High Compression??????

[RedneckChainsawRepair]

Why not make a 2 piece head and get all the compression you want. Nice by the way.

 

[Trx250r180]

What about lowering the jug and cutting squish band? You add displacement if you don't raise the exhaust to match the distance the jug lowered.

no ,it will be the same still ,displacement is bore times stroke ,if you moved the big rod pin outwards and raised the jug ,you could increase it ,or a bigger mm piston will increase it example 50 to a 52 mm either longer crank stroke ,or bigger piston is only way to increase size that i know of .

 

[Jacob J.]

I see where squarefile is going with this. I too have wondered if compression can be high enough for the pressure to slow the piston down right before tdc. But then again it takes several pumps for a comp tester to reach full capacity, so there might not ever be enough pressure on one stroke to cause that.

That's where flywheel weight, and weight of the reciprocating assembly comes into play. The early cylinders from the 10mm wrist pin 044 made more compression, but the engine used a heavier flywheel. I get better results on the hybrid (044/046) saws when using the early, heavier 044 flywheel. The lighter flywheel is good for acceleration but not much else.

Higher compression does slow the piston more and McCulloch used that to their advantage on their early saws that had the removable head.

 

[Deets066]

What about lowering the jug and cutting squish band? You add displacement if you don't raise the exhaust to match the distance the jug lowered.

You would actually be decreasing volume of the cylinder anyway, that's why it increases compression

 

[jmssaws]

It don't matter what u do to the cylinder displacement is the same unless u increase the bore size. U can prune a cylinder that's 10 feet long on it with the exhaust anywhere and the piston will still travel the same distance. Nothing other than a crankshaft with a longer throw or a bigger bore changes displacement. Rod length or cylinder length, combustion chamber size will not change how far the piston travels.

 

[Rx7man]

I've been told by an experienced builder (he's rare around here) that you don't need high compression to make power, and I tend to agree, though high compression does have it's place.

My 'wild' husky 65 only has 140 PSI, and my 'manhattan project' saw has 180.

A lot depends on the goal for the saw.. High compression can be a byproduct of other mods.. Like if you delete the base gasket you're increasing it in 2 ways.. the first is because you're reducing the squish and thus volume at TDC, but the second reason is because you're closing the exhaust port sooner.. if you're base gasket is .6mm on a 52mm bore that comes to about 1.3cc reduction in total chamber volume.. if it was 9 cc you go down to 7.7cc.. that's a about a 15% reduction in volume in addition to the additional 1.3cc trapped at exhaust port closing.. no doubt it's going to have much higher compression! With the lowered exhaust port you make power for longer, so it'll be a torquey saw, but probably won't scream at the top end unless you go with a very wide exhaust port to handle the flow.

However, if you maintained the same compression as the stock saw had, but did a base gasket delete, you could probably move the exhaust port up 10*... Now in that case you'll lose torque, but the saw will come alive and SCREAM at the top end, and will make more POWER, though have less grunt.

It goes without saying that the transfers and intake system have to be matched to the desired performance characteristics of the top end... An intake timing of 70* will work for a torque monster saw, and 90* for a very fast running saw, but if you mismatch it you'll choke the heck out of the high speed saw, and have an impossible-to-tune bottom end on what should be the torquey saw.

Speaking of self destructing from high compression.. I had a 3.5HP briggs engine I milled the head and deleted the gasket on.. I broke the pull cord the first time around.. it had 190 PSI... ran great but after a half hour the oil was totally full of aluminum dust.. I wish I had kept it and wired the governor open to see it explode.. it would have been fun!

 

[EvilRoySlade]

Well, throw rod angle in there with removeable heads and using different Pistons with different pin to top distance. Best part about engines is someone's done it. It's just learning from them and getting the combo right for what you want, Rod angle, compression, torque/inertia relationship, how good your chain sharpening skills are. How fast are you willing to spend?
Or are you working with what you've got?
In my opinion @chadihman dyno was brilliant. Find the right combo for any saw then cut your chain to compliment the engine.

 

[jmssaws]

Rod angle is very important in race motors but I've never gave it thought for a chainsaw. We always switch the 5.7 inch rods out for 6.0 rods when we had high compression big Inch chevys cause the long rod takes angle away. But we're all turbo now and nobody does it anymore. No reason to on these ls motors there bout perfect when they leave gm

 

[EvilRoySlade]

When I was young I had a Honda spree moped. I was young, had many parts engines, access to a lathe. Well I turned the head down, don't remember but too much. Running 112 it wouldn't shut down, just kept firing until it cooled a bit. Would've been a good candidate for alky but didn't know what I was doing then. Hell, still don't.

 

[Chainsaw Jim]

no ,it will be the same still ,displacement is bore times stroke ,if you moved the big rod pin outwards and raised the jug ,you could increase it ,or a bigger mm piston will increase it example 50 to a 52 mm either longer crank stroke ,or bigger piston is only way to increase size that i know of .

Did I confuse displacement with combustion area?

 

[Chainsaw Jim]

You would actually be decreasing volume of the cylinder anyway, that's why it increases compression

I should have described what I meant in a better way. Lowering the jug and raising the roof to match. No tightening the squish, no increase of compression. No raising the exhaust. Simply lengthening the combustion area of the stroke. I thought that little bit extra was an increase in displacement.

 

[Deets066]

Any chainsaw jug I've seen has a domed combustion chamber, if you take .030 off the base and .030 out of squish the domed area is smaller and compression increases.
Now theoretically if they did stay the same you still don't trap more. It just starts compressing earlier

 

[Trx250r180]

I should have described what I meant in a better way. Lowering the jug and raising the roof to match. No tightening the squish, no increase of compression. No raising the exhaust. Simply lengthening the combustion area of the stroke. I thought that little bit extra was an increase in displacement.

I think what you are describing is timing ?

 

[Rx7man]

I think a clarification needs to be made here between 'compression ratio' and 'corrected compression ratio'... the first is the ((total cylinder displacement + head volume)/head volume).. though it's widely used, it's not very indicative of anything... the latter is the volume from exhaust port closing, and that's what your compression tester will read.

If you do nothing but lower the exhaust port (somehow), you'll get more compression on your gauge, the saw will be torquier, but you probably won't make any more power. If you raise it you'll lose grunt, but gain top end power (in other words, torque at high RPM)

Now if you leave the ports where they are and decrease the head volume, you get a higher reading on your gauge, but the truth of it is you aren't trapping any more air in the cylinder, or helping it breathe better.. Any gains to be had would be from a better, more compete burn. Decreasing squish (or any other method of reducing the surface area to volume ratio such as well designed chambers) are good for that, but I think the number on a gauge isn't all it's cracked up to be. It's not the be all and end all of making power.

 

[Deets066]

I think a clarification needs to be made here between 'compression ratio' and 'corrected compression ratio'... the first is the ((total cylinder displacement + head volume)/head volume).. though it's widely used, it's not very indicative of anything... the latter is the volume from exhaust port closing, and that's what your compression tester will read.

If you do nothing but lower the exhaust port (somehow), you'll get more compression on your gauge, the saw will be torquier, but you probably won't make any more power. If you raise it you'll lose grunt, but gain top end power (in other words, torque at high RPM)

Now if you leave the ports where they are and decrease the head volume, you get a higher reading on your gauge, but the truth of it is you aren't trapping any more air in the cylinder, or helping it breathe better.. Any gains to be had would be from a better, more compete burn. Decreasing squish (or any other method of reducing the surface area to volume ratio such as well designed chambers) are good for that, but I think the number on a gauge isn't all it's cracked up to be. It's not the be all and end all of making power.

Agreed, when talking about exhaust height it really depends on what you want to do with the saw, limb or cant race or pull a long bar stumping. Same goes for intake height as well but it's just the opposite with where you gain torque or speed

 

[wigglesworth]

....but I think the number on a gauge isn't all it's cracked up to be. It's not the be all and end all of making power.

Yup.

 

[Moparmyway]

I should have described what I meant in a better way. Lowering the jug and raising the roof to match. No tightening the squish, no increase of compression. No raising the exhaust. Simply lengthening the combustion area of the stroke. I thought that little bit extra was an increase in displacement.

There can be no increase in a motors displacement unless the stroke is lengthened or bore diameter increased.
No matter whats done to the combustion chamber, the piston still displaces the same volume if you dont change the bore diameter or stroke length

 

[Moparmyway]

I think a clarification needs to be made here between 'compression ratio' and 'corrected compression ratio'... the first is the ((total cylinder displacement + head volume)/head volume).. though it's widely used, it's not very indicative of anything... the latter is the volume from exhaust port closing, and that's what your compression tester will read.

If you do nothing but lower the exhaust port (somehow), you'll get more compression on your gauge, the saw will be torquier, but you probably won't make any more power. If you raise it you'll lose grunt, but gain top end power (in other words, torque at high RPM)

Now if you leave the ports where they are and decrease the head volume, you get a higher reading on your gauge, but the truth of it is you aren't trapping any more air in the cylinder, or helping it breathe better.. Any gains to be had would be from a better, more compete burn. Decreasing squish (or any other method of reducing the surface area to volume ratio such as well designed chambers) are good for that, but I think the number on a gauge isn't all it's cracked up to be. It's not the be all and end all of making power.

It certainly is an important ingredient though, and works well when other ingredients are combined properly with it.

A little vanilla extract can make a cake taste great, too much vanilla extract and nobody will eat it.

 

[Moparmyway]

Agreed, when talking about exhaust height it really depends on what you want to do with the saw, limb or cant race or pull a long bar stumping. Same goes for intake height as well but it's just the opposite with where you gain torque or speed

I know my 066 wont win any cant races, matter of fact, I dont want that freak spinning past 13k, especially when she wears a 36" .404 setup.

I did not want a cant racer (people around my area dont even know what a cant is), I wanted a torque monster for getting through root blooms at the base, and for making fast work out of the big ones around me. She is all go and no show

I got exactely what I wanted and more than I asked for !!!!!

 

[jmssaws]

If I can get a crankshaft to take it my new flat top will b quite spunky. I wanted to use a aluminum flywheel but they just can't take it.