Pistons and Cylinders

[Four Paws]

I have been thinking about piston and cylinder geometry. Looking at all the saws that really run good out of the box, nothing stands out as being a trend.

Husky 372/385 use a full circle, solid piston and feed transfers out of the case.
Dolmar 7900 used a windowed piston, now a slab sided piston, and feed transfers out of the case.

Question - what would happen to a 372 if a windowed or slab sided piston was used and nothing else was changed? Or, if a full circle, solid piston was used in the 7900?

Husky 181/2100 used a full circle, windowed piston.

Question - what would happen if we ditched the full circle and ran a now traditional windowed piston? Or, a slab sided piston?

Really just want to have a discussion about whether or not the style of the piston has any measurable impact performance.

What say you?

 

[srcarr52]

Many have modified the 372 piston with windows or made it more slab sided with good success. Personally I make them more slab sided by grinding off the full skirt potion back to the wrist pin wall.

 

[Wagnerwerks]

Many have modified the 372 piston with windows or made it more slab sided with good success. Personally I make them more slab sided by grinding off the full skirt potion back to the wrist pin wall.

Pic?

 

[srcarr52]

Pic?

Sorry. I don't have one.

 

[Hedgerow]

I can tell you a full skirted piston in a 7900 is NOT the ticket.
Lol.

 

[huskihl]

I can tell you a full skirted piston in a 7900 is NOT the ticket.
Lol.

Nor in a jred 670/268xp. Too much resistance. Running 1 ring instead of 2 unleashes about 500 rpm. That full skirt is probably worth 1000

 

[Four Paws]

Pic?

Tree Monkey posted a picture of a 385 piston he did. Basically milled the sides flat and level with the flat area surrounding the pin. Then he windowed it.

I can tell you a full skirted piston in a 7900 is NOT the ticket.
Lol.

I would imagine based on the small case

Nor in a jred 670/268xp. Too much resistance. Running 1 ring instead of 2 unleashes about 500 rpm. That full skirt is probably worth 1000

Didn't the 268XP only have 1 ring in the piston?

I agree the full circle would add some friction. I would be curious to see actual data as to what effect it would have on RPM. It would definitely help if a guy wanted to run really WIDE ports.

Good discussion - hopefully some more folks will get involved.

 

[huskihl]

Tree Monkey posted a picture of a 385 piston he did. Basically milled the sides flat and level with the flat area surrounding the pin. Then he windowed it.



I would imagine based on the small case



Didn't the 268XP only have 1 ring in the piston?

I agree the full circle would add some friction. I would be curious to see actual data as to what effect it would have on RPM. It would definitely help if a guy wanted to run really WIDE ports.

Good discussion - hopefully some more folks will get involved.

Yes. One ring in the 268xp. I was referring to running only one ring in a piston designed for 2. Everyone who has done it has claimed higher rpms with only 1 ring.
And my 670 with a full skirt piston got smoked by a 272xp with a normal piston. Can't be having that lol

 

[295 tramp]