Porting 101

[TraditionalTool]

70% as measured along the curve of the cylinder wall.

Thanks Ed and Brad.

I put the old piston in the cylinder and marked the sides of the current intake and exhaust with a magic marker, then pulled it out and measured with a flexible rule. Currently both are about 25mm (approx.), so I have about 5mm of width I can take it. I won't touch the roof or bottom of the ports, per your recommendation.

 

[edisto]

70% as measured along the curve of the cylinder wall.

That explains it...I thought 70% was getting a little on the edge.

Just out of curiosity, why do you measure along the curvature instead of measuring actual port width? My trig might be off, but the end result of this is that as bore size increases, you wind up with smaller ports relative to making port width, i.e., chord, a direct function of diameter.

 

[sawinredneck]

TT if you read the threads I widened around .120" on the intake and exhaust. The .020-.040" is STRICTLY up and down! Open the throat from the intake tube as much as you dare, I'm not sure where Brad came up with this information? MAKE IT FLOW!!!
Brad without a base gasket he exaust need raised to keep timing in check. I also like to lower the intake for more airflow. RPM's are not my thing I want tourqe, which may be why I despise the 346xpoe so much, but I want usabel power, which is what the mods to the 350 gave me.

 

[blsnelling]

Open the throat from the intake tube as much as you dare, I'm not sure where Brad came up with this information? MAKE IT FLOW!!!

I'm talking about the transition point where the intake elbow meets the spigot of the cylinder. You can open it up as big as you want, but it's not going to flow any more than what the size of the elbow can flow. From there I taper it to the port wall, keeping the walls as straight as possible from spigot to wall.

 

[matt9923]

This has some dam good info.
anyone have a shot cylinder i could practice grinding on?

 

[sawinredneck]

I'm talking about the transition point where the intake elbow meets the spigot of the cylinder. You can open it up as big as you want, but it's not going to flow any more than what the size of the elbow can flow. From there I taper it to the port wall, keeping the walls as straight as possible from spigot to wall.

You and I are in the same book, just on the wrong page Brad. I'm talking about where the intake tube meets the cylinder, there is a huge wall that creates a swirl there, the more you can thin those walls and utilize the gasses coming in, the better ANY saw will run. It's all about transitions, the smoother the transition the happier anything will be, agreed?
That "elbow" is easily flowing 3/4's more air than the cylinder can take on these saws. It's a bottleneck that gains huge amounts if you think about it.

 

[Tzed250]

You and I are in the same book, just on the wrong page Brad. I'm talking about where the intake tube meets the cylinder, there is a huge wall that creates a swirl there, the more you can thin those walls and utilize the gasses coming in, the better ANY saw will run. It's all about transitions, the smoother the transition the happier anything will be, agreed?
That "elbow" is easily flowing 3/4's more air than the cylinder can take on these saws. It's a bottleneck that gains huge amounts if you think about it.

You have to be careful porting the intake back near the manifold...real easy to port to daylight in that area...

 

[TraditionalTool]

70% as measured along the curve of the cylinder wall.

Brad,

For clarification, this means 70 percent of the piston diameter being measured along the circumference of the cylinder wall. Is that right?

 

[TraditionalTool]

Got the cylinder pretty clean, using draino, I still have just a tad left, there's one alu bugger left in there I can feel...you can barely see it about 3/4 of the way to the right of the exhaust port. There is also a bit of smegma in the top of the cylinder above the exhaust port (towards the bottom in this pic).

This is the before pic:

And after cleaning up:

And I did this muffler mod, which I copied some of the ones Brad has done with these deflectors I got from Baileys.

Tomorrow I will finish getting the cylinder prep'd and widen the ports.

 

[edisto]

Brad,

For clarification, this means 70 percent of the piston diameter being measured along the circumference of the cylinder wall. Is that right?

That amounts to a port width of about 50% of the bore diameter, which is relatively mild.

A port width that is 70% of the bore diameter could get you into trouble with short-circuiting from the transfers to the exhaust.

The problem I have with Brad's 70% rule, being measured along the circumference, is that (unless my math is wrong), in a relative sense, porting by that rule will get progressively milder for larger bores, which seems counter-intuitive to me.

 

[TraditionalTool]

That amounts to a port width of about 50% of the bore diameter, which is relatively mild.

A port width that is 70% of the bore diameter could get you into trouble with short-circuiting from the transfers to the exhaust.

The problem I have with Brad's 70% rule, being measured along the circumference, is that (unless my math is wrong), in a relative sense, porting by that rule will get progressively milder for larger bores, which seems counter-intuitive to me.

Ed,

I think it has to be bigger than 50%, it seems to be closer to 70%, because the radius is not very severe. Maybe Brad will comment on this????

I got my piston today. Looks ok, nothing to write home about...

 

[blsnelling]

All I can tell you is what I do. I measure the bore diameter and then multiply that by 70%. I then mark that distance on a piece of tape. Then I lay that tape on the cylinder wall and mark the wall.

 

[TraditionalTool]

On the bevel...

Brad,

I can see a slight bevel around the exhaust port, as came from the factory. I need to put a bevel similar, around the edges?

I see mostly bevel on the sides and floor, but just ever so slightly on the roof where it would seem needed the most.

I guess as you widen the port, it exposes the roof more, since the sides of the port wrap around the cylinder more. As one widens them, it seems that the roof may be more concern to keep the ring from snaggin' on it, as the sides will not be supporting it as well, after widening. Is this suspicion correct?

 

[edisto]

Ed,

I think it has to be bigger than 50%, it seems to be closer to 70%, because the radius is not very severe. Maybe Brad will comment on this????

I got my piston today. Looks ok, nothing to write home about...

Yup...had a parenthesis in the wrong place on the computer. Redid it by hand, and it comes to about 64.42%, which is a healthy number.

 

[TraditionalTool]

Yup...had a parenthesis in the wrong place on the computer. Redid it by hand, and it comes to about 64.42%, which is a healthy number.

Darn parens (and semi-colons), they will fsck a program up quick...oh gee, am I allowed to type fsck on here? The mods will think I'm trying to beat the word censor by using a common term for file system check on unix/linux...*lol*

Makes me yearn to write code once again...maybe I'll find a job and do that...but in the meantime...might as well play with 2-strokes...

 

[blsnelling]

Yes, the roof of the exhaust is the most critical port edge. You must have a bevel there. The wider you go, the more critical it is.

 

[super3]

This has some dam good info.
anyone have a shot cylinder i could practice grinding on?

Pm me your address,i'll send you one.

 

[edisto]

Darn parens (and semi-colons), they will fsck a program up quick...oh gee, am I allowed to type fsck on here?

I blame Excel...it works in radians, so I had to convert, but misplaced a parenthesis, so I was dividing sine theta by 2 instead of calculating the sine of 1/2 theta.

Once I did it by hand, it became pretty clear...70% is a constant proportion of the circumference of a circle, so the degrees occupied by that arc (theta) is a constant as well (approx 80 degrees). If theta is a constant, then sine of 1/2 theta is a constant, which means chord length varies directly with the hypotenuse (the radius).

So...Brad's method of using 70% along the curvature of the cylinder gives a port width equal to 64.4218% of the bore, for all bore diameters.

 

[TraditionalTool]

Yes, the roof of the exhaust is the most critical port edge. You must have a bevel there. The wider you go, the more critical it is.

Yes, as I suspected...when you look at the cylinder it seems pretty obvious that as it looses support along the cylinder wall, that there would be a tendency for the ring to want to push out. I will make sure I have a decent bevel, do you do your bevel as a flat, or as a soft curve around the lip? I'm going to take a stab at this tonight. Still have some cleaning yet...

So...Brad's method of using 70% along the curvature of the cylinder gives a port width equal to 64.4218% of the bore, for all bore diameters.

This is a good number to know, and gives a second point of reference.

A small pair of outside calipers should confirm that, yes, however, I'm about 27mm on the stock exhaust port, and I can only go to 28.16 with your number, so that doesn't seem like I can go too much wider there. Seems I'll need to take most of the removal on the exhaust on the shoulder without widening the port too much, if I understand the numbers.

On the intake I have just a tad more to work with, it's about 24mm. While that is more, that is not too much...and the skirt is about 35mm (chord) on the piston. Seems like I have plenty of skirt, but the amount of widening may be limited, if I'm measuring correctly...(I'm still doing that, seems like I need to go over these numbers better).

 

[TraditionalTool]

All I can tell you is what I do. I measure the bore diameter and then multiply that by 70%. I then mark that distance on a piece of tape. Then I lay that tape on the cylinder wall and mark the wall.

Yep, this is the way to do it, for certain...I have been trying to measure it, and I can get a reading with some very small calipers, but I like the idea of putting the tape on it.

Oh where or where can that blue tape be, oh where or where can you be...

EDIT: Just finished the muffler:

Side:

Front: