026 lower exhaust port question

[yo2001]

I notice that on 026 (I doubt it but could be just mine) the piston goes inset about 1.5mm lower than the lower part of exhaust port. Would that not be a place for an improvement? To me seems like it is a bad spot where end gas can trap and cause detonation problem when modded. On my 044, the piston stays above the bottom of the exhaust port. Any thought?

I'm still learning. So thanks for responding stupid question. If anyone can lead me to a good read that'll be great too. I have been looking through "2 stroke guide" some.

 

[drmiller100]

in general, the when the piston is all the way down, the exhaust port should be open all the way to the exhaust port.

however, it is REALLY bad if the exhaust port is open when the piston is all the way up. So, take the muffler off, run the piston to TDC, then mark the bottom of hte exhaust port on the side of the piston.

disassemble, and make sure there is plenty of room to lower the exhaust.

hope that helps,

 

[timberwolf]

If you ever plan to modify this saw the absolut last part you want to touch is the bottom of the exhaust port.

90% of the flow out of the exhaust port happens in the top 10% of the port, well not exactly but close. By the time the piston gets to BDC there should be almost zero flow out of the port and any flow that there is is often as much fresh charge as exhaust.

As doug pointed out it's bad news if the piston skirt clears the exhaust port floor, the 026 has a short piston and its not to hard to do that, esp if any modifications are made to improve compression.

One other point is the cylinder contact offered to the ring below the exhaust port helps cool the ring aiding in ring life.

 

[romeo]

:agree2: 99% of my builds never see any work to the bottom of the exh port short of smoothing things up. Remember that the power comes from pushing the piston into the combustion chamber, when you do this it will lower the bottom of the exh. port automatically.

 

[yo2001]

Thanks guys. So many other things that have not considered. I always forget to think about the bottom of the piston when I'm thinking about the top of the piston vise versa.

Right now, I widen the port to 1mm from the edge of the piston on intake and exhaust ports, clean the lower transfer and pull the upper transfer to intake side some but didn't raise much. Also took up the exhaust port little and re-chamfer the port. With no base gasket, the compression was at 195psi which was little too much for 93 oct. pump gas and ended up sanding down the piston few thou and chamber roof and drop the compression to 180-185psi. what do you think? seem like it's on right track?

 

[timberwolf]

Think I would have left it at 195, unless you were be really hard on the 026 it should run fine on pump gas and that much compression. May get just a little touchy on idle to get it tuned just right. But it will be good at 180 psi, trade off a little power for some easy of use.

 

[yo2001]

At 195psi, I guess squish was too tight I was getting meteors on the top the piston on the exhaust side as you described in the other threads. The squish test came out in the 0.016"-0.018" at 195psi but who knows.
Now measuring around .020" seems no more meteors.

 

[timberwolf]

Yep squish and RPM also factors into the whole picture. The slower the saw turns the less compression it can take.

026 has and off set combustion chamber so that can put squish velocity up very high over the exhaust side.

 

[trappermike]

I believe in any 2-stroke the bottom of the exhaust port should be even with the piston at BDC.
However,before lowering the bottom edge of the ex. port to match up,I would first move the piston to TDC,then with a pencil trace the exhaust port on the piston skirt,particularly the bottom edge.
Then remove the cylinder and look at the pencil imprint on the piston skirt and look-if you lower the ex. port will the piston skirt still close the ex. port at TDC? Probably so,but just something to check.
Yes,lower the exhaust port if it's higher than the piston crown at BDC,you will get a larger port with no adverse affects.:msp_wink: