Part Four: Compression

[mdavlee]

Compression is Silly!!! Most of my race saws run less than 170psi
Here is a Mac that has the power to impress anyone. @mdavlee and I had to tag team the saw to get it started and neither of us are exactly small guys
Mike took the first cuts and then @Lee H took the reins of this beast (it's his by the way) and gave the cant some hell. If you are wondering how strong the saw is, watch uncle Lee's upcut and how despite plenty of experience and a good solid stance it started to push him back a little.
Trust me, a well built McCulloch is a thing of beauty!!

That saw is just so much fun to run with good ear plugs[emoji23]

 

[Lightning Performance]

I run 12 quench in my work saws under 50mm bores. Most will tell me that's wrong. I'm ok with that.

 

[Al Smith]

That deal on the 10 series design could be analyzed .Some dumped the exhaust straight out the bottom where it hits a squared cornered muffle that shoved it into another square corner then out some baffle plates .That's like hitting two brick walls one after another .A better design some used depending on the year was the rounded bottom that shot it straight out the side .You can remove the baffles but I suggest you get some real good ear muffs because they can wake the dead and make dogs howl .That will wake them up though .Now I've got one, a 70 cc 6-10 that will out cut an 850 over the blocks and maybe runs straight up with a 372 Husky .Plenty of people have seen it but I don't use it except to fiddle around at a GTG .That pot licker hurts my ears . Another big kids toy more or less .

 

[Maintenance Chief]

I have a small 3 lobe brass roots pump intended for natural gas pumping. I've thought about putting it on a saw motor for supercharging?
I always think about the automotive industry going to hell in the late 70s early 80s and how they've put a million controllers in vehicles to compensate for the lack of displacement and compression.
Compression makes grunt no matter how you slice it , but if you can't feed it its pointless.
When we were discussing parasitic loss due to compression, I think the model would show that on a motor with no load but saws are working under load .
What impresses me with a saw is its functional working range compared to CC.
I run alot of McCulloch saws ,but not the fire breathing ones , 10-10, pm55,Pro Mac 800 and pm700. I've been very impressed with the range at which I can push these saws , 8 pin sprockets, .404 and whatever bar length (with in reason). They'll never win a piss rev contest but it'll do what you need it to do with out complaints.
Sorry for the derail, I also really like the 034 I rebuilt with a pop up piston.

 

[moparnut88]

I have a small 3 lobe brass roots pump intended for natural gas pumping. I've thought about putting it on a saw motor for supercharging?
I always think about the automotive industry going to hell in the late 70s early 80s and how they've put a million controllers in vehicles to compensate for the lack of displacement and compression.
Compression makes grunt no matter how you slice it , but if you can't feed it its pointless.
When we were discussing parasitic loss due to compression, I think the model would show that on a motor with no load but saws are working under load .
What impresses me with a saw is its functional working range compared to CC.
I run alot of McCulloch saws ,but not the fire breathing ones , 10-10, pm55,Pro Mac 800 and pm700. I've been very impressed with the range at which I can push these saws , 8 pin sprockets, .404 and whatever bar length (with in reason). They'll never win a piss rev contest but it'll do what you need it to do with out complaints.
Sorry for the derail, I also really like the 034 I rebuilt with a pop up piston.

I was taking to Dave about this on a 500i today! I think it would be a cool project if the injection can keep up.


Sent from my iPhone using Tapatalk

 

[ferris]

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!

 

[Mastermind]

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 thinking was the prevailing thought process on saw building for many years. It came from the two stroke motorcycle world where tuned pipes were used.

But.....over the years we found that a shorter exhaust duration seemed to build a stronger saw for actual work related use.

Now....I'm getting outside my comfort zone here. This sort of talk leads to disagreements between people almost as much as political and religious discussion.

 

[deye223]

That thinking was the prevailing thought process on saw building for many years. It came from the two stroke motorcycle world where tuned pipes were used.

But.....over the years we found that a shorter exhaust duration seemed to build a stronger saw for actual work related use.

Now....I'm getting outside my comfort zone here. This sort of talk leads to disagreements between people almost as much as political and religious discussion.

Randy people just need to respect other people's opinions I know it doesn't happen very often though. I will say this much that 460 that you built for me blows 225 psi Tunes at 14200 rpm and works in our hardwood all day with a 32 inch bar it's got more talk then I've got thank you very much

 

[Maintenance Chief]

This is literally the first I've ever heard about compression costing you horsepower, but I mostly have played with 4 strokes.
I think the theory is sound when we're talking about a finite amount of fuel and air being able to get into the cylinder and an infinite amount of rpms being unlimited.
But anyway to ram more mix and compress it into a tighter mass produces power ( heat being the limiting factor) there are far too many examples of turbo charged Engines including motorcycle that prove that point. Supercharger Engines are an even better example of how compression is pretty effective at raising power, though the fuel air delivery system has to support the demand of that power.
The down side to a high compression motor is its always high compression unlike the power added turbo/ supercharged, so idle and fuel efficiency is a crap shoot.
Ford literally did away with the flathead based on the fact that it couldn't be designed for the better fuel being produced and run higher compression.

 

[Mastermind]

This is literally the first I've ever heard about compression costing you horsepower, but I mostly have played with 4 strokes.
I think the theory is sound when we're talking about a finite amount of fuel and air being able to get into the cylinder and an infinite amount of rpms being unlimited.
But anyway to ram more mix and compress it into a tighter mass produces power ( heat being the limiting factor) there are far too many examples of turbo charged Engines including motorcycle that prove that point. Supercharger Engines are an even better example of how compression is pretty effective at raising power, though the fuel air delivery system has to support the demand of that power.
The down side to a high compression motor is its always high compression unlike the power added turbo/ supercharged, so idle and fuel efficiency is a crap shoot.
Ford literally did away with the flathead based on the fact that it couldn't be designed for the better fuel being produced and run higher compression.

It's not that compression costs horsepower, even in a two stroke engine.

In a single cylinder engine, the pumping losses on a high compression engine can keep the engine from making the rpm that the user is wanting to see. Horsepower is torque x RPM / 5,252 so, if the engine only makes 11,000 rpm, it can't make the hp of an engine that can turn 16k.

 

[Maintenance Chief]

It's not that compression costs horsepower, even in a two stroke engine.

In a single cylinder engine, the pumping losses on a high compression engine can keep the engine from making the rpm that the user is wanting to see. Horsepower is torque x RPM / 5,252 so, if the engine only makes 11,000 rpm, it can't make the hp of an engine that can turn 16k.

Ok that makes sense, I guess I just use a camshaft to dictate the rpms of my engines in vehicles.
Are the ports like valve timing of a 4 stroke?

 

[Mastermind]

Are the ports like valve timing of a 4 stroke?

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

 

[Al Smith]

Here's one I know because I dealt with it recently regarding RPM .A Partner p-100 is factory rated at 6 HP at what ever it runs which I don't know exactly .The very same engine was used on the K1200 concrete saw but because of the rev limited coil only made around 4 or so HP .Because it's the same parts it could be assumed the comp ratio should exactly the same there fore RPM's have to be the limiting factor .

 

[timberjack90]

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

 

[Mastermind]

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

I feel like 9500 - 11000 rpm is the sweet spot. Make the torque curve peak at 10500, and you just built a saw that a man can make a buck with.

 

[Nutball]

That thinking was the prevailing thought process on saw building for many years. It came from the two stroke motorcycle world where tuned pipes were used.

But.....over the years we found that a shorter exhaust duration seemed to build a stronger saw for actual work related use.

Now....I'm getting outside my comfort zone here. This sort of talk leads to disagreements between people almost as much as political and religious discussion.

And I was about to say, there's a lot I disagree with there.

 

[Mastermind]

And I was about to say, there's a lot I disagree with there.

A lot where that you disagree with?

 

[Nutball]

The AS quote.

I think too many variables have been left out, rpm and fuel/air volume intake at the very least.

 

[Maintenance Chief]

One thing that seems to be pretty universal is that high compression does beat the piss out of an engine . Bearings, rods, and any part that takes the blunt force of the compression . I think thats why power adders and variable valve timing became popular.
Im not sure if reed valve saws deal with this better.
I always think about the lowly 318 dodge Engines and there low compression, last forever but don't win any races.
I wonder if the Comet diesel chainsaws had ridiculous compression like a regular diesel engine?

 

[Terry Syd]

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.