Fingers and Bridge ports

[Al Smith]

Well again you'll never know until you try .Start out on something simple and cheap .Fact my virgin attempt was on a 46 CC Craftsman/Poulan, clam shell design.

It improved it a great deal until the seals went funky .Never got around to replacing them,will some day .From there moved on to Stihls and a few McCullochs .Every one was an improvement .

Here of late with the Monarch 10" EE lathe on the blink plus working 7-12's for the last few years I haven't had the time .I'll get back into it some day .

 

[Terry Syd]

If the finger ports are deep enough to really flow, then it may be helpful to take a page from the 5-port two strokes and use a timing figure that is a bit less than the side transfer ports. If you look at a 5-port, the back port is a bit lower than the side ports. This is to get 'underneath' the side transfer flows and direct it up to the combustion chamber.

The angle (and width) of the back port is also very important to the way the cylinder is scavenged. A more upward angle generally helps with the top end and a more horizontal flow helps with a stronger power peak with less '0verrun'.

I'm wondering if someone ever came up with a decent finger port configuration on 'open port' saws to help an open port jug to flow better on top end. If so, I'd love to see the pictures.

As near as I can see, the 'bridge port' configuration is a neat compromise when you don't have a four-port jug. Instead of having a back port that is slightly angled upward, the 'bridge port' relief will 'lift' the flow from that area of the transfer to a different angle than the rest of the port flow. Think of an open port jug and how the flow doesn't hug the top of the piston, that's what the relief of the bridge port does. The flow along the side of the piston lifts the flow so that it flows at a steeper angle in the cylinder. In other words, the back part of the flow is angled more upwards than the front part.

 

[drf256]

Bridge ports must aid in direction. Once the transfer cracks open, they are creating a quick jet of charge before their pressure is released by the mains shooting their load.

They really make little sense otherwise.

Fingers should have a lot of velocity until the mains open.

But it's all happening 250 times a second. So I call BS on all of it.

I think the different ports change the ribbon of intake flow direction as it loops through the cylinder. I think they also change shape, and when done correctly, fill the cylinder with more fresh charge to burn.

I plan on an adjustable outlet muffler. A sliding piece of metal. Keep opening more and more on each model saw until power falls off. Then I'll know the best exhaust surface area for the muffler.

 

[mdavlee]

Timberwolf did the exhaust deal on an 026 about 8-9 years ago. It was huge before he had idle problems and excessive fuel usage. I don't remember the thread name but it's on another site.

 

[Mastermind]

Timberwolf did the exhaust deal on an 026 about 8-9 years ago. It was huge before he had idle problems and excessive fuel usage. I don't remember the thread name but it's on another site.

Remember the 80% rule that someone came up with on exhaust outlet size?

Timberwolf blew that outta the water. lol

 

[Moparmyway]

Timberwolf did the exhaust deal on an 026 about 8-9 years ago. It was huge before he had idle problems and excessive fuel usage. I don't remember the thread name but it's on another site.

Remember the 80% rule that someone came up with on exhaust outlet size?

Timberwolf blew that outta the water. lol

Yup, blown out of the water and still no idle problems or "excessive" fuel use

 

[Redfin]

Remember the 80% rule that someone came up with on exhaust outlet size?

Timberwolf blew that outta the water. lol

Timberwolf did the exhaust deal on an 026 about 8-9 years ago. It was huge before he had idle problems and excessive fuel usage. I don't remember the thread name but it's on another site.

Wasnt it at 120% or so when the idle was affected?

 

[jmssaws]

I've never noticed any idle trouble even with the front cover off.

 

[Mag Craft]

Timberwolf did the exhaust deal on an 026 about 8-9 years ago. It was huge before he had idle problems and excessive fuel usage. I don't remember the thread name but it's on another site.

I had that book marked for a long time so I could refer back to it. It was a good read with real testing and real results.

 

[czar800]

I've never noticed any idle trouble even with the front cover off.

The few times I had the covers off my saws they idled also.

 

[jmssaws]

80% is bogus. I've tried it,take the cover off your stihl all it will be is louder.

 

[tree monkey]

if you go back to the saws of the 70's and put a degree wheel on them you will find that they had shorter exhaust duration and less blow down, and more open mufflers.
saws today have very restrictive mufflers to reduce noise. this forces the oem's to raise the exhaust port duration and longer blow down to give more time for the exhaust gas to escape.
in a stock saw the 80% rule is about right, but most of us don't run stock saws.
the size of the muffler also plays a part in this. the 562 has a large muffler for it's size, and has a short blow down.
opening a muffler more then needed will only make it louder.

 

[jmssaws]

I run less blow down than builders post on here,my 064 hybrid only has 14 degrees. Runs pretty good too

 

[srcarr52]

I run less blow down than builders post on here,my 064 hybrid only has 14 degrees. Runs pretty good too

I've ran as far down as 10 degrees blowdown, so I don't think you are out of the norm.

 

[Mastermind]

I've ran as far down as 10 degrees blowdown, so I don't think you are out of the norm.

On the 562XP and the 550XP I run 10 degrees blowdown.

 

[drf256]

if you go back to the saws of the 70's and put a degree wheel on them you will find that they had shorter exhaust duration and less blow down, and more open mufflers.
saws today have very restrictive mufflers to reduce noise. this forces the oem's to raise the exhaust port duration and longer blow down to give more time for the exhaust gas to escape.
in a stock saw the 80% rule is about right, but most of us don't run stock saws.
the size of the muffler also plays a part in this. the 562 has a large muffler for it's size, and has a short blow down.
opening a muffler more then needed will only make it louder.

What are your feelings on exhaust roof geometry, specifically the acuteness of the arch, and necessary blowdown adjustments?

 

[jmssaws]

I accidentally found that a 066 with 14 to 16 degrees is where I like them.

 

[Deets066]

I accidentally found that a 066 with 14 to 16 degrees is where I like them.

How do you figure blow down with fingers

 

[David Young]

if you go back to the saws of the 70's and put a degree wheel on them you will find that they had shorter exhaust duration and less blow down, and more open mufflers.
saws today have very restrictive mufflers to reduce noise. this forces the oem's to raise the exhaust port duration and longer blow down to give more time for the exhaust gas to escape.
in a stock saw the 80% rule is about right, but most of us don't run stock saws.
the size of the muffler also plays a part in this. the 562 has a large muffler for it's size, and has a short blow down.
opening a muffler more then needed will only make it louder.

Scott I couldn't agree more, another is look how big the muffler is on the 7900.

to add a thought. look at the cat in a 359 muffler basically it cuts the muffler in half. a bunch of holes in it defeats the decrease in muffler area.

I chuckle when you see guys clean out under the deflecter and then hammer the deflector shut again, its as open as your smallest opening.

 

[Deets066]

I accidentally found that a 066 with 14 to 16 degrees is where I like them.

Right close to where I'm at,

14* BD on fingers
17* on mains