Stihl 361 Junkyard Zombie Mutant

[Ketchup]

205 psi at 5600’ is in the realm of “too much compression” in my opinion. That would be around 240 at sea level. One of the first saws I did machine work on is a Poulan…that thing blows like 225 at my elevation of 800ish ‘. I haven’t gotten it to run right….some day I will get back to it and fix it.

I think I agree. I don’t really trust the specific number, but it is a bear to start. Usually I can only get it to turn over once on the first pull. It takes a few tries to get it to turn 3 or 4 revolutions. Luckily it pops pretty fast.

Caleb, is there a reason you went with a 54mm big bore cylinder instead of a 52mm OEM one?

I think you have a metric *s-wordton of case compression, so I would want your uppers more numerically higher than usual so that you don’t over scavenge. Pretty much the same thinking as doing a 446 hybrid. Good numbers for a 446 vs a 460 are quite different. I usually want more BD on the 446.

It would be quite interesting to know where your ignition timing is. To reduce compression, you could add another angle in the band up towards the chamber and then hand blend it. I still believe that dishing a piston after the squish band interface shouldn’t affect scavenging much. Even Husky decided to do it on the newer 562.

You have increased heat from all that compression and fin area reduction. The 036 flywheel may be adding to the mismatch. I wonder if the ignition timing is as well.

As you know, stacking gaskets is going to change many things at once. It’s too bad it’s not a 52mm, where you would have readily available slugs to play with. My gut tells me you need more intake and exhaust duration, and the uppers need to come up a bit. I was thinking 99/122/78.

Saw sounds pretty damn good in that last video. Thanks for taking the time to do this.

I did the 54mm for three reasons. Cost, risk and displacement. I know the problems of AM jugs, but in this case I was curious if I could overcome the performance issues. That isn’t clear yet.

I’m pretty surprised by the compression. I didn’t take that much out of the band (0.020”?) and the squish is a healthy 0.022”. These AM jugs have a relatively large chamber. You’re right about the heat. Lucky I didn’t take more fins off.

I think I like the current ignition timing. I haven’t put a light on it but I tried a whole range. The current setting was the smoothest. I want to time it eventually but my instincts say thats not the main issue.

I started conservative with the port numbers. I like 99/122/78. Stacking gaskets should land somewhere close except the intake, so it’s an easy experiment. I could see this saw landing even a little higher eventually. On the 446 hybrids, what is the typical blowdown? 20 degrees?


Raising the exhaust should reduce the compression but I’m not sure if it will be enough. I’ve been experimenting with reshaping some chambers. That would be the next step if I can’t get the compression down. I’m a little biased against dished pistons, but that would be a less permanent option.

Great thoughts fellas. Thanks for all you’re insights.

 

[Maintenance Chief]

When I put an AM 066 piston in a clone 288 the piston sits about .065 too tall , most folks machine the piston or chamber.
I didn't have those immediate options (suppose, I could have used my sanding table?) ,instead I cut an .080 gasket which in turn raises all the port things. The saw responded quite well and still has substantial compression.

 

[Maintenance Chief]

 

[Maintenance Chief]

@jacob j. told me about this piston swap.

 

[bogieboy]

Glad to s

Glad i am not the only one who has lauched a scabbard off a bar at mach jeeebus

 

[thedude74]

Seems to be oiling just fine. Teeth are sharpish. Pretty sure it’s the numbers.

View attachment 421386
View attachment 421373View attachment 421372View attachment 421371

Right now it feels similar to my ported 7910. Strong but not fast. Pulls over hard.

Ported 7910 territory sounds impressive to me.
Looks like you're making good progress.

 

[drf256]

I think I agree. I don’t really trust the specific number, but it is a bear to start. Usually I can only get it to turn over once on the first pull. It takes a few tries to get it to turn 3 or 4 revolutions. Luckily it pops pretty fast.



I did the 54mm for three reasons. Cost, risk and displacement. I know the problems of AM jugs, but in this case I was curious if I could overcome the performance issues. That isn’t clear yet.

I’m pretty surprised by the compression. I didn’t take that much out of the band (0.020”?) and the squish is a healthy 0.022”. These AM jugs have a relatively large chamber. You’re right about the heat. Lucky I didn’t take more fins off.

I think I like the current ignition timing. I haven’t put a light on it but I tried a whole range. The current setting was the smoothest. I want to time it eventually but my instincts say thats not the main issue.

I started conservative with the port numbers. I like 99/122/78. Stacking gaskets should land somewhere close except the intake, so it’s an easy experiment. I could see this saw landing even a little higher eventually. On the 446 hybrids, what is the typical blowdown? 20 degrees?


Raising the exhaust should reduce the compression but I’m not sure if it will be enough. I’ve been experimenting with reshaping some chambers. That would be the next step if I can’t get the compression down. I’m a little biased against dished pistons, but that would be a less permanent option.

Great thoughts fellas. Thanks for all you’re insights.

Personally I’ve found that 446 require more BD, like 20-22. I believe it’s from all the case pressure. It changes the torque curve as well.

That’s using a 460 piston. When I’ve used a 272 slug, I found that they need less because there is less case pressure. They tend to do better with numbers like a good 460.

Great job on the build. I’m sure you’re correct on the ignition timing, but I’d still want to know where I was. Would be interesting to take a temp shot with an IR thermometer of the cylinder vs muffler while it’s running. If the timing is too retarded, you’ll make less power but the muffler will get hot as all hell.

 

[David Young]

Love this thread.
one thing that is supposed to help cooling is more (wider) squish band. If you cut more out it would likely make it wider. but running .030 squish will still reap the benefits of increased squish velocity.

is there anyway to make the air cooling more effective. more outlet area. more inlet area. many saws use a heat shield on the exhaust side that is also aluminum so added heat sink area but also helped increase flow of the cooling air by being more directed.

 

[Ketchup]

Love this thread.
one thing that is supposed to help cooling is more (wider) squish band. If you cut more out it would likely make it wider. but running .030 squish will still reap the benefits of increased squish velocity.

is there anyway to make the air cooling more effective. more outlet area. more inlet area. many saws use a heat shield on the exhaust side that is also aluminum so added heat sink area but also helped increase flow of the cooling air by being more directed.

I’m thinking a combo of higher exhaust and looser squish.

A divider plate should happen. Something to keep air blowing from the FW to the PTO.

Hopefully I can get this thing disassembled tonight.

 

[Ketchup]

Found some things I don’t like.

I think the FT mesh filter is headed for the trash. It lets a lot of fines through.
And this wash pattern is pretty disappointing. The uppers opened at dead on the same time, but I think there was a hump in the roof of the PTO side.


But they’re dead flat now. 77/100/121
I also relieved the chamber substantially.


And I found these in a drawer, so why not?


I’m headed to the yard shortly. Hopefully I’ll have time for some cuts.

 

[Ketchup]

Welp.

 

[Mastermind]

Welp.View attachment 422219

I think we know why it was getting hot.

 

[jacob j.]

@jacob j. told me about this piston swap.

You gotta watch that guy - he's a loose cannon.

 

[bogieboy]

That sucks @Ketchup... heat related, or an air leak somewhere?

 

[Loony661]

That sucks @Ketchup... heat related, or an air leak somewhere?

Looks to me like it caught a ring..?

 

[bogieboy]

Looks to me like it caught a ring..?

Ahh i see it now....phone screens are a fickle thing.... sometimes you can see the small details, sometimes you cant... i didnt look close enough.

 

[Woodslasher]

You gotta watch that guy - he's a loose cannon.

Yep, he's constantly firing parts and good info all over the place.

 

[Ketchup]

Yeah. Big sad. Caught the bottom ring. Destroyed the piston, busted the bottom of the port. Jug is probably destroyed. I’ll post up more carnage tomorrow.

 

[drf256]

Shìt, sorry pal.

If you need a 52mm jug, let me know.

 

[Ketchup]

Not much more to see really.


If you look close, the ring deformed the roof of the port as well.

This is a first for me. I’ve seen hung rings, but never had it happen to a saw I messed with. I’m not sure why it happened. The floor of the exhaust was stock. I did a very light chamfer, but otherwise it was factory. This jug had been scored before I got it. Some of the scoring was in that area and there was a minor spot just below the lip of the port that was too deep to sand away. I disassembled, raised the ports, radiused the chamber and reassembled in about 4 hours yesterday. It’s possible I got funk behind the ring, though I’m really careful about that. Chock it up to haste and junk parts. I’m definitely bummed about it, but not surprised. I wish it had made it a few weeks though. I was hoping to bring it to @davidwyby’s G2G.