Squish

[Danger Dan]

There are pics of the pipe saws in the Pacific NW get together thread. One was a 7900 and the other a Stihl but I'm not sure what model, 440 or 460 I would guess. As for deans saws the 390xp was impressive but the 346xp was a shocker! That little thing TORE through the wood!

Was the the 346xp the ehp saw? opcorn:

 

[RiverRat2]

Was the the 346xp the ehp saw? opcorn:

Nope,,, Dean did it!!!

 

[drmiller100]

dean's 390 was indeed faster then the stock 681.

and dean's saws were indeed easy to start.

ian's 372 was faster then everything other then the alky saws.

 

[timberwolf]

Back to squish.

Here is a little lookup table to figgure out how much difference squish height will make on total head volume.

Just go down to the squish height you have and over to the right piston size to get the volume used up by the squish height, then if you want do the same for a tighter squish and subtract it from the first to get how much the head volume will be reduced.

Example: 50mm piston .040 in height if reduced to .020 will reduce the head volume by very close to 1.0 cc as the volume used up in the squish height will drop from 1.99cc to 1.00 cc.

 

[Danger Dan]

Another Puzzle Piece

TW, Thanks for that.
I've been working on a 44mm jug and the squish is loose, (~ .050 w/o base gasket). I've been thinking about sending it out to get the base milled, but the combustion chamber is deep, (offset full hemi) so I'm not sure the gains would be worth it. :smoking:

 

[drmiller100]

try measuring it without any basegasket at all.

also, dean got the 681 running really well. for sure faster then stock.

 

[Danger Dan]

It's good to see that Dean got the 681 sorted out.

The .050 squish is without the base gasket.

 

[cdog]

First post guys, so please go easy on me here..............
I am a newb to all of this chainsaw modding, and I recently did the exhaust mod on my 026 and 290, and I must say I am somewhat annoyed that the extra power has been trapped inside for no reason!!!!
As far as raising compression goes I was of the understanding that the added compression had more to do with fuel economy than extra horsepower?
As in a diesel?????
I was always of the understanding that it is simply an air pump powered by gas, so the most gains were made by letting the air in and out more effectively?
Cdog.

 

[drmiller100]

yeah, generally true.

however, chainsaws are designed to be so incredibly anemic so that joe average home owner doesn't burn it down with 5 year old 81 octane gas.

so, raising the compression will add some top end power, and a fair amount of midrange snap.

 

[timberwolf]

Compression does improve fuel efficiency in general as it improves the engines thermal efficiency and by helping burn the fuel more efficiently and transfer that power to the crank. Also having the fuel all compacted into a nice tight centeralized head makes for complete combustion, reduced emmisions as well as improved output and fuel efficiency.

Increased compression on a saw hits on many angles though, effectivly it is much like advancing the timing a little which helps get all the fuel burnt in the most effective part of the crank rotation and before the exhaust port opens. Also reduced squish height increases combustion turbulence which makes for a good well mixed charge, improved flame speed and maximum efficiency. Also bumping up a saws compression reduces the end-gas ratio maximizing how much fuel is available for productive combustion in the first 20-30 deg of crank shaft rotation after TDC (when the power is made).

Down side is heat, more compression makes more heat and more output always put more load on engine components, not to mention the wear and tear on rewind and starter rope.

 

[drmiller100]

there is an argument that increased compression leads to more bad emissions.

apparently the added heat means the explosion gets hot enough to allow nitrogen to bond with oxygen forming "bad stuff."

obviously i'm over my head talking about stuff i barely get.

 

[Crofter]

No Doug, I think you are right. When you burn fuel with an abundance of oxygen you get an efficient combustion, energy wise, but it does create more nitrogen oxide compounds that can react in the atmosphere to form smog. Exhaust gas regeneration is used on both gasoline and diesel engines to attempt to reduce the super lean conditions. Pumping hot exhaust gas into my intake manifold seemed to cake and plug things up quickly on my VW diesel and a strange accident befell the EGR system, Lol! It does not plug up any more though.

 

[timberwolf]

High compression can indeed put cyl temps up, and if they get high enough ^2400 deg and there is an excess of O2 then NOX will go way up. But for the most part 2 strokes are burning rich compaired to 4 strokes where NOX is a major issue.

The big issue with 2 strokes is particulate mater, un-burnt hydrocarbons, and other semi burnt hydrocarbons that become volitile organic compounds. Easy 20-50% of fuel that goes into a simple two stroke can come out the muffler as bad stuff. Seems that is what the EPA is after when they drive the manufactures to put locks on the carbs so you cant richen the carb up. I have seen a few stock saws now that if the carb is richened to the limit it's still on the virge of beeing too lean.

 

[Wild Knight]

When reducing the squish on a work saw, do you compensate for the increase in heat by running it a little rich or adjusting the oil:gas mixture?

 

[timberwolf]

No, I do prefer to run at least 40:1 oil in ported saws, 32:1 even better.

 

[Meadow Beaver]

I have heard the only way you can go .010 squish or less is if you use a carbon piston, I haven't heard of anyone that has used one yet.

 

[046]

very nice free tools! thanks,

Here is a simple squish calculator I found, It does not cover angled squish bands or off set heads, but maybe useful to someone. http://www.torqsoft.net/squish-velocity.html

Here also a link to piston position calculator http://www.torqsoft.net/piston-position.html