Dished Pistons?

[tgerloff92]

What are the downfalls to dishing your piston and cutting your squish band to match(taller on the outer walls of cylinders)? It seems like you could get a better burn. Or is the gains you get from the better burn minuscule compared to the ease of cutting your squish band almost flat for the compression?

 

[w8ye]

The loss in compression ratio will negate any gains in combustion efficiency

 

[ROWDY]

I would think a poor flow of air and fuel flow would also be an issue with a dished piston compared to a flat piston.

 

[tgerloff92]

I would think a poor flow of air and fuel flow would also be an issue with a dished piston compared to a flat piston.

Please explain, I am just spouting off ideas but I think air flow wouldn't be as big of a concerned with a dished piston because it would settle in the pocket of the dish and the combustion chamber.

 

[Cantdog]

The loss in compression ratio will negate any gains in combustion efficiency

Yes.........and the perfect living example is the 261 Husky with a dished piston compared to the 262XP with a flat top...using the same cyl...

 

[w8ye]

Or the 45mm bore Husqvarna 350 compared with the 45mm Husqvarna 353 which also uses the same cylinder but the piston is dished on the 350. The 353 is much more powerful.

 

[sachsmo]

the SD 133 had a dished piston as did a couple old Jonnys.

If you ever pulled them over, the loss of compression would never cross your mind.

 

[steve316]

dish vs flat

As long as your compression was the same the dish will perform better. I base my statement on nascar engine's and other four stroke race motors. The dish shape collects the mixture around the plug and give a more complete burn; also you can run less timing. This is in four stroke motors, IMO it would apply here. Steve

 

[Eccentric]

the SD 133 had a dished piston as did a couple old Jonnys.

If you ever pulled them over, the loss of compression would never cross your mind.

What model Jonnys? The old Jonsereds I've worked on have had domed pistons (and stout compression).

 

[tgerloff92]

As long as your compression was the same the dish will perform better. I base my statement on nascar engine's and other four stroke race motors. The dish shape collects the mixture around the plug and give a more complete burn; also you can run less timing. This is in four stroke motors, IMO it would apply here. Steve

This was along the lines I was thinking, maybe it's not practical to cut your combustion chamber in a way to keep the compression up. But just something that floated by in the void between my ears.

 

[Cantdog]

What model Jonnys? The old Jonsereds I've worked on have had domed pistons (and stout compression).

Yeah I have to ask that question too.........I have just about every example of older Jonsereds until you get back to the XF, XD etc. era and have never seen a dished piston....Domed yes but not dished.....

 

[w8ye]

As long as your compression was the same the dish will perform better. I base my statement on nascar engine's and other four stroke race motors. The dish shape collects the mixture around the plug and give a more complete burn; also you can run less timing. This is in four stroke motors, IMO it would apply here. Steve

But with performance modifying chainsaw guys, they always set the squish at .020" whether the piston is Flat or domed or if part of the rim of the combustion chamber is cut out. I've never seen a chainsaw with a dished piston have enough compression for any kind of super performance.

The problem with a two stroke, the compression ratio is figured only from the volume above the top of the exhaust port compared with that remaining volume at the top of the stroke and not from the entire volume of the stroke times bore.

 

[Cantdog]

But with performance modifying chainsaw guys, they always set the squish at .020" whether the piston is Flat or domed or if part of the rim of the combustion chamber is cut out. I've never seen a chainsaw with a dished piston have enough compression for any kind of super performance.

The problem with a two stroke, the compression ratio is figured only from the volume above the top of the exhaust port compared with that remaining volume at the top of the stroke and not from the entire volume of the stroke times bore.

Yes and the other main difference is the fuel charge comes in the top of the cyl through the valves located in or very near the combustion chamber. The dished piston in these 4 strokes allows the valves to remain open at very near TDC....Dished pistons flow better than valve pockets cut in the top of the pistons for this application....in a saw motor the fuel charge comes from below and the exhaust has to leave quickly ahead of the incoming fuel charge....every stroke...totally different situation....saws don't have the luxury of a separate (dead) exhaust stroke......intake and exhaust very nearly simultaneous....or overlapping actually.

 

[tgerloff92]

It seems like cutting the combustion chamber the same shape as the piston so that the out edges and working toward the middle are still the same squish clearance up until the combustion chamber opens to form your pocket for air and fuel. I do see a harder time pushing out the exhaust with this though as the transfers would seem to push over the pocket in the piston

 

[Zombiechopper]

I see where this would be good in theory, but in practical terms it's probably not a great idea. It would be very tricky to cut the squish band like that, and piston crowns on many saws are not terribly thick...

Plus, the builders doing flat cut squish bands are already getting phenomenal performance using that method. Your suggestion would complicate the squish machining, and add an additional step by needing to also machine the piston. I doubt the same amount of compression could be achieved without dropping the jug considerably more which then would cause issues with intake timing.

This might be a good plan on a cut off head saw, but i bet if it was worth doing then the insane racing nuts that build those saws would have already tried it

 

[ROWDY]

Please explain, I am just spouting off ideas but I think air flow wouldn't be as big of a concerned with a dished piston because it would settle in the pocket of the dish and the combustion chamber.

I'm also spouting thoughts and ideas. I'm looking at the idea that the "intake and exhaust" (I said fuel and air flow earlier) does not have a straight in, and straight out flow. Their for, you are restricting it by the combustion chamber design.

 

[Kenskip1]

What a great post. You guys really got me to thinking.Best post in a log time. Ignition timing? Thanks Ken

 

[tgerloff92]

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[tgerloff92]

Ya just a thought on theory, as state before not really a practical idea.