Part One: The Exhaust Port

[rattler]

I have done a step exhaust on 372 before

 

[Nc grease monkey]

I have done a step exhaust on 372 before

Any noticeable gains?

 

[CR888]

There was a builder on EBay doing that for 660/066 exhausts. He really believed in it too after reading his description. He sold mufflers that were ported.

 

[Nc grease monkey]

Ill play around with it some and see if there are any noticeable gains

 

[rattler]

Any noticeable gains?

Not th a t I could tell but it had alot of other work done on it

 

[Redfin]

I ground a step on my chico saw for Randys buildoff. Mainly because there was just so much material to remove. I did not try it without.

 

[breese]

You have to wonder if some of the older saw makers knew something back when..
Take my 1958 Homelite EZ.
The floor of the exhaust is flat from the cylinder on out.
The roof on the other hand makes 2 curved upward transitions along with a splitter of sorts.
What is interesting with the splitter is that it is Not directly down the center of the exhaust port so one would assume its not there as a support... But who knows...

 

[David Young]

great info in this thread

 

[Cracker Boy]

No it isn't upside down.

View attachment 71945 View attachment 71948

Is this a ks cylinder off 034s

 

[Terry Syd]

You have to wonder if some of the older saw makers knew something back when..
Take my 1958 Homelite EZ.
The floor of the exhaust is flat from the cylinder on out.
The roof on the other hand makes 2 curved upward transitions along with a splitter of sorts.
What is interesting with the splitter is that it is Not directly down the center of the exhaust port so one would assume its not there as a support... But who knows...

View attachment 95264 View attachment 95263 View attachment 95262

Same concept - get a quick pressure drop just outside the exhaust port to aid in the blowdown. The manufacturing process made it necessary to use the step on the top versus the bottom.

 

[Chainsaw Jim]

Is this a ks cylinder off 034s

I do believe so.

 

[Cracker Boy]

I do believe so.

I got one on my 360 pro ported it was a runnin sob before port.now its a beast

 

[NCFarmboy]

I put a step in a 55 Husky a while back it did run really well. Got the saw back last week scored piston. Still has the chain on it I put on it. Owner let somebody BORROW it.
Shep

 

[David Young]

I put a step in a 55 Husky a while back it did run really well. Got the saw back last week scored piston. Still has the chain on it I put on it. Owner let somebody BORROW it.
Shep

pics?

 

[tree monkey]

 

[David Young]

View attachment 95766

thank god for those suspenders...

 

[Chainsaw Jim]

Here's another example of a factory step in the exhaust. The Jonsered 111 super.

 

[XP_Slinger]

Here's another example of a factory step in the exhaust. The Jonsered 111 super. View attachment 98330

View attachment 98331

I like how straight that port is. No slope on the roof would maximize flow IMO

 

[nohoff]

The Stihl FS 460 has a step in exhaust too

 

[wcorey]

No slope on the roof would maximize flow IMO

Maybe...

There are also some who believe a downward sloping ex port to be an advantage.

Could be a design consideration in the newer generation of saws with these back sloping cylinders.

I picture it in terms of how loop scavenging is working at that point in the cycle, not that I have a handle on the dynamics that go on in there at 10k rpm.

When the intake charge comes in up and behind the soon to be pushed out exhaust, the boundary between the two would go up at an angle toward the intake side/wall and down toward the ex port.
It seems like a downward exit slope could encourage a more orderly/linear expulsion of the similarly angled layer of exhaust gases and maybe reduce loss of intake charge.

The step on the exhaust floor could also be a benefit toward this end...