Part One: The Exhaust Port

[drf256]

@Miller Mod Saws

I personally think that the curved vs straight up and down edges have something to do with skirt wear. I try to make sure that the sides when the crown is at BDC aren't curving back in yet.

They say better than 90% of exhaust pressure, but not volume, is vented in the first 10* of crank opening. Leaving the port more elliptical than trapezoidal should create more area where you need venting but less overall area of opening the skirt needs to ride over.

 

[drf256]

Has anyone tried forming more of a bell shape where the port and the cylinder wall meet? So instead of the exhaust flowing past a sharp edge into the port, it flows around a bit of a curve?

Good thinking. That's my personal Texas crutch.

If I think I made a roof a bit to flat, I can chamfer the center upwards more.

The more curved the roof, the functionally lower it is. So a flat 100 roof will vent a lot at 3000 rpm and 14000 rpm. The viscosity of the exhaust gasses will make a curved 100 roof vent pressure slower, so it will functionally act as a lower roof at higher rpm. Hope you guys get what I'm thinking.

 

[drf256]

A smaller exhaust port should keep velocity high and cause better Venturi effect than a huge one. I think most expansion occurs in the muffler and not the port.

How big I make the exhaust port matters on how the factory shaped it originally. Also depends on how big the opening in the plating is.

@blsnelling taught me about port shape and diameter, even if he doesn't remember.

I take a clay imprint of my port opening in the jug and then compare it side by side vs an imprint of the flange. In most cases, I just fully blend the opening in the plating into the port leveling no curves. The exhaust flange opening is generally 30-40% larger than the plating opening.

 

[drf256]

I feel that the exhaust roof creates the size of the saw in some respects. The exhaust roof closing point measured to the band is the swept volume of a saw. If I have a saw with transfer design that will allow me to maintain velocity and flow enough volume to fill a larger area, I go with a lower ex roof. I'm creating a smaller case on a larger jug in some respects, almost like a hybrid.

If I can't fill that area effectively, I'll go with a higher roof. Kinda another hybrid of what @mdavlee is saying he does. If I have to raise the transfers a mile, I'm gonna lose velocity.

@David Young posted torquesoft software that can be used to help one estimate before you cut away and can't undo it.

 

[XP_Slinger]

View attachment 68376

I was thinkin similar to this, for a work saw

I think this has some real potential. But I don't know jack, I just like drawings...lol

 

[drf256]

Just my personal theories. Every model is different and every intended use is different.

FWIW, the 262 above has a KS jug. Factory timing, 100% stock without the .040 base gasket is 106/128/68. I checked it 3 times.

It's also a head slapper by about .003.

 

[XP_Slinger]

Just my personal theories. Every model is different and every intended use is different.

FWIW, the 262 above has a KS jug. Factory timing, 100% stock without the .040 base gasket is 106/128/68. I checked it 3 times.

It's also a head slapper by about .003.

I always enjoy your theories Doc. The intake in that 262 must be positioned pretty high in the cylinder or the piston has a long skirt. Never timed one, interesting to learn how it's laid out.

 

[mdavlee]

cast ring or steal ring?

Steel

 

[drf256]

I always enjoy your theories Doc. The intake in that 262 must be positioned pretty high in the cylinder or the piston has a long skirt. Never timed one, interesting to learn how it's laid out.

Thanks.

The piston skirt is a mile long on a 262. There's literally about .125 of skirt left at TDC at the ex floor, even after dropping the jug .070.

The Stihl 036, OTOH, can be dropped about .055 off the base sans gasket before you see light under the skirt.

 

[paragonbuilder]

I feel that the exhaust roof creates the size of the saw in some respects. The exhaust roof closing point measured to the band is the swept volume of a saw. If I have a saw with transfer design that will allow me to maintain velocity and flow enough volume to fill a larger area, I go with a lower ex roof. I'm creating a smaller case on a larger jug in some respects, almost like a hybrid.

If I can't fill that area effectively, I'll go with a higher roof. Kinda another hybrid of what @mdavlee is saying he does. If I have to raise the transfers a mile, I'm gonna lose velocity.

@David Young posted torquesoft software that can be used to help one estimate before you cut away and can't undo it.

I see it this way too. The sooner the exhaust closes, the more mix you are compressing for a bigger flame. And the longer the gases have time to expand, pushing on the piston, before being released into the exhaust port.

I haven't found a low exhaust to limit top end rpm on the saws I've built, but they seem to gain some grunt. I also don't really care about top speed rpm on a work saw. For me it's about power in the cut. I would rather have a broader torque band and gear up, than a super fast saw that has a very narrow band.

It looks like I've been very conservative in my exhaust roofs compared to what some have posted. Time to up my game!




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[mdavlee]

I'm a small transfer higher exhaust kind of guy

 

[EvilRoySlade]

Good thinking. That's my personal Texas crutch.

If I think I made a roof a bit to flat, I can chamfer the center upwards more.

The more curved the roof, the functionally lower it is. So a flat 100 roof will vent a lot at 3000 rpm and 14000 rpm. The viscosity of the exhaust gasses will make a curved 100 roof vent pressure slower, so it will functionally act as a lower roof at higher rpm. Hope you guys get what I'm thinking.

If I'm reading correctly, I think you are talking about a different topic than the bell shaped comment. I'm reading that not only does one chamfer the port edges but they also grind with a radius entering the port. Not grinding the port straight into the cylinder then only chamfering the lip.

 

[paragonbuilder]

262xp exhaust

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[drf256]

Talking about two topics in one post.

If I make a roof too flat for my comfort, I can cheat with my chamfer to make the rings see a different opening than what's viewed from the outside.

 

[Deets066]

I'm a small transfer higher exhaust kind of guy

I'm bout in the middle between the 2 of ya

 

[drf256]

262xp exhaust

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Looks great to me.

Also depends on how big your port is. On most Stihls, I struggle to hit 60% of width because of the piston skirt. They can be flatter than a wider husky at 70% width.

If you drew 2 lines at 60% width on that 262 port Dan, I bet it would look really flat between them.

 

[drf256]

Here's @huskyboy 044. The port came out a bit square on the roof for my liking. Runs great though.

 

[drf256]

Best pics I've got. It's symmetric.

I guess I shouldn't be a skeered as I've been.

 

[Deets066]

Best pics I've got. It's symmetric.

I guess I shouldn't be a skeered as I've been.

Looks fine to me

 

[drf256]

Looks fine to me

It's got nearly 20 tanks through it and hasn't consumed itself.