Part One: The Exhaust Port

[huskihl]

200 degrees exhaust duration is the zone where most 80-120cc chainsaws are timed.

Just a clarification…200° would be 100° down and 100° back up. It would have to open and close at 80°. That’s more like a piped dirt bike maybe. Saws in that range have about 160° exhaust duration. I think that’s what you meant, but you need to subtract the opening from 180 and multiply it x 2

 

[huskihl]

The larger opening right behind the plating in the exhaust of the 5 series huskies is for performance without making the opening in the plating so large. They made room for hot exhaust gases to expand

 

[Ketchup]

Just a clarification…200° would be 100° down and 100° back up. It would have to open and close at 80°. That’s more like a piped dirt bike maybe. Saws in that range have about 160° exhaust duration. I think that’s what you meant, but you need to subtract the opening from 180 and multiply it x 2

Dead on. I botched that. Cerberus had it right.

The larger opening right behind the plating in the exhaust of the 5 series huskies is for performance without making the opening in the plating so large. They made room for hot exhaust gases to expand

Lots to think about there. I guess it didn’t seem like enough room to me.

 

[Cerberus]

Wow great posts and didn't realize there was an exhaust thread it's been the better part of a year since I read the series Will certainly be over there w/ appropriate topics

Huskihl are you positive Re those "lipped exhaust ports"? That is amazing if so, I mean it's hard to imagine a quick 'dump' opening into a larger exhaust (by the size of the lip) exhaust port would be superior to simply doing a multi-angle diverged port.. Actually as-mentioned it's unwise to make assumptions on this stuff, but the reality is multi-angle divergence (and divergence in general) most-definitely help both with exhaust-release and with 'pull' (due to ability to expand) but it may simply be easier mass-producing a "lip" at the inner port than a multi-angled exhaust port.

Can't help noting the super long (relatively), straight and non-diverging exhaust port of the 2511, found that interesting especially when the muffler allowed for a much bigger exhaust-flanging (heck even grinding-out "flowing" that area gives about 20% more volume or surface-area at that interface)

But Re assumptions & EPA-compliance I still can't understand Stihl enlarging, then reducing, their exhaust port divergences....I'm not saying I doubt "It's EPA" I'd just like to better understand mechanisms!

Awesome posts here!!!

 

[MustangMike]

There is often more than one way to skin a cat!

Just compare the changes in Ford and GM V-8 engines. Both companies developed a lot more power with lower emissions, but they did it in very different ways.

Ford went with Overhead Cam multi valve engines. GM stuck with the pushrod V-8, but did extensive research on intake runner and combustion chamber shape and size.

Both companies came out with great products that met their goals, but they did it in completely different ways.

 

[Ketchup]

The larger opening right behind the plating in the exhaust of the 5 series huskies is for performance without making the opening in the plating so large. They made room for hot exhaust gases to expand

I guess I wonder if it’s worth retaining the lip or just grind it out. What about adding it to a port that didn’t have it originally?

 

[huskihl]

I guess I wonder if it’s worth retaining the lip or just grind it out. What about adding it to a port that didn’t have it originally?

I have no experience in creating it. But a saw that was ported like that with a narrow exhaust port ran near as good as another with a wider ex port.

Usually once you’ve widened or raised, there isn’t enough meat left in the flange to do more

 

[Duce]

How is this?

 

[Nutball]

 

[PogoInTheWoods]

Uhhh..., Trick-or-Treat?

 

[pbillyi69]

Didn't realize this one was this flat, but she's got 10 gallons of mix through it.

034 super

View attachment 68354

i knkw this is an old thread. on that flat port how much bevel is there and has it stayed together or has it grabbed a ring

 

[Bjorn]

Here is a nice exhaust port, nice curve to avoid ring snagg.

 

[Mastermind]

The floor of the exhaust port is where I've always hung a ring. Just sayin.

 

[huskihl]

i knkw this is an old thread. on that flat port how much bevel is there and has it stayed together or has it grabbed a ring

It was beveled well. I’d imagine it’s still running. Showed no signs of anything bad happening while I had it. Sold it to @trooney

 

[huskihl]

The floor of the exhaust port is where I've always hung a ring. Just sayin.

Easily overlooked

 

[Leafy]

put a nice arch on the port and match the piston to the port, you loose a little squish but the effective valve opens faster than a flat port of the same width.

 

[Mastermind]

put a nice arch on the port and match the piston to the port, you loose a little squish but the effective valve opens faster than a flat port of the same width.

Grind the piston crown to match the exhaust port? Just at the port, or from edge to edge?

 

[Leafy]

Grind the piston crown to match the exhaust port? Just at the port, or from edge to edge?

Just at the port. Same way that some people grind on the piston to move the transfers.