Chainsaw Porting Theory

[drf256]

So how would you fix that? Would you have to raise the roof to round it off whitch would change timing?

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There is no fix needed. If Stihl made it that way, it will last.

I'm wondering if we are too hell bent on the roof arch.

Remember, even with widening of the port, it's still opening gradually. It's only wider for the first 20* if it's squarer. I hope that makes sense.

There likely a fine line between how flat you can go and how hard it is on the rings.

 

[Stihlbro]

Here's a pic that intrigues me. Look at the roof of the exhaust port on the new 261 Stihl. I wouldn't let one this square leave my bench. View attachment 12987

This would allow for some seriously fast exhaust venting and a lot of port area vs. time.

How did the saw run? It's not nice to judge a port until you get to know her........



The staining in a transfer can be merely from how the saw is operated. In other words, ain't nothing wrong with the porting. It may have lived it's life working at lower rpm than it originally designed for.

 

[drf256]

Good morning Joey.

Glad you're here.

Care to comment on intake shape/duration and throttle response?

 

[Keith Gandy]

There is no fix needed. If Stihl made it that way, it will last.

I'm wondering if we are too hell bent on the roof arch.

Remember, even with widening of the port, it's still opening gradually. It's only wider for the first 20* if it's squarer. I hope that makes sense.

There likely a fine line between how flat you can go and how hard it is on the rings.

I agree . I see getting it tooo square in the corner being the turning point to being a problem. I can say that from an experience that went South quickly!!! Lol!!

 

[mdavlee]

On an exhaust port raised 3/16" or more I could see arch being a bigger problem than that port.

 

[Stihlbro]

Good morning Joey.

Glad you're here.

Care to comment on intake shape/duration and throttle response?

Absolutely! The shape of the intake can be determined by the intake track and carb size. A lot is saws I've been into the intake port opening at the piston is larger that the exhaust. Not sure why but I think I know.
The concept on squaring a port up for the most area makes sense. But I've found the tooling used can determine your shape. I see all kind of shapes. Same area just different.
It has already been determined the lesser the intake duration the better fuel efficiency, the more duration the less fuel efficient. When you start getting too much intake duration the throttle response and tuning gets harder. With saying that, if you go back to the time/area theory, a port widened to 65 percent of the bore and has a opening point of 72 can likely be the same area as a port width 55 percent and at 74 opening point.

Finally you have to have a plan. Map things out, study the saw design. Compare bore to stroke and crank case volumes. It's not all about the numbers. It's shape and area and volumes.

Thank you, now I gotta go find something to port. WOOOOOOOO! (Riv Flair voice)


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[Ironworker]

Is it possible to put a slight bevel on the ring if leaving the exhaust port square. I'm thinking if the r&d thought a square exhaust is good then maybe it is.

 

[drf256]

I'll see if I can dig up a some intake port pics. Stock vs. ground.

 

[Keith Gandy]

Is it possible to put a slight bevel on the ring if leaving the exhaust port square. I'm thinking if the r&d thought a square exhaust is good then maybe it is.

I put a small bevel on the edge of my rings with 2000 grit

 

[drf256]

044

Same duration. Better throttle response.


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[Stihlbro]

You got any 044/046 jugs, gosh them thing from certain manufactors, I still wonder what to the hell was they doing.

New 550 and 562 with tiny intake ports, wth going on there?

 

[drf256]

036. Widened and squared and just widened. Widened and squared ran better.



The center bottom of both ports is stock and at 84*

 

[mdavlee]

You got any 044/046 jugs, gosh them thing from certain manufactors, I still wonder what to the hell was they doing.

New 550 and 562 with tiny intake ports, wth going on there?

They sure do have a lot of variation.

The 550,562, and 576 all have the tiny intake ports. Not sure why but they do make good power.

 

[Mastermind]

This is one of the better threads I've read in. Good discussion makes a feller think.

Going back a minute to Blair....

He said the first 10° of transfer opening would be back pressure. I wonder what opening points he was seeing......that's very interesting.

In my mind I see a pressure wave....

 

[Keith Gandy]

This is one of the better threads I've read in. Good discussion makes a feller think.

Going back a minute to Blair....

He said the first 10° of transfer opening would be back pressure. I wonder what opening points he was seeing......that's very interesting.

In my mind I see a pressure wave....

Delayed Scavenging Stratification Technology that Stihl stole from Blair!!!

 

[drf256]

If the pressure waves are only for 10*, how does it backstuff a transfer that opens 20* later?

 

[Keith Gandy]

If the pressure waves are only for 10*, how does it backstuff a transfer that opens 20* later?

Like a venturi effect?

 

[XP_Slinger]

Where are you located XP?

I wouldn't believe any inversion at all, but Blair's book does show a pressure change in the transfers upon initial opening in a chainsaw engine.

Does that pressure change just compress the clean mix that's at the transfer tip, or does actual reversion occur? That I'm not clear on.

That's what I'm not sure of. Does it actually revert normally? Or does the new transfer charge push it fast enough to keep exhaust out of the transfer. Still learning.

I'm in central NY, a couple miles from the eastern end of Oneida lake if you're familiar with the region.

 

[drf256]

I see that he measured it. I cannot wrap my head around the flow reversal at 250 times a second overcoming inertia.

 

[drf256]

Like a venturi effect?

Awesome thought.

Piston uncovers port fast, would cause sudden negative pressure wave. Wow.