Part Four: Compression

[Maintenance Chief]

When you really start building compression 200+lbs is it worh ditching the decomp button at the risk of destroying the starter?
Some decomp valves may bleed but definitely not a constant source of compression loss.

 

[Ketchup]

Awesome thread-series, LOVING it Have some Q's after reading this thread and may as well hop-on right with the squish-band and combustion-chamber:

Quick Q first: Decomp valves-- how much compression can be lost from keeping these? Or is it just a worry of the valve eventually failing? In any event I plan to remove mine, it looks as simple as screwing it out & replacing it BUT I hear 2 options, and am actually strongly pondering my own 3rd option:
#1- JB Weld to plug the hole, or 'more professionally':
#2- plugs that are purpose-built for this, to plug the hole, or my thinking:
#3- take the cylinder to hardware store, find a proper fitting plug that extends-into the chamber when it's fully seated, that way you can actively bump compression by reducing chamber-volume!! Have heard of "TIG a drop of 4130 steel into the chamber", this ^ extended bolt would serve same purpose I think! JB Weld would be used "as Loctite" when putting the plug in, an idea I like EVEN IF the plugging-bolt doesn't actually enter the combust.chamber!



After reading this thread, I'm finding I still don't have a good plan to boost compression in my 590 after having removed the gasket.... IF you're just looking for 2-3 thousandths, is it possible to just do a really good sanding of the cylinder base and the corresponding top of the casing? I'm looking at how rough mine is, after gouging-off that sticky OEM base gasket, and can't help thinking "a realllly good proper sanding job on both sides IE cylinder base and case's top,would probably be enough to give 1-3 thousandths", would love to hear thoughts on this as I know it is very "hack" but I can't fault the idea
(Is there anything one could buy at a local hardware store -- or a dremel-accessory -- that could be used for doing true/plumb grinding of the cylinder base? Cannot help but think how simple an apparatus it should be, but just too unfamiliar, like I have no clue what the actual gear for any of this CNC stuff looks like, am just using grinders but would totally run to the hardware store if there was a "fixed-in-apparatus" grinder that let me do a cylinder base properly!)

What about regular porting & compression? I've finished the thread (even took notes) and cannot actually correlate any common porting work to compression except exhaust, where it seems any enlargement will *lower* compression (and torque, but boost RPM usually -- I'm uncertain if compression cares as much about the port's height as it does port-volume or port surface-area..)

Re pop-up piston kits, I see I can swap my 590's "1 ring, domed" piston for a "2 ringed, pop-up" aftermarket piston...
-How much compression would one see from this swap? Would it be entirely due to the changing of domed, to pop-up? Does the 2nd ring add compression, or just help alleviate/spread loading inside the cylinder?
- I've realized my 1.75yrs old 355t's crazy high compression is probably from all the carbon on-top of the piston--- won't this type of buildup effectively make it so all saws are effetctively "lowering squish"(and thus bumping compression) as they're used?


"Volume" in the band? I have to imagine this is in-reference to the concept of carving-into the combustion chamber's perimeter, making a wider (and higher-in-cylinder) squish band?* This is done solely so that one can do further drops of cylinder, height than they'd otherwise be able to, correct?
[To be clear, squish band work is not possible with "cheap handheld or countertop" gear, but only pro-level CNC stuff, right?]



Does "cutting pop-ups into the piston" come into play here? I'm loving the concept of simply swapping my piston to a pop-up, seems it'll do much of what I need (so far as that part of the engine ie squish-band and compression of chamber), however while I see the aftermarket pop-up pistons and understand how they work, I am still baffled by "cutting" pop-ups into a piston, cannot even picture (or find pics) of what this could be..

Thanks a ton for any insight into ANY of this, gonna - hopefully - be doing round-2 on my 590 today and still trying to work-out a couple things on how my approach will go (I did not get the pop-up piston kit yet as I was afraid of my ability to install it, am new to this lol, but am now thinking that if I can't I'll just bring it to the shop and pay them to!)

Just plug the decomp with a standard decomp plug.

Sounds like you boogered your cylinder base. You can sand the base if you’re careful. Scribe a line with a razor blade at the depth you want and then use adhesive sandpaper on a pane of glass or granite surface plate. It’s easier to mark out 0.010” than something tiny like 0.001”. Don’t use a belt or disc sander. Do it by hand and be careful.

An aftermarket popup piston will raise your compression, but the remaining squish (and popup height) will determine how much.

Dual ring may hold more compression but mostly it’s for piston stability. Dual rings also add friction. Singles normally have a very minor edge in performance but less longevity.

The lower and smaller the exhaust, the more compression the saw will have. But exhaust needs time and area for other reasons as well. Exhaust height and width greatly effects torque and rpm, not to mention exhaust and heat loss. It all works together and compression is only one factor.

I was just thinking about optimal Squish volume. Too much compression becomes resistance. Maybe it also effects the chemistry of the explosion? Probably not much as long as you stay below detonation pressure.

If your saw builds more than 0.005” of carbon, you’ve got problems. That’s not not how to build compression.

Most Pop-ups are normal pistons with the crown trimmed down. It can be done on a lathe in about 20 minutes (faster if you do it a lot). Some saws have a common bore that many pistons fit. If you can find a taller piston from a different model then you just trim it down to create you desired squish. Otherwise, if cutting a stock piston, you will have to cut the cylinder base to get your desired squish.

 

[Cerberus]

Just plug the decomp with a standard decomp plug.

Sounds like you boogered your cylinder base. You can sand the base if you’re careful. Scribe a line with a razor blade at the depth you want and then use adhesive sandpaper on a pane of glass or granite surface plate. It’s easier to mark out 0.010” than something tiny like 0.001”. Don’t use a belt or disc sander. Do it by hand and be careful.

An aftermarket popup piston will raise your compression, but the remaining squish (and popup height) will determine how much.

Dual ring may hold more compression but mostly it’s for piston stability. Dual rings also add friction. Singles normally have a very minor edge in performance but less longevity.

The lower and smaller the exhaust, the more compression the saw will have. But exhaust needs time and area for other reasons as well. Exhaust height and width greatly effects torque and rpm, not to mention exhaust and heat loss. It all works together and compression is only one factor.

I was just thinking about optimal Squish volume. Too much compression becomes resistance. Maybe it also effects the chemistry of the explosion? Probably not much as long as you stay below detonation pressure.

If your saw builds more than 0.005” of carbon, you’ve got problems. That’s not not how to build compression.

Most Pop-ups are normal pistons with the crown trimmed down. It can be done on a lathe in about 20 minutes (faster if you do it a lot). Some saws have a common bore that many pistons fit. If you can find a taller piston from a different model then you just trim it down to create you desired squish. Otherwise, if cutting a stock piston, you will have to cut the cylinder base to get your desired squish.

Such a thorough reply, thank you!!

"boogered my base"? If you mean "baseline #'s" I agree, but the mating surfaces of bottom-of-cylinder and where it meets the casing are both nice&shiny, I used 220g on both surfaces before applying Motoseal (and, after opening-up my cylinder again for 'round 2/final round' porting, thankfully I did not get any Motoseal into the cylinder, did a good job of at least ~.125mm gap of bare metal at innermost-perimeter of the union!)

Re "machining a lower cylinder by-hand", I just ended up ruling that it wasn't worth it...I want it lower, I've no doubt I could do a good enough job that it'd mate-up just as well / nearly as well as OEM, but the time to do it properly it's just easier for me to keep asking-around til I find someone who can CNC stuff (as I have other saws, climbsaws, where the porting and power//weight is far more important to me, the 590 was just 'easy project' the 355t is the real goal but there's not volumes of info on porting 355's like there is on 590's ;D )

Re exhaust, and exhaust///compression...will admit that I was kinda fearing that, by raising exhaust height (106.125* final height on 60cc CS-590), I'd have problems because while some extra RPM's are great I already felt my particular 590 was anomalously low on compression, I mean when I got it its starter-cord pulled easier than any other echo on the shelf, although the cylinder/piston look just fine...

I fear it may be too late now, after I've gone and ported it twice, to go and drop the cylinder and/or get a pop-up kit....I know the saw's weak-point (or one-of them) is the low compression / high squish (26 or 27thous after the gasket delete) so expect I'd get some of the best gains with lower cylinder and/or pop-up replacement piston BUT fear that, since I already ported it, that I blew that chance....am gonna ask in the 590/600/620 thread to see if that's the case, I was game to order the popup piston for my last porting-session but I still hadn't (even til now!) figured-out whether you just get the pop-up and simply swap it, "plug&play", or if these pop-ups come "a bit under/oversized", meaning that to use them it's expected/requisite to do in-conjunction-with CNC'ing (lowering cylinder height, raising the squish-band of the combustion chamber, etc etc)
(I was never planning the type you describe IE home-brew pop-up's, to be clear it's a pre-made 590 popup piston for like $50 I'd be buying, just to be crystal clear )

Thanks again for all the info it's greatly appreciated!! BTW, the way you talk of CNC/lathes....any tips for someone who's literally never seen this machinery in real life, probably couldn't even recognize it in-photo, and has zero clue who to ask (literally would just type "CNC" into google hoping to find "CNC store" which I know does not exist ) Are there commonplace-enough operations that'd have this type of equipment? Like, if I just knew where the machinery was, then it's just a matter of how much $$ someone there needs to do it for me! Am presuming VERY few people actually have such equipment in their home-setups?

 

[Ron660]

Compression…I like it.

 

[telejed]

Many times I want a lower exhaust than stock.......in that case a cut squish is the best way to get there.

That also increases the time the expansion/burn/power stroke/ distance the piston moves down from ignition? More fully burning and using the energy created by the charge?

 

[Ketchup]

Such a thorough reply, thank you!!

"boogered my base"? If you mean "baseline #'s" I agree, but the mating surfaces of bottom-of-cylinder and where it meets the casing are both nice&shiny, I used 220g on both surfaces before applying Motoseal (and, after opening-up my cylinder again for 'round 2/final round' porting, thankfully I did not get any Motoseal into the cylinder, did a good job of at least ~.125mm gap of bare metal at innermost-perimeter of the union!)

Re "machining a lower cylinder by-hand", I just ended up ruling that it wasn't worth it...I want it lower, I've no doubt I could do a good enough job that it'd mate-up just as well / nearly as well as OEM, but the time to do it properly it's just easier for me to keep asking-around til I find someone who can CNC stuff (as I have other saws, climbsaws, where the porting and power//weight is far more important to me, the 590 was just 'easy project' the 355t is the real goal but there's not volumes of info on porting 355's like there is on 590's ;D )

Re exhaust, and exhaust///compression...will admit that I was kinda fearing that, by raising exhaust height (106.125* final height on 60cc CS-590), I'd have problems because while some extra RPM's are great I already felt my particular 590 was anomalously low on compression, I mean when I got it its starter-cord pulled easier than any other echo on the shelf, although the cylinder/piston look just fine...

I fear it may be too late now, after I've gone and ported it twice, to go and drop the cylinder and/or get a pop-up kit....I know the saw's weak-point (or one-of them) is the low compression / high squish (26 or 27thous after the gasket delete) so expect I'd get some of the best gains with lower cylinder and/or pop-up replacement piston BUT fear that, since I already ported it, that I blew that chance....am gonna ask in the 590/600/620 thread to see if that's the case, I was game to order the popup piston for my last porting-session but I still hadn't (even til now!) figured-out whether you just get the pop-up and simply swap it, "plug&play", or if these pop-ups come "a bit under/oversized", meaning that to use them it's expected/requisite to do in-conjunction-with CNC'ing (lowering cylinder height, raising the squish-band of the combustion chamber, etc etc)
(I was never planning the type you describe IE home-brew pop-up's, to be clear it's a pre-made 590 popup piston for like $50 I'd be buying, just to be crystal clear )

Thanks again for all the info it's greatly appreciated!! BTW, the way you talk of CNC/lathes....any tips for someone who's literally never seen this machinery in real life, probably couldn't even recognize it in-photo, and has zero clue who to ask (literally would just type "CNC" into google hoping to find "CNC store" which I know does not exist ) Are there commonplace-enough operations that'd have this type of equipment? Like, if I just knew where the machinery was, then it's just a matter of how much $$ someone there needs to do it for me! Am presuming VERY few people actually have such equipment in their home-setups?

0.027” squish isn’t terrible. Put an AM popup in it and you should be in the 170-190psi range. It should be plug and play but check your squish after install.

CNC is an automated system of robot arms and tooling that are preprogrammed to specific cut patterns. Usually to make very complex or multiple identical components. There are CNC lathes, routers, mills, grinders, welders and all kinds of other automated tools. You don’t need anything that fancy. A good old analog metal lathe will cut your cylinder base just fine. A milling machine or surface grinder will also do the job in competent hands. It should be easy for any practiced machinist.

 

[srcarr52]

Such a thorough reply, thank you!!

"boogered my base"? If you mean "baseline #'s" I agree, but the mating surfaces of bottom-of-cylinder and where it meets the casing are both nice&shiny, I used 220g on both surfaces before applying Motoseal (and, after opening-up my cylinder again for 'round 2/final round' porting, thankfully I did not get any Motoseal into the cylinder, did a good job of at least ~.125mm gap of bare metal at innermost-perimeter of the union!)

Re "machining a lower cylinder by-hand", I just ended up ruling that it wasn't worth it...I want it lower, I've no doubt I could do a good enough job that it'd mate-up just as well / nearly as well as OEM, but the time to do it properly it's just easier for me to keep asking-around til I find someone who can CNC stuff (as I have other saws, climbsaws, where the porting and power//weight is far more important to me, the 590 was just 'easy project' the 355t is the real goal but there's not volumes of info on porting 355's like there is on 590's ;D )

Re exhaust, and exhaust///compression...will admit that I was kinda fearing that, by raising exhaust height (106.125* final height on 60cc CS-590), I'd have problems because while some extra RPM's are great I already felt my particular 590 was anomalously low on compression, I mean when I got it its starter-cord pulled easier than any other echo on the shelf, although the cylinder/piston look just fine...

I fear it may be too late now, after I've gone and ported it twice, to go and drop the cylinder and/or get a pop-up kit....I know the saw's weak-point (or one-of them) is the low compression / high squish (26 or 27thous after the gasket delete) so expect I'd get some of the best gains with lower cylinder and/or pop-up replacement piston BUT fear that, since I already ported it, that I blew that chance....am gonna ask in the 590/600/620 thread to see if that's the case, I was game to order the popup piston for my last porting-session but I still hadn't (even til now!) figured-out whether you just get the pop-up and simply swap it, "plug&play", or if these pop-ups come "a bit under/oversized", meaning that to use them it's expected/requisite to do in-conjunction-with CNC'ing (lowering cylinder height, raising the squish-band of the combustion chamber, etc etc)
(I was never planning the type you describe IE home-brew pop-up's, to be clear it's a pre-made 590 popup piston for like $50 I'd be buying, just to be crystal clear )

Thanks again for all the info it's greatly appreciated!! BTW, the way you talk of CNC/lathes....any tips for someone who's literally never seen this machinery in real life, probably couldn't even recognize it in-photo, and has zero clue who to ask (literally would just type "CNC" into google hoping to find "CNC store" which I know does not exist ) Are there commonplace-enough operations that'd have this type of equipment? Like, if I just knew where the machinery was, then it's just a matter of how much $$ someone there needs to do it for me! Am presuming VERY few people actually have such equipment in their home-setups?

There are many people on this site who will machine a cylinder for you. I do it occasionally as people ask, it's usually less than a 2 week turn around and shipping is not terrible for just a cylinder. To machine most cylinders all you need is a manual lathe and basic machining knowledge.

 

[telejed]

Good topic.


Many times I want a lower exhaust than stock.......in that case a cut squish is the best way to get there.

That also increases the time the expansion/burn/power stroke/ distance the piston moves down from ignition? More fully burning and using the energy created by the charge?

Sorry that my response there is so scattered. I don't know the two cycle terminology of what I would call the power stroke in a 4 cycle since Intake/Compression and Intake/Exhaust overlap in 2 cycles.

So by cutting the squish and lowering the cylinder (even if squish is the same after) you have now made a chamber that pushes piston down longer before that force is allowed to escape (unused/lost/wasted) to the exhaust port. Provided there was a waste in the first place. I'm not an engineer but, I'm sure all of that could be calculated with burn rates, some application of the ideal gas law, and actually knowing the volume of charge when the exhaust closes. The first two of those I could probably do the third well, CALCULUS IS HARD!.

 

[Ketchup]

Sorry that my response there is so scattered. I don't know the two cycle terminology of what I would call the power stroke in a 4 cycle since Intake/Compression and Intake/Exhaust overlap in 2 cycles.

So by cutting the squish and lowering the cylinder (even if squish is the same after) you have now made a chamber that pushes piston down longer before that force is allowed to escape (unused/lost/wasted) to the exhaust port. Provided there was a waste in the first place. I'm not an engineer but, I'm sure all of that could be calculated with burn rates, some application of the ideal gas law, and actually knowing the volume of charge when the exhaust closes. The first two of those I could probably do the third well, CALCULUS IS HARD!.

You can do the engineering math, but it’s not really necessary. There’s a small range where the exhaust height can produce a spectrum of rpm and torque. Too low or high doesn’t make a good power band. The sweet spot may be slightly above or below stock height. Usually within 6 degrees. Adding compression and enlarging the muffler outlet will effect where that lies.

The transfer height also plays a key role. If you’re exhaust is low, your transfers will be as well. You can shorten blowdown some by widening your exhaust port, more by piston matching the exhaust roof, but a low exhaust roof will still dictate lower transfers.

 

[telejed]

You can do the engineering math, but it’s not really necessary. There’s a small range where the exhaust height can produce a spectrum of rpm and torque. Too low or high doesn’t make a good power band. The sweet spot may be slightly above or below stock height. Usually within 6 degrees. Adding compression and enlarging the muffler outlet will effect where that lies.

The transfer height also plays a key role. If you’re exhaust is low, your transfers will be as well. You can shorten blowdown some by widening your exhaust port, more by piston matching the exhaust roof, but a low exhaust roof will still dictate lower transfers.

It's been a while, and by no means to I really understand all this stuff, but when I was reading 2 Stroke Tuners Handbook, the author mentioned tracing the ports to determine volumes. He said something about the math. . . I realized it's calculus. In the one class I took we traced one of those tissue paper bells that fold up onto graph paper. We then had to figure out the equations to represent the curves, add some calc magic and determine the volume.

I didn't get the right answer.
I also need to reread that book. Not the calculus one.

 

[Mastermind]

Fellas, if you think calculus is what it takes to get a simple saw engine to run as you like, you might be overthinking it.

 

[Ketchup]

It's been a while, and by no means to I really understand all this stuff, but when I was reading 2 Stroke Tuners Handbook, the author mentioned tracing the ports to determine volumes. He said something about the math. . . I realized it's calculus. In the one class I took we traced one of those tissue paper bells that fold up onto graph paper. We then had to figure out the equations to represent the curves, add some calc magic and determine the volume.

I didn't get the right answer.
I also need to reread that book. Not the calculus one.

I can’t say I found the 2stroke Tuners Handbook to be a page turner. I should also read it again.
If you want to post up some maths here, please do. Everybody here loves variables.

 

[huskihl]

Fellas, if you think calculus is what it takes to get a simple saw engine to run as you like, you might be overthinking it.

Pfft. Even spaghetti noodles are beyond my willingness

 

[Mastermind]

Pfft. Even spaghetti noodles are beyond my willingness

I remember the spaghetti noodle thingy.

But.....I like pie.

 

[Stump Shot]

Fellas, if you think calculus is what it takes to get a simple saw engine to run as you like, you might be overthinking it.

Maybe some calculus would help explain directing spent exhaust gas back into the combustion chamber makes more compression, sadly it would seem everyone but me already knows this and I can't figure it out.

Pfft. Even spaghetti noodles are beyond my willingness

I like spaghetti.

 

[Stump Shot]

Fellas, if you think calculus is what it takes to get a simple saw engine to run as you like, you might be overthinking it.

On models we're quite familiar with I bet any one of us could port it by eye from muscle memory and have a satisfactory result. Not that we should give up our precision measuring tools and simple math, you are correct in your assessment that you don't need the know how to make the motor from scratch to hot rod on it.

 

[Ketchup]

I like Nachos.

I also like turtles.

 

[farminkarman]

I like Nachos.

I also like turtles.

Mmm, turtles

 

[Mastermind]

How about turtle nachos?

 

[Stump Shot]

How about turtle nachos?

Mutant turtles?