Chainsaw Porting Theory

[beaglebriar]

I really found the benefits of a good muffler mod today. When I originally ported my 353 I just gutted the cat and opened up under the existing deflector. It ran good but I wasn't super impressed. After looking at Treemonkeys 2153 build I decided to add another hole. It runs night and day different with the second opening. Spools up faster and has a lot more balls.

 

[Terry Syd]

Stratos are the hot set-up. Whereas a conventional two-stroke may have a limit on intake width of say 70 degrees of bore, stratos can easily exceed 100% of bore. Not only does the cumulative width of the ports exceed a conventional two-stroke, the multiple ports don't need to have a curvature on the port, they are flat. The result is a huge intake opening that opens NOW.

 

[paragonbuilder]

I may have picked at JMS a little bit but otherwise I have been trying to contribute. As far as strato ports go this is a start. . . .

That's pretty cool! I hadn't had a chance to really study one yet. That helps a lot!!


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[merc_man]

Has anyone ever tried porting two saws the same but one stock top end and other cheap aftermarket top end. Just curious if you can get same gains with a cheapy vs stock top end.

 

[Basher]

Both top ends can be ported with the same numbers but they won`t be even power wise, it would take extra work on the aftermarket jug to get it to make the same power as an OEM cylinder does. Usually the transfer tunnels of the AM cylinders are not the same.

 

[paragonbuilder]

Has anyone ever tried porting two saws the same but one stock top end and other cheap aftermarket top end. Just curious if you can get same gains with a cheapy vs stock top end.

@Mattyo is doing this with 346 jugs on 350's. He is getting them ready for the CT gtg.


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[drf256]

Has anyone ever tried porting two saws the same but one stock top end and other cheap aftermarket top end. Just curious if you can get same gains with a cheapy vs stock top end.

There's other bads about the AM as well.

Firstly, many have the plating included over the band and chamber. So you need to cut bands on a lathe with a carbide bit. No using a mandrel and tool steel. So that option gets limited immediately.

Second, the plating is cheap and does strange stuff. Like pop off around the ports while grinding.

And then there is the huge resale issue. I build a saw and sell a saw for fun. I'm lucky if I recoup my costs. This is a fun hobby for me and will never be my day job, nor would I want it to be. But it's nice to not lose money on a saw. After all those hours I'd feel a bit cheated if I could'nt sell the saw for at least my costs. So the option of letting a saw go gets kinda eliminated as well.

 

[drf256]

@Mattyo is doing this with 346 jugs on 350's. He is getting them ready for the CT gtg.


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I was planning on it as well, but may run out of time.

I can't compete with Mattyo on the 350. I doubt anyone can. He's got countless hours on those saws and knows his stuff. Plus he grinds all day for a living. That amount of experience is tough to quantify.

 

[Mastermind]

Aftermarket top ends are an odd thing to say the least....some will never run with a oem jug, but some will surpass.....

I won the 066 build off with a aftermarket jug.

Crap shoot is a term that comes to mind.....

 

[Iron.and.bark]

Welcome, and nothing is off topic.

It seems that Dolly's have super tight cases. Hope the guys correct me if I'm wrong here, but I see larger bore jugs on the same case (6400/7900) running more blowdown because there will be proportionately less case volume to swept volume ratio.

So in your case, we'd really need your timing numbers to opine. I'd venture to say you'd want to keep your transfers physically lower than what you would expect to be optimal in your saw with the standard bore.

That's all I got. Hope someone jumps in to confirm the statement about dolmar transfers I made above.

Thanks it is appreciated. So from what I inferred from what you said re the ratio, not only would I lower the transfer floor but also enlarge the surface area (volume) of them?

 

[drf256]

What's the "transfer floor"?

I'd leave them alone.

 

[mdavlee]

Bottom of the upper will be covered by the piston top normally.

 

[Iron.and.bark]

What's the "transfer floor"?

I'd leave them alone.

On port openings at cylinder wall

Top = roof
Bottom = floor

Always used these terms :/

 

[shorthunter]

Not sure if he is a member here but here is a video from mweba on stratos

 

[drf256]

Thread bump.

On exhaust roofs.....

I'm sure it varies from model to model and by bore vs. stroke ratios, but what's too physically LOW for an exhaust roof
(Numerically high)?

At what point is there no longer enough combustion chamber pressure to create any torque?

Many of you know, many echo builds are going on. They seem to have longer than customary strokes and much more than customary ex roof degree opening. So a lot less duration.

Being they have long strokes, a height of 110 would be more like a placement of 115 with a shorter stroke crank.

So any discussion on what you all feel is the point of diminishing return here?

Pretty sure I'm gonna raise my exhaust roof a bit more, from 106.5 to 105. I need to bleed off more compression. It too high for my liking and the saw isn't broken in yet.

 

[smokey7]

I sure wish I had more to add here . I would bet that a divot would help your comp some and not really effect timing much. Either that or enlarge the combustion chamber. I don't really ever want a saw with over 200#. The one I have that's at 200 is a trick to start unless warm. Please continue this adventure into ports, I am following as best I can.

 

[mdavlee]

I would think over 110 for most stuff is too low of an exhaust for making power. A 346 is the saw I run with the lowest exhaust. They get a ton of rpms at 107-108

 

[drf256]

I'm not looking for super high comp. My lathe is a POS. I need to overhaul my shop to see if I can fit a better one.

In many cases, I overshoot my cuts and wind up needing to take a bit more, resulting in comp that's a bit higher than my liking.

A divot is a good idea, but I'm more worried about heat and saw durability from the dynamic compression than starting.

 

[drf256]

I would think over 110 for most stuff is too low of an exhaust for making power. A 346 is the saw I run with the lowest exhaust. They get a ton of rpms at 107-108

That's what I was looking for.

What comp do you shoot for in that saw?

 

[Moparmyway]

I'm not looking for super high comp. My lathe is a POS. I need to overhaul my shop to see if I can fit a better one.

In many cases, I overshoot my cuts and wind up needing to take a bit more, resulting in comp that's a bit higher than my liking.

A divot is a good idea, but I'm more worried about heat and saw durability from the dynamic compression than starting.

Then raise the exhaust by a degree or two and try it out.
You will have lower compression and be able to tell if you went forwards or backwards