Real talk about saw porting theory (no arguments)

[Keith Gandy]

At 1* ABDC most of the exhaust has been purged and the transfers have completed filling the chamber, other than inertia. So where is the exhaust gasses coming from at this point?


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Restricted exhaust and bad rings blowing by or never seated properly

 

[Keith Gandy]

Stihl 461 is bad about this with that baffle in place in the muffler trying to stuff

 

[paragonbuilder]

Restricted exhaust and bad rings blowing by or never seated properly

Wouldn't that happen when the transfers first crack open though as opposed to ABDC?


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[Keith Gandy]

Wouldn't that happen when the transfers first crack open though as opposed to ABDC?
Yes. Freeporting would be an issue there also. I think?

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[drf256]

Wouldn't that happen when the transfers first crack open though as opposed to ABDC?


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I believe so Dan.

The loop of charge starts at the exhaust side of the uppers.

So the space between the exhaust port and the ex side of the uppers is theoretically still filled with burnt gasses. It starts getting pulled towards the intake by siphon effect of the uppers flowing forward. The cylinder is never 100% purged. There's always some charge dilution.

Flow the uppers too much and too big, combustion can sneak down the transfers at some point. Likely ABDC.

If there's a lot of transfer space, a saw could conceivably ignite charge in the tunnels.

Saw might "pop" or backfire when this occurs.

 

[Keith Gandy]

I believe so Dan.

The loop of charge starts at the exhaust side of the uppers.

So the space between the exhaust port and the ex side of the uppers is theoretically still filled with burnt gasses. It starts getting pulled towards the intake by siphon effect of the uppers flowing forward. The cylinder is never 100% purged. There's always some charge dilution.

Flow the uppers too much and too big, combustion can sneak down the transfers at some point. Likely ABDC.

If there's a lot of transfer space, a saw could conceivably ignite charge in the tunnels.

Saw might "pop" or backfire when this occurs.

Sounds like a good recipe for blown seals

 

[jmssaws]

As we are starting to get into some theory now, I have a question.

I hear it commonly debated that carbon deposits are found in the uppers and tunnels from "backstuffing" of them by exhaust gasses at the point where they open.

Anyone ever think about what's happening at 1* ABDC?

There will be negative pressure being generated in the case then. The uppers will cease to have pressure, but there will still be charge inertia.

I postulate that some exhaust gets sucked in then, and if we can change what's sucked back then, we may find a little extra.

This may be why carbon is found in the transfers.

An exhaust finger in the right place would make that backfill be less diluted.

Ive seen pics of carbon in the transfers but I've not seen it first hand.

1 degree abdc the fresh charge is in the chamber and has already pushed the old out and in a efficient motor there should be very little burnt charge left to get pulled back into the transfers.

I know it happens but I don't see how it can in a healthy saw.

 

[Keith Gandy]

I still see porting like this. Look at what the factory gave u, run the saw and ask what does it need? Cutting squish and bumping compression helps in most all cases . Next maximizing fuel flow and air intake would be what Id look at to come up with a porting plan and as far as #s I sorta think of what I want the attitude of the saw to be. Just my brainfart!!

 

[jmssaws]

The transfers in my mind need to be a one way street and if there not something is wrong

 

[Keith Gandy]

The transfers in my mind need to be a one way street and if there not something is wrong

I agree. Correct blowdown wouldnt allow scavenging into the transfers? Restricted exhaust could though

 

[jmssaws]

A 661 cylinder is a good one to run water in because of the way it's fed,it helps me to do that now and then to see what's happening.

 

[jmssaws]

When the transfers are right there's a harmony/free feel to the saw That's unmistakable and that's what I'm looking for.

 

[Keith Gandy]

A 661 cylinder is a good one to run water in because of the way it's fed,it helps me to do that now and then to see what's happening.

I usually use the water through them trick on all mine

 

[Keith Gandy]

When the transfers are right there's a harmony/free feel to the saw That's unmistakable and that's what I'm looking for.

Gives that free rpm in cut

 

[jmssaws]

The only way I can see it happening is for it to be blown past the rings.

 

[Keith Gandy]

The only way I can see it happening is for it to be blown past the rings.

How does Stihl achieve their delayed stratification scavenging technique on the 461? I think the big baffle they have in the exhaust port stuffs the transfers partially. Ive seen the transfers all sooted up on the 461 I had

 

[jmssaws]

How does Stihl achieve their delayed stratification scavenging technique on the 461? I think the big baffle they have in the exhaust port stuffs the transfers partially. Ive seen the transfers all sooted up on the 461 I had

I don't know what delayed stratification scavenging is but I've been inside a few 461's and not noticed any carbon in the transfers.

I started porting a 036 this morning and after saying I never seen carbon in the transfers this saw is jammed full.lol
But it had a smear on the exhaust side of the piston that had the rings held in and ran that way for years it looks like.

Muffler restriction surely would play a role in it but to carbon them like the exhaust port has to be from blow by.
When the transfers are open the fresh is mixed with the old so if it was back stuffing them I don't know if there would be enough burnt charge to leave carbon or the next fresh charge could also wash it away.
Things are going so fast that the transfers are flowing a stream that's barely interrupted so in my mind I can't see them being back stuffed.
That's my thinking anyway

 

[Keith Gandy]

Im not sure either Jason but thats Stihls fancy term for the 461 design!! Lol! Mine wasnt bad but I did notice soot in them until that big baffle was removed from the exhaust. I just assumed and chalked it up as restrictive exhaust. Maybe unseated rings caused it

 

[jmssaws]

I'm sure exhaust restriction can cause it but I don't think it should be doing it.
Seems counter productive to me

 

[EvilRoySlade]

Does anyone else feel that strato saws need to be approached differently? Seems to me from staring at them that adding fuel into the strato ports is 2-fold. One being increase life of saw by bringing a bit more oil up top, two being if you're upping performance you can take advantage of using it like a larger intake.