Author Topic: Squish  (Read 2477 times)

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Offline joe_indi

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Squish
« on: August 11, 2012, 06:57:51 am »
I have a cylinder that has too much of squish area, that is, vertically.
The resulting lower compression is preventing a clean burn of the 2 stroke oil.
I am think of removing some metal from the cylinder base and thus get the piston to go higher for more compression.
The side effects are advancing of the exhaust port and retarding of the inlet port.
Is this going to have a negative effect?
I think I need to grind off about  1mm from the cylinder base.

Joe

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Offline Cut4fun .

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Re: Squish
« Reply #1 on: August 11, 2012, 07:14:03 am »
Joe are you in that area where stihl used 1mm base gaskets instead of the .5mm gasket like we use here? From my readings the 1mm gasket was used in some places to lower compression in some country's.

What I'm getting at it may be as easy  as switching base gaskets out or just removing gasket altogether and then using threebond 1194.

The poulan 475 I just put together was in the low 40's squish band range with gasket.
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Offline Al Smith

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Re: Squish
« Reply #2 on: August 11, 2012, 08:05:12 am »
You won't change the port timing that much by lowering the cylinder .The degrees will remain the same as far as "blow down " between the opening of the exhaust and opening of the transfers .The intake will advance slightly and the exhaust will be retarded slightly .Probabley around three degrees which if anything will increase the torque but might slightly decrease the top end speed .

With the increased torque most likely it won't even be noticed .

FWIW most older Stihls set the exhaust port opening at around 98-100 degrees .If you wanted to you can raise it up slightly to maybe 96 degrees which will give you more blow down and slightly more RPM's .If you only raise the compression slightly I personally would not change the actual port timing numbers but might widen the ports slightly .

More info the older models used maybe 45-46 thou clearance or squish and the gasket is a flat 20 thou thick .If you use sealant that might account for 2 thou thereby reducing the clearance by around 18 thou more or less .

There's a certain formula that takes into account the port time open ratios depending on the bore size and RPM's .MacDizzy ,Bell ,and Gorden Jennings might be some help as I can't remember them exactly myself .

Offline joe_indi

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Re: Squish
« Reply #3 on: August 11, 2012, 08:48:17 am »
Joe are you in that area where stihl used 1mm base gaskets instead of the .5mm gasket like we use here?
Nope. We get our saws with the .5mm gaskets too.
The current issue is with a saw that is currently under trial ;).

You won't change the port timing that much by lowering the cylinder .The degrees will remain the same as far as "blow down " between the opening of the exhaust and opening of the transfers .The intake will advance slightly and the exhaust will be retarded slightly .Probabley around three degrees which if anything will increase the torque but might slightly decrease the top end speed .

I had just completed the job and like Al said, the torque seems to have increased, and the top end has reduced.
Not a bad thing because the crank big end rollers will last longer.
The best part is that the fuel is burning cleanly now.

Joe

Offline Al Smith

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Re: Squish
« Reply #4 on: August 11, 2012, 09:42:35 am »
Early on I basically did about the same and it did cut faster .Although instead of sealant I cut the cylinder bases on a lathe .It was only later with a couple of models I later went in and did some tweeking to the ports to jack up the  speed a tad bit .

You know this method really isn't a bad way to jack some power with very little effort .This and a little muffler work would provide very satisfactory to 90 percent of folks who want just a little more grunt but don't feel like ponying up a couple hundred to get someone to soup it up .Once again proving you don't have to be a bloody genious to pep up a saw engine .

What the heck if it didn't work out just put the gasket back in and you're right back to where you started .Full circle so to speak . ;)

Offline EHP

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Re: Squish
« Reply #5 on: August 11, 2012, 07:55:46 pm »
Joe, lowering the cylinder will retard the ext. port and transfers not advance them, and you will advance the intake port timing , First thing you need to know is how much squish does your motor have to start with then you can figure out what needs to be done

Offline joe_indi

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Re: Squish
« Reply #6 on: August 11, 2012, 11:33:14 pm »
Okay Al, EPH,
I did get my ports crossed.
The exhaust port will be retarded because the piston top opens it a bit later now.
The Inlet port gets advanced because it is now a bit lowered and the lower end of the piston opens it a bit earlier now.

Now, coming to the transfer ports, they would have a retarded timing because the piston top opens them, right?
EPH, I have no clue to how much squish this saw should have. I used the squish of the MS460 for a comparison. All this was done in my regular shop where I had a vernier handy to take measurements.
So I used a steel scale instead and found the squish is nearly double that of a MS460.
I often use the 0.8mm gaskets instead of the 0.3mm standard ones to reduce piston damage on old 460s, but at the cost of compression, which often leads to unburned 2 stroke oil.
For the sheer lack of tools at that moment I used up  a couple of depth gauge files and emery paper to lower that cylinder.Well the result was not something to put up in a beauty contest, so I used some Three bond as the leveler and allowed it to dry for about 20 minutes before installing the cylinder.
It was a great relief that there were no leaks.
The first hint of things turning for the better was that the oily carbon blackened spark plug came out looking dry and dark gray after about 5 minutes of test cuts. And, unlike earlier, not a hint of exhaust smoke at anytime.
I tried it with different mixes of two stroke oil, 2% of Stihl oil, 3% of the the local oil and finally 4% of the local oil, the last being the standard out here.
Even with the 4% oil the results are very good.
From a slightly tinny exhaust note the sound has become more throatier.
It has also made the revving more 'linear' (if that is the proper word)
Joe
 

Offline Al Smith

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Re: Squish
« Reply #7 on: August 12, 2012, 07:57:03 am »
"Quote " Now, coming to the transfer ports, they would have a retarded timing because the piston top opens them, right?

Depends on how you look at it and who's theory you subscribe to .By lowering the cylinder and leaving the top of the transfers as they were in theory the crankcase pressure will be slightly higher when they open .

For example many Stihls have a transfer opening 20 degrees after the exhaust port opening .Ample for the stock running RPMS but not enough time for complete blow down if running at speeds exceeding the factory designed speeds .

If the compression is raised high enough by the initial lowering of the cylinder and the exhaust port raised a few degrees it gives a longer blow down and thusly a more "cleansed " tranfer of fuel and purging of the residual exhaust fumes .Therefore making the next power stroke more effective .---at least in theory  ;)

Now things can be improved by using external secondary transfers or internal finger ports but that gets into the catagory of race saw engines .

The biggest challange in the conversaion of transfer ports is the abilty to "sweep " the area above to intake port .Now on that my theory or one I use is with a raised dome piston which alters the flow as the transfer passes over it .Others use flat top pistons and have great success .So it just comes down to theory and experimentation .

Offline EHP

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Re: Squish
« Reply #8 on: August 12, 2012, 02:04:42 pm »
you say you got a vernier , just get some soft solder and place solder in the cylinder thou the spark plug hole , place the solder along the same direction the crankshaft is laying in the saw and turn more over to get a squish reading , If plug is grey I would richen it up abit , more a brown colour is better 

Offline Al Smith

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Re: Squish
« Reply #9 on: August 12, 2012, 03:55:31 pm »
That soft solder method is what most of us use only in my case with a set of Starret micrometers as I don't trust verniers .--old school  ;)

It works but I for one wish somebody would come up with a better method .

Odd for some reason it seems I manage 9 out of ten times to miss the correct amount to take off the cylinder base and invaribley resort to changing the thickness of the base gasket .

Fact just yesterday I noticed while changing the bearings and seals on a souped up 200T I had managed to miss that one too and used a 10 thou thick gasket .It isn't that I don't know how to run a lathe because I can cut class one tight threads with out as much as measuring  them .Just can't cut length worth a hoot . ???

 

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