Piston wash patterns

[Definitive Dave]

for whatever it is worth I have seen several pistons at low number of tanks with a beautiful symmetrical wash pattern on pure race built saws
Dave

 

[Stump Shot]

Here's a picture of the piston in my 272 as I thought it was interesting to see. This after a short run time from being installed. Crankshaft had plans of retirement... deservedly so, being a rode hard put away wet loggers saw for most of it's life. Otherwise I don't really see many.

 

[srcarr52]

So should the wash pattern be uneven then? To some extent. Perhaps angle of the wash the same but pto side being “dirtier” is ok?

The PTO side of the cylinder is always going to be running hotter than the flywheel side and thus have more carbon build up. The angles of the piston wash should show up pretty similar per side but the size and distance of the wash is going to be different due the the temperature gradient. Unless the saw only makes 3 cuts and gets to cool down.

 

[drf256]

The PTO side of the cylinder is always going to be running hotter than the flywheel side and thus have more carbon build up. The angles of the piston wash should show up pretty similar per side but the size and distance of the wash is going to be different due the the temperature gradient. Unless the saw only makes 3 cuts and gets to cool down.

Exactly what I'm dealing with, compounded by an assymetrically placed exhaust port. Even though I centered it, the entire length isn't symmetrical.

046 jugs have offset Ex ports, inside and out.

 

[Wolverine]

The PTO side of the cylinder is always going to be running hotter than the flywheel side and thus have more carbon build up. The angles of the piston wash should show up pretty similar per side but the size and distance of the wash is going to be different due the the temperature gradient. Unless the saw only makes 3 cuts and gets to cool down.

My pic was after lots of mill time.

 

[Lightning Performance]

I always try to index my spark plugs.
They seem to burn cleaner over a longer time. Open spark gap area tends to give more ignition lead than shrouded. The ground strap runs hotter in the pocket. That can be a draw back racing if you get into too much heat. It will melt sooner. Gapless plugs run cooler imo.

 

[srcarr52]

Exactly what I'm dealing with, compounded by an assymetrically placed exhaust port. Even though I centered it, the entire length isn't symmetrical.

046 jugs have offset Ex ports, inside and out.

Aren't 044 cylinders worse to narrow the case further?

 

[drf256]

Aren't 044 cylinders worse to narrow the case further?

Sorry, Not understanding.

 

[Keith Gandy]

Cylinder cooling from the flywheel fan certainly can’t be symmetrical, could that effect internal temps enough to affect wash pattern?

I agree and this was discussed in a thread awhile back but cant remember which. Think it was the oil thread

 

[Bigmac]

It has to be a heat issue, liquid cooled two strokes just don’t have the same issue. Most have better port layouts and tuned pipes. To me the question is...is it hurting power? Is the cooler fan side charge...just less atomized than the pto side and washing more carbon because it’s more raw fuel than the hotter pto side ? Or is the heat causing it to build up? Would changing the port timing from side to side a degree or two change the wash? Would that make more power? Should more heat from one side to the other cause more buildup on the hot side or the cool side? Wouldn’t the hotter side burn better? Or is it an atomization issue with heat? Most of my liquid cooled motors have super clean squish bands and more center carbon, and little of that, but a lot of that is a good pipe and scavenging. More questions than answers! Lol

 

[srcarr52]

Sorry, Not understanding.

Doesn't the 044 have an even more offset exhaust port vs. the 046?

 

[drf256]

Doesn't the 044 have an even more offset exhaust port vs. the 046?

I'd have to check, but I believe its centered in the 50mm jug.

 

[blsnelling]

.....are over rated.

 

[Brewz]

Stihl 084 I’m porting
This is the old piston.
Transfer on the side my fingers are on was found to open just before the other side, about 1 or maybe 2 degrees at most

 

[Terry Syd]

Scavenging in chainsaws aren't as critical as a high performance two-stroke, the scavenging ratio is only around 75% as compared to a high performance engine at 95%.

That said, we need to save as much charge as we can to bump up the power. If you loose 5cc of a charge in a chainsaw, then relatively to the same size high performance engine, that is a bigger loss than in a high performance engine.

We've seen some engines where there is some 'short circuiting' out the exhaust port from the forward transfer ports. The piston wash will illustrate it as it turns and goes out the exhaust port. On such an engine we don't want to make the exhaust port any wider.

Likewise some engines show a strong 'tongue' formation in the wash which indicates a short circuiting being pulled out under the escaping exhaust gases. We can change the transfer angles to minimize the 'tongue' that gets pulled out.

Balancing the flows from side to side is also desirable as it tends to reduce the 'mixing' of exhaust and intake mixture and thus the resultant loss of mixture out the exhaust port with the exhaust gases.

The piston wash is the only real indication that we can study of what the flow is doing in the cylinder. I pay attention to it.

 

[wcorey]

242 piston, pretty much stock cylinder, intake had been lowered.
The lighter appearing right side is at least partly due to lighting/glare...

 

[Terry Syd]

For those who haven't seen it, Blair's book on Two-stroke design is chock full of good information.

http://www.dragonfly75.com/motorbike/2StrokeDesign.pdf

 

[drf256]

Scavenging in chainsaws aren't as critical as a high performance two-stroke, the scavenging ratio is only around 75% as compared to a high performance engine at 95%.

That said, we need to save as much charge as we can to bump up the power. If you loose 5cc of a charge in a chainsaw, then relatively to the same size high performance engine, that is a bigger loss than in a high performance engine.

We've seen some engines where there is some 'short circuiting' out the exhaust port from the forward transfer ports. The piston wash will illustrate it as it turns and goes out the exhaust port. On such an engine we don't want to make the exhaust port any wider.

Likewise some engines show a strong 'tongue' formation in the wash which indicates a short circuiting being pulled out under the escaping exhaust gases. We can change the transfer angles to minimize the 'tongue' that gets pulled out.

Balancing the flows from side to side is also desirable as it tends to reduce the 'mixing' of exhaust and intake mixture and thus the resultant loss of mixture out the exhaust port with the exhaust gases.

The piston wash is the only real indication that we can study of what the flow is doing in the cylinder. I pay attention to it.

So keep Exhaust port smaller to limit short circuiting?

If a motor is losing some charge via the Ex port, at what point of the cycle is it occurring at? Are we sure that we aren’t just doing a better job loop scavenging the entire volume? Do we know that the loss isn’t at least in part to a free flowing muffler?

There will have to be some loss for max power, just how it goes. There will be no way for charge dilution to be minimized without some losses.

So a tiny little ex port might make beautiful wash, but would be want that? A tight muffler would cause high pressure by the port opening and stall charge from exiting, effectively causing carbon deposits that would make the wash look better.

I’m just not sure that with the changes we make, a pattern on the crown can accurately predict all that’s going on in a cylinder.

I’m interested in max power at the expense of some fuel economy. Interestingly enough, most of the saws I do are only slightly less fuel efficient than a stock saw. For the life of me, I still am not sure why. I think it’s combustion efficiency.

 

[CR888]

The illusive Tree Monkae did a good 461 thread doing a little more than a basic port job. He was tuning the transfers via a complex visual report viewing the piston wash. The saw would not have time to build excessive heat during cut sessions. That was a good read for saw transfer flow, short circuiting etc. Like reading a spark plug for tune, yeah its kinda crude but it will tell you what's going on. Short of complex tech to read 2T transfer flow, direction and combustion I think starring at the piston wash pattern is a highly effective tool for 2T tuners, it won't lie to you.

 

[RI Chevy]

That was a cool thread. Each change he made he pulled the jug to check piston wash. [emoji106]
Scientific. [emoji16]