Fingers and Bridge ports

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[Backyard Lumberjack]

Not boing one bit. I can talk porting theory all day. Thinking outside the box and spewing out thoughts that sound dumb in retrospect are both things I try to do frequently. If you don't try things, you'll never know what worked.

drf255 sed:I can talk porting theory all day.

theory, priciples, postulates and practical application.

I know just what you mean. this thread could maybe be a chapter in the textbook on subject - Contemporary 2-Stroke Porting - Application OPE Theories. by Numerous Posters.

funny how the subject knows no limits of interest, yet there seem to be no lengthy threads on removing crank seaks, splitting cases, or how to clean behind your flywheel. etc

plenty good stuff on the art of cylinder modicatons...

 

[tickbitintn]

All those "other things" can be covered pretty quickly and easily....
Porting and flow can be discussed indefinitely....

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[Backyard Lumberjack]

drf255, post: 51219, member: 319"]I'm VERY interested in port geometry. How shapes affects performance and flow at different RPM.

BL: for discussion purposes, only...

The way I see a port line is more dynamic. I try to see it with the inertia of charge more than absolute opening and closing. I may be very wrong on this.

doubt anyone would say u r wrong... the further u 'see' things... the better to understand. can't change anything for the good, if u don't understand the principles.

So the exhaust port is where the most stress on the rings occur. There's compression and decompression occurring there.

.true, but the rings are designed to take it, with reliable repeatability. not so sure I would say stress. its hot for sure, but parts normalize and compression and decompression takes place all around the loaded ring. hard to imagine the events at 200+ times per second...

The roof arch allows the rings to slide out of the gap slowly. Round is better for rings. Flat/square is better for venting.

You've got about 10-15* of engine rotation to vent the exhaust enough to be lower than the transfer pressure. But at rpm, you've also got a small amount of intertial affect to pull exhaust out, even without a pipe. You also have transfer inertia to fill the cylinder against a small amount of exhaust pressure.

.have thot about this quite a bit. the piston's intake stroke is visually short. and the transfer event short too in terms of piston travel. other than the 14.7 atmospheric for a normally aspirated engine... the 2-stroke performs so well. but how? to me it's like a supercharger packing the intake manifold. most folks focus on psi gauge, but ideally... that is just back pressure... some blower cams reduce that and help ensure a higher % of intake charge consumption, ie more in cyl and less in manifold. seems to me the crankcase is 'packed' continually by the 2-stroke function. my question I ask myself is - is the column of air stopping and starring each intake/fill cycle... or are the parts mechanical and set and the air column is a gas so can self-compress due to inertia and thus... it does not stop and start. since it has density cannot see the mass stopping and starting to move again... 200+ times a second. stopping and starting takes time... and not much time is available... lol

You also have more "effective" compression with a rounder exhaust roof. Air has sone viscosity to it. So there a point at higher rpm where the gasses will get cleaved off and compression will actually start before the exhaust port is fully closed. Think old McCullogh T slot ports. I think Stihl still uses this design on their cutoff saws.

.not sure I can buy into this theory. I can see where you will have add'l cyl filling as intake uses exh inertia... but open port, no compression. compression is not going to take place until cylinder sealed. and imo, exhaust roof will have no effect on compression, effective or mechanical. imo, compression is a mechanical event that addresses a density change to a gaseous volume.

A lot to think about. And don't get me started on intake floors...

.well, that's my story... stickn to it...

good stuff!

time to be constructive for me, think I will head off and see if I can pin that cylinder....

 

[GoBigBlue1984]

I'm liking where this is going.

I've got wood and 700 acres to play on.

Is Kentucky too far away for you guys? I nominate idiotwithasaw (Ben) to host. We could do several locations at once. They only have around 7,000 acres.

 

[GoBigBlue1984]

I'm liking where this is going.

I've got wood and 700 acres to play on.

It would be a mega GTG. Complete with loads of food and a Saturday night band. Country music/southern rock. It could work fellas.

 

[Mastermind]

I'm up for it. Just check with the other GTGs before y'all decide on the date.

 

[tickbitintn]

I'm in probably be closer too...

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[Toad22t]

It would be a mega GTG. Complete with loads of food and a Saturday night band. Country music/southern rock. It could work fellas.

What part of Kentucky? Some parts of Kentucky just scare me! But man was there some southern belles down there!

 

[GoBigBlue1984]

What part of Kentucky? Some parts of Kentucky just scare me! But man was there some southern belles down there!

We're in Hodgenville (birthplace of Abe Lincoln) an hour south of Louidville and 2 hours north oh Nashville.

 

[tickbitintn]

That would cut my drive time in half !! This just keeps getting better !!

 

[Stihl working Hard]

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


View attachment 10116 View attachment 10117 View attachment 10120

Nice job on the muff mod great pics

 

[Backyard Lumberjack]

Nice job on the muff mod great pics

I agree, as well. thx for the postings. I have been working on my 044 muffler, mostly in port matching to exhaust port... it has one exit opening, stock position but cut back and I want a 2nd one. was planning on maybe something from out in marketplace... but I like what u have added to side. looks like high flow to me!

I certainly am influenced by your pix...

 

[Toad22t]

Just north of bowling Green then? My brother used to live east of bowling Green and worked in scottsville.

 

[Mastermind]

Looks like I dragged this thread into the ditch.

My apologies.

I cleaned it back up.

 

[czar800]

^^^Thread killer... ^^^

Bump for a good read.

 

[Mastermind]

^^^Thread killer... ^^^

That's me.

 

[T.Roller]

Very interesting stuff