Fingers and Bridge ports

[Backyard Lumberjack]

Lots of area in that ex. Looks good, but does it have enough arch in the roof? Hard to tell from that angle

also, the way I see it, and in part influenced from previous comments, Mr. MM I believe... in several of his "here is how I do it" indpeth threads" posted over 'there'...and also treemonkey's comments here on exh port timing, BD can enhance exit velocities... is that the exhaust exit from the area sealed by cyl after combustion... rapidly vents the cyl as piston cracks open the top of the exhaust port... hot gas!!! and most is scavenged out in part from pressure release and also kinetic energy of the exiting column of air... and most is out by 65% or so open... it can be seen, and I note it in OE exh port design is kinda megaphone shape, to include the roof of exh port. while in cylinder the exhaust is very hot, and exiting hot, too... but cooling fast. getting denser. so I will make some mods to help that along. so that is about all the raised roof on this project I will mess with. but for today's R&D and further thots on timing as presented here, specifically in the BD period... I would not have had enough information to recognize what I feel will be opportunistic benefical tweaks to the work there already done...

but I am interested in anything pro or con anyone mite want to contribute. my mission plan is already laid out, but I like any and all add'l Intell.... can use it all one way or another... thanks for the comment.

 

[Mastermind]

What you are doing looks real good to me.

 

[Backyard Lumberjack]

What you are doing looks real good to me.

Thanks! appreciate the feedback...

when I am following general guidelines and some of the voodoo magic of the Master, hisself and such... I am reminded of the Disney movie...

The Sorcerer and The Apprentice... lol

"now then before you can port, you need to understand port timing theory... and application!... first things first... listen to me now, here is how it workx...."

 

[Mastermind]

 

[stihlman441]

If that sticker was on my saw and you were looking up at me all day with that sexy hat on i dont know if i could cope with that ?.

Cheers

 

[Backyard Lumberjack]

Lots of area in that ex. Looks good, but does it have enough arch in the roof? Hard to tell from that angle

do you mean curved arch? that is what an arch is to me. I would be interested in what you would do. the roof is basically flat across the top, aside from radius turns in corners.

>but does it have enough arch in the roof?

enough arch to do what?....

 

[Terry Syd]

To slowly/easily move the rings back into the groove of the piston before they hit the top of the port. When the ring can get slipped back smoothly into the ring groove of the piston, not only does it extend the ring life, but the top of the exhaust port doesn't get worn out.

Now, if you want to push the envelope, you can shape the top of the piston to conform to the shape of the nicely contoured exhaust port. It takes a bit of work as you have to look through the exhaust port with a light though the spark plug hole as you move the piston, you shape the opening of the top of the piston to conform to the shape of the top of the exhaust port. It does drop the compression a small amount, but not as much as you might think. However, when you shape the piston crown to the shape of the exhaust port, it is worth about 2 degrees of blowdown (on a saw with about 12 degrees of blowdown, if you have 25 degrees already I wouldn't bother with it).

When you shape the piston crown, it is in an ARC, that is, it is actually a longer distance than if the roof of the port was exactly FLAT. In other words, you can INCREASE the initial blowdown area a fair amount by shaping the top of the piston next to the exhaust port.

 

[drf256]

lots and lots... and you prob still wouldn't get this kind of information... but if one does that lots and lots, and then comes across this kind of information... he learns a lot! all that and some parts massaging, etc. too... several comments here are, imo... of significant importance given one is focused on optimization and can deal with the basics so that the pushed or leaned on mods do not alter the saw's perferformance in a negative manner... good stuff!

I spent no less than 4 hours this afternoon and early evening mocking up components to ports, (assembled piston on rod, or in bore at least half dozen times, observing and studying the events) studying piston to bore's components and noting further what, where and with who... the piston meets and hangs with as it does it vertical and descending travels... in part, taking in to account some of the comments made in this thread. and the result of it all is I clearly see numerous additional tweaks, kisses and massages that I can do to improve the cyl porting work I am doing... and this does not even start to address all the opportunities that the piston alone offers... well, you know the old saying... "speed costs money, how fast do you want to go!" lol...

chain saw verson: "speed costs time and knowledge, how many hours do you have... ? " lol

some of the places I went today:

View attachment 10348 View attachment 10349 View attachment 10350

BYLJ,

Port work looks great. I'm having a hard time reading your posts though. Try to space out your sentences if you don't mind (I'm interested in what you have to say).

 

[smokey7]

I understand you well read and spoken people notice a ton or errors is posts. I'm sure it's hard to read with punctuation out of place but the info is usually there somewhere. I'm terrible with which spelling or certain words all thevtime. I apologize I'm just a motor head and did terrible in school.

 

[Backyard Lumberjack]

Terry Syd:

To slowly/easily move the rings back into the groove of the piston before they hit the top of the port. When the ring can get slipped back smoothly into the ring groove of the piston, not only does it extend the ring life, but the top of the exhaust port doesn't get worn out.

thanks for your comments. I understand what you are saying. I feel the arch is more than adequate for getting the rings to slide easily over the top of the exhaust port roof. I have not materially changed that shape, not at all actually. if it looks so, its coincidental. I am deflashing not porting... although the two terms have their own extensions in meaning. I am ok with that relative to arch, assuming its what Deets meant. I thot he meant more so for exhaust flow. I read square peaks ups top end rpms, at possible cost of ring/cyl wear. the rings move up n down and don't hang. I have looked hard at ring and ring in cyl... and have thot about ring catching. but, to me its like a rigid I-beam, support it on both ends and it wont bend in middle, thus as I see my parts slide, the exposed ring is not entering the exhaust port... it is rigid and supported on either side of the port well.

Now, if you want to push the envelope, you can shape the top of the piston to conform to the shape of the nicely contoured exhaust port. It takes a bit of work as you have to look through the exhaust port with a light though the spark plug hole as you move the piston, you shape the opening of the top of the piston to conform to the shape of the top of the exhaust port. It does drop the compression a small amount, but not as much as you might think. However, when you shape the piston crown to the shape of the exhaust port, it is worth about 2 degrees of blowdown.

I have seen this and thot about it, too. I see how it is worth a couple of blowdown degrees. issue is blowdown has to dump the exhaust pressure, closer to -0- the better. too high and the transfer port pressures will have to fight against it... I like the idea of piston contouring... and while imo this a stock with gentle mods top end... lol... I might just contour the piston top. your explanation has helped me see better what I was seeing other day for the first time. I fully understand the concept, or the porting tool don't touch it.! iykwim. that is why I am going to register the items that bolt to the exhaust port flange so as to be smooth and not 'mill ridged'... hmm, relieved piston tops... hmm... I like it. to me, that exh port flage exit point as a trasition to and thru the exhaust components is critical, or it will hinder exiting exhaust flow, thus hurting the blowdown effect...

When you shape the piston crown, it is in an ARC, that is, it is actually a longer distance than if the roof of the port was exactly FLAT. In other words, you can INCREASE the initial blowdown area a fair amount by shaping the top of the piston next to the exhaust port.

got it, do you have any pix of piston crowns shaped as such? thanks again...

 

[Backyard Lumberjack]

BYLJ,

Port work looks great. I'm having a hard time reading your posts though. Try to space out your sentences if you don't mind (I'm interested in what you have to say).

ok, see what I can do to slow it down some...

ps: how was that?

lol, j/k

seriously, not exactly sure what you mean.... 'try to space out your sentences....' can u provide an example? I reread them, make some changes, correct this or that if I see it... reads ok to me. but, perception is everything... these fingers just go into WOT mode... and the words jump up to the screen... thoughts to thoughts. if you don't like the ... I can try to change that... mite not post as much though... its just how I think.

 

[drf256]

It's hard to read a post that's that lengthy in one paragraph. It may just be me.

I read it on a tablet.

Carry on as you would. I'll do my best to follow.

 

[Backyard Lumberjack]

BYLJ, (I'm interested in what you have to say).

thank you. a compliment considering the audience. hope its not too boring... lol

 

[drf256]

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.

 

[paragonbuilder]

To slowly/easily move the rings back into the groove of the piston before they hit the top of the port. When the ring can get slipped back smoothly into the ring groove of the piston, not only does it extend the ring life, but the top of the exhaust port doesn't get worn out.

Now, if you want to push the envelope, you can shape the top of the piston to conform to the shape of the nicely contoured exhaust port. It takes a bit of work as you have to look through the exhaust port with a light though the spark plug hole as you move the piston, you shape the opening of the top of the piston to conform to the shape of the top of the exhaust port. It does drop the compression a small amount, but not as much as you might think. However, when you shape the piston crown to the shape of the exhaust port, it is worth about 2 degrees of blowdown (on a saw with about 12 degrees of blowdown, if you have 25 degrees already I wouldn't bother with it).

When you shape the piston crown, it is in an ARC, that is, it is actually a longer distance than if the roof of the port was exactly FLAT. In other words, you can INCREASE the initial blowdown area a fair amount by shaping the top of the piston next to the exhaust port.

Interesting...


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[drf256]

I'm VERY interested in port geometry. How shapes affects performance and flow at different RPM.

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.

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

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.

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.

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

 

[paragonbuilder]

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.

A mentor once told me, " If you immediately dismiss ideas that seem dumb or wrong, you will miss the game changer!"


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[Keith Gandy]

Everyone is invited.

What part of Missouri?

 

[jmssaws]

Bollinger county

 

[srcarr52]

Correct me if I'm wrong but from what I gather the first portion of the transfer process bypasses the carb.

Yes, the strato ports fill the transfer port tubes with fresh air (no fuel).