Part One: The Exhaust Port

[Johnmn]

That pic leads me to another question.

Angle of the floor?

Steep downward angle?

Relief step?

Thoughts?

Also what about taking material off the floor? I have seen some of you guys do this

 

[srcarr52]

That pic leads me to another question.

Angle of the floor?

Steep downward angle?

Relief step?

Thoughts?

Steep or relief step if you can't make it steep.

 

[XP_Slinger]

I like to open it up maintaining the factory floor angle or steepen it if there's enough material, also retain the step near the window. If the step isn't there I grind one in.

 

[EvilRoySlade]

Youre drawing a Husky 262 Jug.

I've not handled one. No wonder why they're such good runners.

 

[drf256]

I've not handled one. No wonder why they're such good runners.

You would have to put a 2" block under the front base of the jug to stare at the exhaust roof straight on.

 

[drf256]

Also what about taking material off the floor? I have seen some of you guys do this

Almost never needed and almost always bad.

The floor stops freeporting of the skirt at TDC. I adds stability to the piston, and rarely does the crown of the piston descend past it.

 

[CJ Brown]

Okay so back to port shape for a second time. If some curvature is required on the port roof at the cylinder wall to prevent ring snags, why not match the edge of the piston to match the curvature of the port, thus allowing the port to open wide instantly rather than gradually with the port curvature? Feasible?

 

[Mastermind]

Also what about taking material off the floor? I have seen some of you guys do this

Not at the plated end, but at the outlet end. Yes, on many Stihls that I'm already matching the muffler shell to, I lower the floor to increase exit angle.

 

[XP_Slinger]

You would have to put a 2" block under the front base of the jug to stare at the exhaust roof straight on.

Though I've never been inside one, I'd bet the reason the 550 and 562 cylinders are laid back toward the operator is partially because of the angle of the exhaust port.

 

[srcarr52]

Though I've never been inside one, I'd bet the reason the 550 and 562 cylinders are laid back toward the operator is partially because of the angle of the exhaust port.

Yup, I mentioned that back on page 3. They also have a steep intake angle towards the crankcase.

 

[XP_Slinger]

Yup, I mentioned that back on page 3. They also have a steep intake angle towards the crankcase.

Thanks Shawn. Sorry I missed your earlier post. This thread is moving fast!

 

[Stump Shot]

Also what about taking material off the floor? I have seen some of you guys do this

Not at the plated end, but at the outlet end. Yes, on many Stihls that I'm already matching the muffler shell to, I lower the floor to increase exit angle.

So is it usually a good idea not to drop the floor level even if there is room to do so on both ends of the piston travel? Does this affect the next part of the process?

 

[Redfin]

Yup, I mentioned that back on page 3. They also have a steep intake angle towards the crankcase.

Maybe not as defined as those but the 261 reflects the same angles.

 

[Mastermind]

So is it usually a good idea not to drop the floor level even if there is room to do so on both ends of the piston travel? Does this affect the next part of the process?

If the piston crown drops below the bottom of the port, and freeporting at the skirt are not an issue, then lowering the bottom of the port might be ok. The Husqvarna 2100XP is the only saw I can think of that fits those requirements,

 

[XP_Slinger]

If the piston crown drops below the bottom of the port, and freeporting at the skirt are not an issue, then lowering the bottom of the port might be ok. The Husqvarna 2100XP is the only saw I can think of that fits those requirements,

Seems like that would be a rare thing to see indeed. I think it would be of benefit if the port floor angle carried through to the muffler instead of making that turn to almost parallel. But that's pretty much what engineers have done on the new saws so nothing new there.

 

[Mastermind]

http://opeforum.com/threads/part-two-the-intake-port.7290/

 

[Stump Shot]

If the piston crown drops below the bottom of the port, and freeporting at the skirt are not an issue, then lowering the bottom of the port might be ok. The Husqvarna 2100XP is the only saw I can think of that fits those requirements,

Maybe that's where I saw it.

 

[David Young]

View attachment 68376

I was thinkin similar to this, for a work saw

This is the shape I have gone to. It will get most of the scavenging work done for you but the benefit of this style is the bottom of the port gently reseats the rings. I have had the top of the piston break out from the bottom being too flat. headed up the whole piston is supporting the ring. Headed down there is .100-.150 supporting the ring.
as far as the need for a flat roof... is the linear speed of a piston constant or does it change? Where is it the fastest?

I like the step in the exhaust as well.

 

[srcarr52]

This is the shape I have gone to. It will get most of the scavenging work done for you but the benefit of this style is the bottom of the port gently reseats the rings. I have had the top of the piston break out from the bottom being too flat. headed up the whole piston is supporting the ring. Headed down there is .100-.150 supporting the ring.
as far as the need for a flat roof... is the linear speed of a piston constant or does it change? Where is it the fastest?

I like the step in the exhaust as well.

Piston velocity is pretty similar to a sine wave. So the highest velocity is when the crank pin is at 90 or 270 deg and it has 0 velocity at TDC and BDC (0 and 180 deg).

 

[RIDE-RED 350r]

Well, linear speed of the piston does change. It is slowest when at and near TDC and BDC, fastest halfway between the two.

As to the mirror polishing of the exhaust, I think there may very well be some benefit to it, but I also think it is temporary unless one cleans the carbon from the exhaust tract regularly. Carbon inevitably will start to accumulate no matter how exotic the premix oil one uses or the mix ratio. Some oils are cleaner than others, but in the end carbon deposits will start to form. That is the main reason I have never tried it on my 3 work saws I have ported. Maybe a different story on a race saw that only sees racing action and regular cleaning of carbon is reasonably expected to be done.