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Carbon after a squish tightening

Simondo

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Hello Gentlemen.
The number to aim for squish seems to be 20 thou from what i see. I went through a cylinder and with no base gasket came out at 19.5 on test. Its such a tinny clearance I was wondering about the long term situation with carbon build on top of piston and squish band reducing that. Might it get to the point of interference or do you get in there once in a while and clean ?
 

smokey7

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Depending on bore size between. 016-.025 . No problems with carbon in squish.
 

Simondo

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If it's tuned right you won't get enough buildup to matter. If you have that much buildup your going to have other issues...
What saw is this your working on?


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This is more about research than actually solving anything on a saw I'm doing. Having taken the cylinders off a wide range of saws of differing ages and seen the general carbon deposit amounts, l have always been interested to know if its a issue for gasket delete before i go down that rout on one of my own saws.
 
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drf256

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I'm not a pro, but I've personally been moving towards a bit looser squish. Like .023-5.

In a ported saw with more compression and timing advance, there's the potential for the piston to expand more than the jug with prolonged cuts.

Using Motul 800, I get no carbon. I mean none. I use 32:1.

How are you measuring squish? Solder can be deceiving and overestimate squish. More so the more the base difference of the solder is vs. intended squish goal.
 

smokey7

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Play dough or plastigauge, solder if it's small enough in 4 spots
 

Simondo

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I'm not a pro, but I've personally been moving towards a bit looser squish. Like .023-5.

In a ported saw with more compression and timing advance, there's the potential for the piston to expand more than the jug with prolonged cuts.

Using Motul 800, I get no carbon. I mean none. I use 32:1.

How are you measuring squish? Solder can be deceiving and overestimate squish. More so the more the base difference of the solder is vs. intended squish goal.
You have hit the proverbial nail on the head …Thats what nagged in the back of the old brain box after measuring the squish on a cold motor. Things move with heat and cold so i figured the 20 thou number must be a combo of ideal gain for compression and a allowance for the slight build of carbon you get and heat . The solder method is what i used ( 4 quarters) on a cold engine and i assume that would have been a different measurement on a hot engine. Mostly i go for 45:1 -- 50:1 mix, semi or fully synth oil.
Im no full on saw TECH like a lot of the very knowledgeable folks here but i have a metalworking background so it got me thinking I should see how others see it.
 

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20 thousandths has been considered the number to shoot for, it allows room for expansion, but also pushes more of the a/f mix into the combustion chamber for max power. It's as much about efficiency as it is about compression. Compression can be achieved with a large squish but it will be inefficient. Also a larger squish is more prone to detonation.


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drf256

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20 thousandths has been considered the number to shoot for, it allows room for expansion, but also pushes more of the a/f mix into the combustion chamber for max power. It's as much about efficiency as it is about compression. Compression can be achieved with a large squish but it will be inefficient. Also a larger squish is more prone to detonation.


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All arguable points.

I think a larger squish is LESS prone to detonation. Maybe one of the PRO's here will chime in. Large squish created better swirl. There is a limit to what is too much though.

Also, the charge in the squish area doesn't burn during the important part of the combustion process. That volume is lost as power producing.

But its a tradeoff, like everything else.

I do think that the squish clearance is partially there because it gets tighter and tighter as the motor heats up.
 

DrewUth

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Agreed that as compression goes up, so does the possibility of detonation.
 

bikemike

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If it's tuned right you won't get enough buildup to matter. If you have that much buildup your going to have other issues...
What saw is this your working on?
My guess is a oh26 they are slappers without gasket

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Simondo

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Just as a overall point of reference for you boys in the US….all our pump gas in the UK is 95 ron as a "standard unleaded". We do have what is known here as super unleaded that is advertised ( by Shell…other brands are available !!:D) as giving" higher performance "(Only if your engine is capable or its pointless extra money otherwise ! ) . I think that is 97 or 98..one or the other. This might move the goal posts on detonation.
 

bikemike

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I think carbon don't help with detonation. If it does hold heat and can glow it can ignite like a diesel
 

Mag Craft

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You know I have seen several posts now that refers to .020" being the ideal squish. Actually the larger the piston and stroke the more the expansion and the more squish that is needed. The smaller the piston and stroke the less expansion and you can you a tighter squish. I have gone down to .017 and .018" on small saws with no issues at all. I would not do that to a bigger saw. You could probably go even tighter on a small saw but I was not comfortable with that besides I was already pushing 190 PSI and did not want any more. So just saying .020" squish might be ideal for some saws but not all.
 

Simondo

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You know I have seen several posts now that refers to .020" being the ideal squish. Actually the larger the piston and stroke the more the expansion and the more squish that is needed. The smaller the piston and stroke the less expansion and you can you a tighter squish. I have gone down to .017 and .018" on small saws with no issues at all. I would not do that to a bigger saw. You could probably go even tighter on a small saw but I was not comfortable with that besides I was already pushing 190 PSI and did not want any more. So just saying .020" squish might be ideal for some saws but not all.
Do you have a rule of thumb as small saw is a subjective thing. Lets say the 40-50cc class and the 50-60cc class ..you may call those small but there bang in the mid range in some folks view.
 

Mag Craft

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I would consider small 50cc and below. I do not have a rule of thumb but I do not think I would set a 85cc or 90cc saw at .020" squish.
The saw I set at .018" squish was a 54cc saw. It is only something to take into consideration when setting the squish on your saw.
 
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Mag Craft

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Do you have a rule of thumb as small saw is a subjective thing. Lets say the 40-50cc class and the 50-60cc class ..you may call those small but there bang in the mid range in some folks view.

I would consider 60cc to 72cc saws in the mid range and .020 to .022" squish would work out great on those. Most of my saws are in that range and are my favorite size.
Those saws have great cutting power without to much weight.
 

Simondo

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I would consider 60cc to 72cc saws in the mid range and .020 to .022" squish would work out great on those. Most of my saws are in that range and are my favorite size.
Those saws have great cutting power without to much weight.
The most popular size general use saw in the UK is the 50cc class and is hottly contested by all the manufacturers . You can see why i said size is subjective can't you :D… 18..22 thou gives a useful reference though.
 

smokey7

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I had deto on premium with 21cc domes on a pwc motor. Same head same bore size tighter squish from .075 to now .037 no more deto and everything else stayed the same. It wasn't just bad fuel it happened to 2 topends then I talk to a old school builder racer guy. Different applications but squish makes a big difference just as dome shape changes alot beyond the compression change.
 
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