Squish question

[Chainsaw Jim]

Who's going to build a saw with so much compression that it fires without a spark? Would the squishband need to be flat all the way across with just a tiny dimple?

 

[Chainsaw Jim]

Piston velocity is a matter of RPM and the lubrication needed is the same regardless of compression pressure. The oil ratio needs to be such that it provides adequate lubrication for the main bearings and doesn't cook off on the cylinder wall at maximum load. How the flame generates in the chamber is more about design than pressure. The oil ratio differences you are talking about are negligible.

Apparently I can't ask the question intelligently enough to be understood for an answer to be possible so I'm over it.

 

[Wilhelm]

I run a tight squish on big saws,017 is a number I use a lot, the 7900 I did yesterday is 015.

That sounds a bit tight.
How does she run?
Did You just delete the gasket or have You done some machining?
Is she a true "man saw" or DeCo enabled - how does she crank either way?

I just dug out the solder pieces I checked squish with on my PS-6400:
- w/o the OEM gasket , 3-point check ~ .014 / .015 / .0125
- with the OEM gasket , 2-point check through the spark plug hole ~ .0245 / .025

I felt the gasket delete was too tight and left the saw stock with the OEM steel gasket.

I noodled and bucked beech yesterday with my PS-6400, 20" bar almost buried with only the chains teeth looking out the other side of the log - felt as strong as ever!

 

[drf256]

This is a good discussion. Thanks to all that responded.

It's an efficiency thing...
You want the squish band to push as much of the charge into the combustion chamber as possible, most of what ever is leftover in the band doesn't get a chance to ignite, so is more or less wasted. Most of the velocity issues with too tight a squish are due to wide bands that aren't possible in relatively teeny chainsaw motors.
I read a lot of different numbers for minimum effective squish, somewhere around .040-.060" is typical, but that is likely dependent on bore size and/or band width and most published info seems to deal with larger bores. So who knows really?

If you go from .015" to .030" it's doubling the loss, and the squish is such a large percentage of the bore size in these little motors....
I'd enlarge the combustion chamber before I'd deepen squish if I wanted the most out of it.

As far as detonation goes, supposedly with these small bores it just isn't an issue at any practical compression rate, even with 87 octane.

It's easier to use an example I feel.

What I do an 026/260 results in a huge squish band. Over 50% of the chamber, easily. This large band and high compression is a side effect of the desired goal, a lower exhaust roof. I'm not intentionally trying to have the comp this high, it's just how it goes.

I did open the chamber significantly on this saw. I also tapered the interface of the band to the chamber. If I hadn't, I'd be 25psi higher, maybe.

How does 50:1 at 170psi compare to 32:1 at 225psi? Is it the same? The higher compression is needed to burn higher oil content more efficiently, so wouldn't lower compression and less oil have the same combustion strength?

You missed my whole point. I'm not talking about oil ratio as everyone is used to discussing on all the other threads, I'm talking about the function of higher compression versus lower compression and what affect the ratio of oil has in that.
It seems like you would risk burning the piston up with 50:1 at over 220 compression.

I get your point. Compression pre-excites combustion, the spark just adds that last drop of energy for the reaction to occur. People forget that the pressure of gas expansion, and not an "explosion" is what drives the piston down.

So with enough compression, the saw will start to diesel.

I think I would keep a tighter squish and open the combustion chamber some if I needed to reduce compression.


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I did try that.

 

[drf256]

Why does everyone think that the factory leaves squish at around .040? Carbon buildup, heat?

I know that exceptions exist to this. Some saws, like the Stihl 066, came with sub .020 from the factory.

Do you think it's just insurance, or that they know something we don't?

I may tighten the squish up on this saw, I'm having a problem dealing with the fact that it's .035. It runs really well as is though.

 

[Simondo]

For those that worry about an increase in compression pressure being hard on the bottom end need to realize the only time the extra pressure is an issue is startup. Once the engine starts the load is negligible. The starter takes more beating than the rest of the engine.

The components are much stronger on a compression ignition engine compared to a spark ignition engine I believe. This would presumably take into account the hike in compression stresses and torque created, suggesting to me there is a need for more strength in a higher comp engine . You don't think there is some transfer of principle over to a increase in compression and power obtained from a engine designed around lower stress ...compression ignition or not ?

 

[Simondo]

Why does everyone think that the factory leaves squish at around .040? Carbon buildup, heat?

I know that exceptions exist to this. Some saws, like the Stihl 066, came with sub .020 from the factory.

Do you think it's just insurance, or that they know something we don't?

I may tighten the squish up on this saw, I'm having a problem dealing with the fact that it's .035. It runs really well as is though.

Safe working margin in work and over time would surly have to be taken into account hence the clearance they leave.. V.. the performance they achieve. They will have a lot more data from testing than Me !!

 

[Mastermind]

I remember a few years back I accidentally cut more out of the squish band than I meant too. So to keep compression down to a tolerable amount, I left the squish clearance wider than "normal". It still ran fine, and even seemed to idle smoother. I posted about it in AS, and before long Dennis Cahoon called me up. He and I had a good long talk on squish clearances...

For maximum performance, and detonation resistance he said to run it as tight as possible without mechanical interference. Now, this advice is for a saw that is not gonna be a dedicated work saw.

Too tight on squish, or too much compression makes a lot of heat, and we all know that ain't good for a real live working saw. Personally, I set squish at .018 - .022 depending on the bore size, and the end use. For a milling saw, I'd go .025 or so. No one size fits all numbers here.....

In Al's 026, I'd just leave it as is and run it. .035 or so is fine. I'd use 91 octane with it that wide though.

 

[mdavlee]

Why does everyone think that the factory leaves squish at around .040? Carbon buildup, heat?

I know that exceptions exist to this. Some saws, like the Stihl 066, came with sub .020 from the factory.

Do you think it's just insurance, or that they know something we don't?

I may tighten the squish up on this saw, I'm having a problem dealing with the fact that it's .035. It runs really well as is though.

Oem 066 gasket kits comes with a low compression gasket for other markets with real low octane fuel. Tighter the better for me. I've run some big saws even on a mill with .017".

How much area is the squish band compared to the chamber? There's an optimal ratio for that. You need to build a head for one of those to try lower compression with the low exhaust.

 

[Mastermind]

I think we could cut the edge of the combustion chamber to lower compression.

 

[paragonbuilder]

Oem 066 gasket kits comes with a low compression gasket for other markets with real low octane fuel. Tighter the better for me. I've run some big saws even on a mill with .017".

How much area is the squish band compared to the chamber? There's an optimal ratio for that. You need to build a head for one of those to try lower compression with the low exhaust.

With as big a squish band as Al has here, could he make the combustion chamber how he wants like you would on a 2 piece head? Or is it just tough to do being that far in there? Also how much meat can be ground out and survive?


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[paragonbuilder]

Seems like the 026,260 would be a good candidate for a custom made jug like Randy was doing with the 066/660. What ever happened with that anyways?


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[mdavlee]

With as big a squish band as Al has here, could he make the combustion chamber how he wants like you would on a 2 piece head? Or is it just tough to do being that far in there? Also how much meat can be ground out and survive?


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Getting into the plug hole would be the problem. Building a head is lroba you as quick as setting up and opening the chamber.

 

[paragonbuilder]

Getting into the plug hole would be the problem. Building a head is lroba you as quick as setting up and opening the chamber.

Ah, that makes sense. Thanks Mike.


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[Mastermind]

Seems like the 026,260 would be a good candidate for a custom made jug like Randy was doing with the 066/660. What ever happened with that anyways?


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Going into production.

 

[paragonbuilder]

Going into production.

Nice! Will they be sold exclusively at hlsupply?


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[Marshy]

...
It seems like you would risk burning the piston up with 50:1 at over 220 compression.

What are you basing this on? Oil is only used in 2 strokes gas because they don't have any other way to lubricate the moving parts. So why would less oil have any effect on burning up the piston?
Less oil = less fuel octane = higher potential for pre-ignition?
Please define "burning the piston up".

 

[Deets066]

I believe he is sayin for high compression, ported saws that it would be wise to run more oil than 50:1. Which I agree with but let's not turn this into an oil thread too.

Optimal squish for me would be between .016 and .022

 

[mdavlee]

This one was .020-.021" cut by hand.

This one was .020" all the way around. Base and head machined so it should be.

 

[Deets066]

This one was .020-.021" cut by hand.

This one was .020" all the way around. Base and head machined so it should be.

That's cheatin