Part Four: Compression

[Nutball]

Left out the big issue - heat soak. The hotter the two-stroke gets, the more power it looses. Do a flash reading on a dyno and then compare it to a run where the engine is hot - there can be up to a 20% difference in power.

Higher compression makes more heat, that heat gets pushed into the engine.

I would have to go back to the old engineering books, but the type of cooling will dictate to a large extent the maximum compression ratio. A simple finned cylinder can't take as much heat as a fan-cooled fin cylinder, which can't take as much heat as a water-cooled cylinder.

As I recall, a fan-cooled finned cylinder does not like much over 200 psi as the power drops off.

Of course, that depends on how it is being run. I would agree with that statement if it was steady state on a motorcycle. However, for a limbing saw that is on and off all the time, there are constantly periods for a cool down.

Why is power lost in a hot engine? Friction from an expanded piston? Preignition or the fuel burning too fast to last much of the power stroke?

As for compression slowing an engine, a heavier flywheel should help that. And, with an increase in speed, I'll call it rpm in this case, comes an exponential increase in stored energy. So, higher rpm should more easily overcome compression, and since rpm is one component of power, as long as the porting supports it, that extra rpm should continue to more easily overcome the compression.

But,,, I just thought how torque is obviously required to overcome compression, and torque can decrease as rpm increases without loosing power right? Hasn't that been shown by the dyno? So, maybe compression can limit the upper reaches of rpm regardless of port timing on a naturally aspirated engine.

There's my overthinking for tonight.

 

[Maintenance Chief]

Why is power lost in a hot engine? Friction from an expanded piston? Preignition or the fuel burning too fast to last much of the power stroke?

Oxygen molecules become less dense creates a situation where less fuel particles can mate up with air particles.

 

[huskyboy]

Is it just me or is it that saws are actually used in a lower rpm band than some think? I only run saws 70cc+ 6 days a week and I can't stand it if I got a 24" bar in a tree and idleing I squeeze the trigger and it doesn't start cutting and bogs, I've built a couple clone 440 hybrid saws to learn with and they really really liked the 220 to 250 psi range but pull ropes didn't, they were only tuned to 12,5 or maybe 13

You sure can tell right away what saws have torque and what do not have torque when bore cutting hardwoods that’s for sure.

 

[Nutball]

Oxygen molecules become less dense creates a situation where less fuel particles can mate up with air particles.

But the air doesn't get hot until it is in the cylinder right? I guess some air could be lost from heat expansion before the ports close. Good explanation.

 

[Terry Syd]

Yeah, the air gets hot in the engine and is less dense. Just like cutting on a hot day or a cold day, only much more pronounced.

I used to suck guys into drag racing my bike when I had a cool engine. Just enough to clean it out and then on with the race. When you can put down 10-15% more power by cheating with a cool engine, they soon figure it out.

Essentially, the piston gets good and hot and the bottom of the piston crown radiates heat into the crankcase. The crankcase also comes up to temperature and heats the charge. With the cylinder hot the cylinder heats up the charge as it enters the cylinder and the expansion allows some of it to escape the cylinder.

There is a reason for 'intercoolers' on super and turbo chargers. The cooler and denser the mixture the more mixture you can cram in the cylinder. I remember blokes using dry ice to chill the water cooled intercoolers at the drags.

The water-cooled engine can keep temperatures down much better and that's why they can produce more power.

 

[drf256]

Torque is the actual kinetic energy produced by an engine. Power is energy over time.

This comes down to physics. Compression simply converts the kinetic energy of the piston/crank to potential energy/thermal energy in the charge as heat. The spark adds the last bit of energy needed for ignition to occur and produce expansion of the gasses to punch down on the piston and produce more kinetic energy. Too much compression and the mix will ignite without a spark (preignition/dieseling).

Hotter engines lose more heat than cooler engines. Heat is power. The charge isn’t as dense in a hotter engine for combustion power, you have less potential energy.

The hotter the engine gets, the more the optimal air/fuel ratio changes. Loading a saw up, when you put the bar into wood let’s say, causes a sudden increase in cylinder heat. That’s why a saw goes from 4 stroking to 2 stroking when you load it. Same reason that hotter engines suddenly lose 4 stroking, same as when compression is too high. That’s why a saw needs to be designed with an intended use in mind. If I build a saw that will be used with a long bar in big wood in a warm climate, I keep compression down on purpose. If it’s a gtg saw or a short bar saw, I can pump compression up a bit.

Auto tune saws can make more power than manual tune saws because they can compensate for heat and compression. I can turn the wick up and know that the MT will fix the tune for me. You get a wider margin of error.

Like mentioned above, compression will fight rpm on top, so it will mechanically inhibit higher rpm and thus affect the overall power equation. The energy the motor is producing can be more, but the overall number on paper will be less.

Like us old guys used to say, Horsepower sells cars but torque wins races.

PS. We all know compression in a 2 stroke is built in the muffler, and it’s just the amount of atmospheric pressure we allow into the engine that matters the most. He he he....

 

[Al Smith]

The old "Widow maker" my Harley chopper was punched 90 over with some other enhancements .On the open road it did just fine but in town it was a TT special .Tavern to tavern because that SOB would over heat and it was a kick start .It flat would not start on a hot engine . I had to park that thing before it killed me .---However back to the subject I could tell the diff how it ran by how cold the air was .It really liked those cool mornings .It was also rather fond of gulping Cam II gas .

 

[Stump Shot]

Port height can be thought of as valve duration, and size as lift.

Yes... I would add that, the exhaust port also controls the power stroke(down stroke) and the swept compression(up stroke).
So... this would be like putting higher compression pistons in our four stroke example. But you still have to pick out a camshaft with the right lift and duration to get the full effect of the piston change.
Just grinding up would be like blindly picking a totally random camshaft out of a barrel, whereas if we pick the grind that allows the engine to breath properly(in and out) we can make a good usable saw that will make more torque, horse power and operate at a higher RPM for increased efficiency, maintain good idle and without being wasteful of fuel. If we put a mis-ground cam in our engine it all goes out the window.
So... a lot of things happening between when the exhaust port opens and closes again, and why there's no one way to go about it as the cylinders are laid out differently to begin with.

Also the guy that brought that over from AS owes you a harrumph.

 

[Al Smith]

If you cut to the chase if it can't get out it can't get in .Start from the exhaust and work backwards .There are more porting schemes than Carters have little liver pills and for the most part they all work .Of course even with the best of theory if the work is done by a ham hand it usually will not work well .You could in theory raise the exhaust so high that if it hung together it could approach the revs of a model airplane glow engine .Problem is it would not have enough grunt to power a pizz ants motor scooter .

 

[Stump Shot]

If you cut to the chase if it can't get out it can't get in .Start from the exhaust and work backwards .There are more porting schemes than Carters have little liver pills and for the most part they all work .Of course even with the best of theory if the work is done by a ham hand it usually will not work well .You could in theory raise the exhaust so high that if it hung together it could approach the revs of a model airplane glow engine .Problem is it would not have enough grunt to power a pizz ants motor scooter .

A fella brought over an 041 he just found at a sale and wanted to get it running, it had been run for some time with the flocking off the filter screen and was worn to the point of almost no compression. If you pulled the starter really fast it would start up, at idle it was running at about cut speed and would rev to no end if you hit the throttle. But if a butterfly landed on the chain, it was all done and over with.

When I asked a snowmobile race technician how much compression they ran in their oval track machines, his reply was, just enough for them to start and stay running, about 100 pounds.

 

[Al Smith]

What I don't know is how much back stuffing a tuned pipe does which most likely is what a racing sled would have .However being a smart aleck in Key West Florida when I was in the navy I had a Yamaha 250 unhorse me when it hit the pipe RPM's .Being a Harley rider I had never been on a 2 cycle scoot let alone with a tuned pipe .Talk about asphalt fever ,goodness .Alcohol was involved----

 

[Al Smith]

You can talk about exhaust port openings until the cows come home .I never took one above 96 after actually some of the 10 series piston ported Macs used that .Copesy would take them up to 92 .They were not torquers they were revers ..To make any fast cuts over the blocks you had to handle them with kid gloves .For a work saw that is not what you want .

 

[pbillyi69]

A McCulloch engine can be a study all of it's own .I know more than most but I'm by no means a expert but I know one .My bud Louie who was a crew chief on a Huey gun ship in 'Nam and I went to apprenticeship with was number 2 in the nation several years in a row in the glory days of 2 cycle karting .He still has his notes and his port maps .Poor guy had a stroke though and I haven't made contact with him for several years .BTW he is exactly one day older than me .
Louie was on the watch for a decent 101 for me that didn't cost a kings ransom .There used to be a zillion of them but they went the way of the passenger pigeon .
Dan Henry sent me the bones of a super 44 I still haven't done anything to .I might be able to throw together a 5 cuber in vintage style .You never know until you try .Big kids toy more or less

I had a mac 101 kart when i was a kid and that thing was a screamer. i sold it really cheap and bought a different one. I am pretty sure that my lil sister still has a mac 92 engine and maybe a 101 sitting in my parents basement

 

[cus_deluxe]

I found this on AS what do u think about it?

I want to put to bed the myth of high compression in chainsaws,having worked on 2 stroke racing engines since the 70's I want to lay some straight facts down. High compression was first made popular during the 60's with big v-8 racing engines(4-stroke) and compression ratios went up to 13-1 or more. But the best 2-stroke racing engines actually reduced the compression ratio from the stock engines. Why,because the more the compression ratio the more horsepower it takes from the engine to crank the engine against that high compression,at some rpm the HP taken to crank that high compression takes more HP than it makes,and then the engine starts to lose HP. Thats why a factory motor making 60 HP at 8.5-1 compression,makes 100HP at 7.5-1 comp. ratio.
Now higher compression does make more power at low and mid range power for sure,but not many saws run in that range. so if you want more low end or mid range power compression is good,but if you need high rpm race power high compression will simply cost you Hp. As my instructor told me in 1975 "high compression fights high rpms",he was right.
So more more compression can help you,but not at high rpm's.
When you yank the cyl. gasket out of a saw you do gain some compression,BUT you also lower the exhaust and transfer ports down,reducing their duration,and reducing higher rpm power. I would much prefer to raise ports than compression for power,much more can be gained.
In many engines I would much prefer to raise the exhaust port and lose compression,I know I will make much more top end power.
Removing the cyl. gasket and lowering the transfer ports is really bad,you reduce their duration and reduce their open time from tens of thousanths of a second to even less!

that sounds like a quote from trapper mike.

 

[moparnut88]

I had a mac 101 kart when i was a kid and that thing was a screamer. i sold it really cheap and bought a different one. I am pretty sure that my lil sister still has a mac 92 engine and maybe a 101 sitting in my parents basement

It would be nice to have those old Macs to build something with!


Sent from my iPhone using Tapatalk

 

[Al Smith]

Again a lot of ado over compression ratios . I've taken readings from stock yesterday of an 024 Stihl and was surprised it was 160 PSI .Like wise a 2100 Husqvarna, stock I assume was 165.A modified Stihl 038 Magnum was 195 .Yet on two Partner P-100's they only blew about 130 PSI, fresh rebuilds .Then comes to mind static and dynamic .Dynamic or under power might boost that up 30 or more pounds per square inch .So there's more to the story than just static readings on a gauge . You need to remember instead of a camshaft it's port time area open .The faster it runs the less it's open but at the same time the internal pressures are much greater moving things along faster .--clear as mud

 

[Maintenance Chief]

that sounds like a quote from trapper mike.

I have a book written about racing 2 strokes and modifications from the 70s and compression does reduce efficiency of higher rpms with no load.
Torque load is different and it doesn't have to be a mystery whether a saw runs stronger with a bump in compression , a simple pop up piston is gonna make your common stock saw stronger .
A saw has a tremendous amount of work to do with that small motor compared to turning 2 wheels.
I don't know why use the word "myth" that sort of implies that there's no contradiction? But I've got an 036 with a pop uo he can run!

 

[Ketchup]

I’ve been experimenting with increased compression in top handles. I’m finding a surprisingly tight squish creates the best torque. 0.014”-0.016”. They don’t free rev very high, but I can push on them.

I was worried about head slapping at first, but now I think the tiny modern engines are built precisely enough and the parts are small enough that heat expansion is slightly less of a concern.

I also think compression is a little different in small displacement saws. I feel like you have to compress 35cc harder to get the same PSI as a larger saw (like a 70cc, which has twice the displacement).

The timing is also a little different. Exhaust roofs seem a good bit lower in most small saws. I started tightening squish hoping to raise exhaust and the power band. It hasn’t worked for me. Medium low exhaust and high compression seems to hold best in the cut.

Heat is a problem though. My saws need a big outlet and they aren’t quiet.

 

[Ketchup]

I was cutting bands tonight and started wondering how elevation relates to compression.

I was wondering if extra compression could be more of a problem up high. Could it inhibit intake? Would more volume in the band allow more fuel and oxygen to bond?

My thinking up to this point has been that I need compression because the quantity of charge is less per case volume, so I need to crush it more to get the same explosion a saw would have at lower elevation. My gut says that’s the way to go. But if I break into too much compression would that be even more of a problem than at sea level?

I need a dyno…

 

[Cerberus]

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?

Would more volume in the band allow more fuel and oxygen to bond?

"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!)