192t porting question

[Ketchup]

Hey guys,
A fellow gave me a nice low hour 192t with a bad clam seal. I took it apart, opened up the muffler, advanced the timing and then decided to port it because there was obvious space to open up the intake and exhaust. I did all that.
My question is, has anyone modified the finger port that comes out of the exhaust port? It's a funny shallow thing. I really don't understand why it's there in the first place. Has anyone filled it or opened it up with positive results?
I know, it's a 192. But it was a freebie, so why not play with it? I also think I can sell it easier if it's got all of the power I can find.
Any porting thoughts on these would be great info! Thanks!

 

[cus_deluxe]

Pics of “finger” port? Ive not seen the inside of one so i dont know what youre saying.

 

[jackjcc]

It’s an open port design not really a finger port.


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[Ketchup]

Not in my shop right now, so I did a screen shot of one off the internet. It's a bit blurry but there is a shallow finger port with a streak of exhaust in the middle.

What this pic doesn't show well is that the exhaust tunnel is also angled almost 90 degrees to the port opening. It enters the muffler on the right side of this picture. Not the top of the photo.

Seems to me that the finger port would effect timing badly and reduce transfer pressure. I'm considering filling it with epoxy.

 

[LegDeLimber]

I can't say if Stihl does this with a small saw, but a number of trimmer and small blower engines have a narrow slot cut into the cylinder wall, that runs from the exhaust port upwards.
This was intended to work as a compression bleed to make pulling the engine over at cranking speed easier.
At running speed the theory is that the compression loss is not fast enough to cause serous power loss.

That void in the web pic looks kinda wide to be a bleed slot.
But every designer & manufacturer will have their own opinion on how to do things.

The one drawback to the slotted relief trick was it causes more unburned gases to blow out and therein lies an emission problem with it.

 

[Ketchup]

That's exactly what it looks like. It's very shallow, seemingly to limit flow. But it has to diminish performance.

This saw blew the clam seal. I wonder if that finger port is partially there to keep that seal intact? It seems quite weak. The finger port was pretty filled up with carbon when I pulled the piston.

If not, filling it could be a way to get more power. Possibly at the expense of easy pull starting.

Filling it is a lot of work for a saw that's $300 new...

 

[LegDeLimber]

The slot only affects the pressure above the piston rings.
Whatever pressure passes through to slot just goes out the exhaust.
It doesn't affect the crankcase pressure.

If the clam seal popped out, I'd suspect loose screws in the bottom pan or warped sealing surfaces.

I believe Mattyo (i may have the spelling wrong) has some youtube videos and talks about checking the bottom ends
for flatness and leaks or sealing issues.

 

[jackjcc]

It sure does have one and it’s clearly machined. I don’t think the picture shows it that well though. I don’t think the meteor replacement had that slot.

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[Ketchup]

I guess I was thinking ignition pressure would effect crankcase pressure and speed. If you bleed off the ignition pressure early, then the piston has less downward force and less speed to transfer.

Raising the exhaust on a port typically gives more RPM and less torque, though. So this port may actually be helping the saw maintain speed.

It's possible that if I fill the finger port I will have to raise the exhaust to compensate.

I guess I was hoping someone had already fiddled here and could save me wasting time.

Of course, I am on a forum talking about porting a 192, so time can't be that important.

I did suspect loose screws or warped surfaces, but it checked out okay.