incoming A/F charge at the very last micro second. Even more heat is dissipated when the A/F charge is heading up the transfers to the chamber. Also, some of that incoming heat grabbed from the fresh incoming cool A/F charge in turn will be given back to the opposing outer transfer port walls and dissipated to the cooling fins and lower case parts. The windowed piston has a lot more overall incoming A/F flow, when it can be utilized by using both sides of the motor when the A/F charge is heading in to the case. All this moving heat around and giving some back to the cylinder on the exhaust side below the port. I dought the incoming A/F through the transfers to the chamber is any hotter on a windowed piston motor but, I'd bet that the piston is cooler.

Different designs like different flow patterns. Some can use increased cross flow and some can't.

What saws can be built with both? That would be an interesting experiment to try.

 

[barneyrb]

Anybody ever try to stick an upside down glass in a sink of water?

Have you ever pulled a full glass of water up with the same glass?

 

[Justsaws]

I do a small amount of work on repairing OPE. I've also done a bit of machining of cylinders, pistons, etc. for friends of mine that port saws. When you're buddies with guys like Andre and Brad, you tend not to be very motivated to learn to port yourself. They tell me what to machine, I do it, they do their thing, and voila, a ported saw. One of the things that I'm having trouble wrapping my mind around is why you would use a windowed piston in a saw who's lower transfers open in the base. Aside from lightening the reciprocating assembly, I see no advantage, yet very successful builders such as Terry and I believe Brad do it. Why?

I like to think think of it as being more capable of a complete charge change over more area. All things never being equal sometimes it might be a benifit sometimes not. I would imagine if I was trying to control unburned fuel blowing out the exhaust that a non-windowed piston be would a better choice, typical slab sides being only marginally better but still better.

 

[wigglesworth]

Quad port husky at approx. BDC. What's not to love?

OEM at BDC

Windowed at BDC.

 

[Andyshine77]

The only thing I can see windowed pistons doing in cylinders not designed for it, is increasing case volume, which intern reduces charge velocity/flow. A larger amount of the charge may in fact just sit in the case or transfers during each cycle, never making it into the combustion chamber. At some point we're not simply porting the engine, we're re-engineering it, and no saw porter I know of has the $$$ backing them to do things the truly correct way. That would take software, flow bench, dyno, thermal imaging and thousands of man hours. Any thing less is just a guess IMHO.

opcorn:

 

[Mastermind]

I just leave the piston as is.......

 

[FATGUY]

quit your babbling Andre

 

[Andyshine77]

quit your babbling Andre

Up yours Nik.

 

[FATGUY]

with what? windowed or non-windowed?

 

[Andyshine77]

with what? windowed or non-windowed?

Windowed!!!!!!!!! Hahahaha.

 

[FATGUY]

babbling bastard

 

[Andyshine77]

babbling bastard

I know I know.:welcome:

 

[barneyrb]

Windowed!!!!!!!!! Hahahaha.

That's gonna let the #### out, were you wanting to let it out or keep it in?????? I never try to stir anything up.....

 

[wigglesworth]

The only thing I can see windowed pistons doing in cylinders not designed for it, is increasing case volume, which intern reduces charge velocity/flow. A larger amount of the charge may in fact just sit in the case or transfers during each cycle, never making it into the combustion chamber. At some point we're not simply porting the engine, we're re-engineering it, and no saw porter I know of has the $$$ backing them to do things the truly correct way. That would take software, flow bench, dyno, thermal imaging and thousands of man hours. Any thing less is just a guess IMHO.

opcorn:

tl:dr

But I'm sure I'd like it...

 

[sun64]

I have a question that hopefully someone may know the answer
I look at the Stihl 460, 660 jugs and the 090 jug and the transfer design seems to be similar / same
The 460,660 are windowed pistons and the 090 is windowless.
Sorry but i havent had anything to do with the 880
I know the 090 runs around 8000 rpm and the others are 13 to 14000 rpm.
Just wondering if the rpm's have any bearing on the design being for a windowless piston.
The same crank case had the 070 ( 106 cc ) jug as well as the 137cc jug so it was not just made for one size cylinder.
One was designed in the 60's and the others 30 / 40 years later.
Is it that they have found as said before that the piston will run cooler windowed ?
Wondering if it is worth trying to modify one ?
Cheers Wayne

 

[wigglesworth]

Anybody window any slugs today?

 

[Mastermind]

Anybody window any slugs today?

Naw......I just ground on a 038 Mag II