Not so Sally saw...

[jmssaws]

A 394/5 has a very wide exhaust stock and at the time I was getting away with it on the 60% 066 exhaust but a 395 is over that stock.

The pic above is 70% and I would have been alright if it wasn't as flat.
I learned a valuable lesson and it hasn't happened in 5+ years.

I've hung a couple in transfers testing 066 cylinders but your gonna have that if you do it enough

 

[jmssaws]

You can go as wide as you want/piston allows if you have enough arch.

But the port might not flow any better than a square 60%.

Another thing is the symmetry to the arch, it it is not very symmetrical the ring will rock back/fourth eventually freeing the pin. More so with extremely wide ports 70%+.

I probably use more arch than most, but I haven't Tryed to find out how far I can push before things start hanging...

The 661 I'm playing with i started with a big arch and have flattened it 3 times so far and it's a gain everytime but you can only go so far.
I tend to run little arch and little bevel on a less wide port. Work better for me.

 

[jmssaws]

A crisp clean opening port makes power same as a wide one.

 

[paragonbuilder]

A crisp clean opening port makes power same as a wide one.

Any idea why that is?


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[jmssaws]

Flow.
I'll take a narrow port that opens faster than a wide one that opens slower.

 

[Red97]

Roundy roundy round.



Made a sleeve for one of my smaller mandrels. No sense making a whole new one.


.018 off the base.

.017 out of the band squish. 023~

Got it all layed out. 99 120 78.5

Grind tomorrow

 

[Time's Standing Stihl]

Nice work Joe. This at home or work?

 

[paragonbuilder]

Roundy roundy round.



Made a sleeve for one of my smaller mandrels. No sense making a whole new one.


.018 off the base.

.017 out of the band squish. 023~

Got it all layed out. 99 120 78.5

Grind tomorrow

Nice work Joe! I like the sleeve. Good idea!


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[Red97]

Nice work Joe. This at home or work?

Shop at dad's house. (300ft from mine) is where the lathes/mills are. All the porting is done in my garage.

Never fails when I'm over doing machiene work, I get roped into doing stuff for dad lol.

 

[Mastermind]

You can go as wide as you want/piston allows if you have enough arch.

But the port might not flow any better than a square 60%.

Another thing is the symmetry to the arch, it it is not very symmetrical the ring will rock back/fourth eventually freeing the pin. More so with extremely wide ports 70%+.

I probably use more arch than most, but I haven't Tryed to find out how far I can push before things start hanging...

Excellent post my friend.

 

[Terry Syd]

Here's a trick to get a faster blow down with the curved roof of an exhaust port. Put in a very sensible curvature to the roof that will guarantee a long service life. Then shape the top of the piston crown to the curvature of the roof.

The port not only opens instantly, it actually has a wider opening than a flat top piston.

The shortest distance between two points is a straight line. If the line is curved, it is a longer distance. In other words, the curvature of the shaped piston creates a WIDER opening. The result is a combination of a faster opening - and with more area.

Most blow down occurs at the initial opening when the exhaust gas pressure is the highest. You can take advantage of that by tweaking the initial shape of the opening.

It takes time to do the job properly. You have to shine a light through the sparkplug hole and look through the exhaust port as the piston cracks open the port. Just keep nipping the edge of the piston (I use a flat needle file) until the entire width of the port opens at the same time.

Polishing the piston top and combustion chamber helps to create plenty of light to work with.

 

[Red97]

Here's a trick to get a faster blow down with the curved roof of an exhaust port. Put in a very sensible curvature to the roof that will guarantee a long service life. Then shape the top of the piston crown to the curvature of the roof.

The port not only opens instantly, it actually has a wider opening than a flat top piston.

The shortest distance between two points is a straight line. If the line is curved, it is a longer distance. In other words, the curvature of the shaped piston creates a WIDER opening. The result is a combination of a faster opening - and with more area.

Most blow down occurs at the initial opening when the exhaust gas pressure is the highest. You can take advantage of that by tweaking the initial shape of the opening.

It takes time to do the job properly. You have to shine a light through the sparkplug hole and look through the exhaust port as the piston cracks open the port. Just keep nipping the edge of the piston (I use a flat needle file) until the entire width of the port opens at the same time.

Polishing the piston top and combustion chamber helps to create plenty of light to work with.

Very interesting idea.

Couldn't you run the top of the piston to the "peak" of the port arc.Scribe it, them file to match?

Only negative I can see to doing this would be losing the squish effect over the exhaust. that is a pretty important area for a squishband.

Seems it would have to be blended back a bit to create a nice smooth transition.

How would it affect the port duration? Act like a lower port?

Wonder what the effects on scavenging would be with an odd shaped piston?

I have seen a few that have cutouts for the transfers. But not a high performance saw.

Might give that a try with the junk piston/cylinder.

 

[Terry Syd]

I do it the hard way because I want the opening to be exact. You could scribe it, file the two notches on each side of the 'peak' of the curve to get close to the line, then finish off the shape.

Surprisingly, it doesn't take that much off the piston to get the shape right, but it is labor intensive. I would also put a thin smear of grease on the piston crown to help trap the filings.

It acts like you increased the blow down timing. In other words, its like you raised the exhaust port. However, the port timing remains the same and you don't loose as much compression or trapping efficiency like you would by raising the port. On my 450 Husky it was like taking the 12 degrees blowdown to 14 degrees. If an engine needs more blowdown to run well, IMO this is a superior method to achieve it.

What do you mean by "loosing the squish effect over the exhaust"? The flame front readily moves under the squish band, in fact, if you read Blair's work you will see that best power is achieve at 50% mass burn at around 10 degrees ATDC. The piston is already 10 degrees ATDC and there is still 50% of the mixture to burn.

 

[Red97]

I do it the hard way because I want the opening to be exact. You could scribe it, file the two notches on each side of the 'peak' of the curve to get close to the line, then finish off the shape.

Surprisingly, it doesn't take that much off the piston to get the shape right, but it is labor intensive. I would also put a thin smear of grease on the piston crown to help trap the filings.

It acts like you increased the blow down timing. In other words, its like you raised the exhaust port. However, the port timing remains the same and you don't loose as much compression or trapping efficiency like you would by raising the port. On my 450 Husky it was like taking the 12 degrees blowdown to 14 degrees. If an engine needs more blowdown to run well, IMO this is a superior method to achieve it.

What do you mean by "loosing the squish effect over the exhaust"? The flame front readily moves under the squish band, in fact, if you read Blair's work you will see that best power is achieve at 50% mass burn at around 10 degrees ATDC. The piston is already 10 degrees ATDC and there is still 50% of the mixture to burn.

Transfering the arc to the piston, would double/triple the squish clearance on both sides of the ex port at tdc

Seems those 2 pockets of slower moving "squish mixture" could disrupt chamber flow/scavenging.

Thinking those gaps, could leave the possibility of end gas detonation in extreme cases.

Arching the piston does make perfect sense, just wondering at what cost?

 

[paragonbuilder]

Here's a trick to get a faster blow down with the curved roof of an exhaust port. Put in a very sensible curvature to the roof that will guarantee a long service life. Then shape the top of the piston crown to the curvature of the roof.

The port not only opens instantly, it actually has a wider opening than a flat top piston.

The shortest distance between two points is a straight line. If the line is curved, it is a longer distance. In other words, the curvature of the shaped piston creates a WIDER opening. The result is a combination of a faster opening - and with more area.

Most blow down occurs at the initial opening when the exhaust gas pressure is the highest. You can take advantage of that by tweaking the initial shape of the opening.

It takes time to do the job properly. You have to shine a light through the sparkplug hole and look through the exhaust port as the piston cracks open the port. Just keep nipping the edge of the piston (I use a flat needle file) until the entire width of the port opens at the same time.

Polishing the piston top and combustion chamber helps to create plenty of light to work with.

Awesome! I like it Terry!


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[paragonbuilder]

Transfering the arc to the piston, would double/triple the squish clearance on both sides of the ex port at tdc

Seems those 2 pockets of slower moving "squish mixture" could disrupt chamber flow/scavenging.

Thinking those gaps, could leave the possibility of end gas detonation in extreme cases.

Arching the piston does make perfect sense, just wondering at what cost?

More for us to test Joe!


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[Terry Syd]

I've already tested it, works fine. However, if you have an engine that already has good blow down, then there is nothing to gain. I tried it years ago on a couple of racing engines and didn't notice anything. However, when I tried it on the very short blow down 450, it was very noticeable.

 

[Red97]

Got the jug Roughed in. Putting a roof on the house this weekend. Won't get it back together till next week.

 

[Sty57]

Got the jug Roughed in. Putting a roof on the house this weekend. Won't get it back together till next week.

Of ya where closer I'd be there Joe......I work cheap.

Beer and pizza

 

[Red97]

Of ya where closer I'd be there Joe......I work cheap.

Beer and pizza

Thanks Brian,

Hired it done, just had to get materials.

Gonna run the loader/dump trailer and air hose is all.