Chainsaw Porting Theory

[smokey7]

I was under the impression that the divot depending on size could effect the dynamic comp just as the cranking comp.
Also I understand the over shooting your nunbers, we got some wore out machines at work that you better sneak up on or really know the machine. Drf you sure seem to have a real passion and drive for these motors. I really enjoy some of the boundary pushing stuff you do. Alot of it I doubt if I'd ever do some of it. But enjoy it all the same.

 

[Mastermind]

Al, take a little off of the middle of the piston dome.....

 

[David Young]

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.

I have suggested it before but a .015-.020 gasket would lower compression and raise the E and T while lowering the I. Nothing wrong with squish at 30-40 thou to test.

 

[jmssaws]

I have been building saws with the squish around 030 and like the room, the saws run just as good.

 

[Mastermind]

They aren't as detonation resistant, but performance is close to the same.

 

[jmssaws]

They aren't as detonation resistant, but performance is close to the same.

I have only seen one saw detonating and it was from bad gas

 

[drf256]

I was under the impression that the divot depending on size could effect the dynamic comp just as the cranking comp.
Also I understand the over shooting your nunbers, we got some wore out machines at work that you better sneak up on or really know the machine. Drf you sure seem to have a real passion and drive for these motors. I really enjoy some of the boundary pushing stuff you do. Alot of it I doubt if I'd ever do some of it. But enjoy it all the same.

I'm thoroughly enjoying my new hobby. I tend to get carried away.

Al, take a little off of the middle of the piston dome.....

It's pretty thin Randy. Nothing like an 026 dome.

I have suggested it before but a .015-.020 gasket would lower compression and raise the E and T while lowering the I. Nothing wrong with squish at 30-40 thou to test.

I have been building saws with the squish around 030 and like the room, the saws run just as good.

They aren't as detonation resistant, but performance is close to the same.

The detonation resistance and lack of unburned volume loss makes a tight squish desirable for me. It's not for compression in my mind.

With my lack of precision with mandrel band cutting, .025 is ok with me.

I don't think increasing squish will give me the amount of comp reduction I may need.

IDK, the last two saws I've done, a 50 and 60cc, are running damn good in the 240 range. I just don't know how long they will run well.

 

[jmssaws]

My 066 is 240lb with a 027 squish, it's cut timber and had the hell beat out of it for a year and is fine,that said a 190lb saw is faster but won't lug with it.

 

[Mastermind]

I use compression to gain torque on these smaller saws. I don't see any failures from it. How does your Echo run Al?

 

[jmssaws]

I talked to awol about that and he said his 562s last a year in the timber regardless of the compression. I believe there 260lb

 

[Mastermind]

When I saw that he was doing a cut squish, and a pop up, I knew they would be wicked high.

 

[jmssaws]

I built a 660 that way once just to be different, it runs ok but not better in any way.

 

[drf256]

I use compression to gain torque on these smaller saws. I don't see any failures from it. How does your Echo run Al?

Surprisingly well Randy. I still got some monkeying (no offense) to do with the jug and carb till I'm happy.

 

[awol]

I talked to awol about that and he said his 562s last a year in the timber regardless of the compression. I believe there 260lb

That's just a wild guess because I don't have an adapter for my comp gauge. They are also running with a VERY low exhaust, around 110 to try and widen the powerband and increase lower end output. When you cut the squish band, and add a popup on the piston, and lower jug without raising the exhaust, compression builds up quick!

 

[jmssaws]

I'm still amazed how they run with the exhaust that low and the huge intake duration.

 

[drf256]

What percentage of trapped volume/actual compression do you guys guesstimate is occurring at, let's say 13,000 rpm?

I bet it's less than half static. I have zero way to prove my theory though. I bet it's different in every saw.

I also wonder whether or not lower comp with more timing advance makes more power than higher comp with less advance. Very high static compression and a lot of advance don't seem to mix well.

Jason, did you try advancing the timing on your 195 psi 066 more than your 240# one? If so, what were your conclusions?

 

[smokey7]

I bet it does. I would worry how long it could survive set up like that. I'd bet with near 100# more then stock would stress things a bit.

 

[drf256]

I'm still amazed how they run with the exhaust that low and the huge intake duration.

My bet is very little blowdown.

AWOL, what's the answer?

 

[awol]

I think the secret of dynamic compression is all in the arrangement and aiming of the upper transfer ports. Proper placement, timing, pressure, and direction should theoretically raise the compression at higher engine speeds simply by the amount of scavenging taking place.

 

[jmssaws]

What percentage of trapped volume/actual compression do you guys guesstimate is occurring at, let's say 13,000 rpm?

I bet it's less than half static. I have zero way to prove my theory though. I bet it's different in every saw.

I also wonder whether or not lower comp with more timing advance makes more power than higher comp with less advance. Very high static compression and a lot of advance don't seem to mix well.

Jason, did you try advancing the timing on your 195 psi 066 more than your 240# one? If so, what were your conclusions?

The 240 saw has 030 off the key and the other is 020.
I don't think detonation is very feasible in a saw, to a point the more compression and ignition advance you can run will yield the most power, depending on the saw of course.