Chainsaw Porting Theory

[Mastermind]

Still have one MS440 That Big Dave did,,, that saw to this day 9 years later never fails to put a a smile on my face... good seeing u posting wigs!!!! oh and GFY,,, and give the gun totin monkey a good kick in the sack for me!!!!

GFY too my friend!!

I'm slowly getting back into the saw game. Between work and mowing last year, I just needed some time off.

I'll be sure to kick him square in the giblets next time I see him....

And these are my friends?

Wow.....just wow.

I'll throw the word COMPRESSION into the game.

No doubt, the highest power output from a charge will be accomplished by the highest compression it can handle before detonation occurs.

But compression builds heat, that's it purpose. Heat, parasitic strains due to higher compression, and detonation are an engine's worst enemy.

Different engine designs can handle different maximum compressions.

Squish band velocity and chamber design play a role.

So what have you guys done in the real world?

I'm running my 036 at 215 psi. It's the saw I've cut the most wood with. I've had zero issues with it at that level.

My last few 026's have been 240 psi with no issues to date.

I know Mopar and JMSsaws run 066's in the 240 range.

Compression can slow a single cylinder engine down though....

I'm thoroughly enjoying the discussion.

Very different from the AS, where if discuss something and there is a disagreement, the lynch mob comes after you.

I'm gonna post freely here. If I'm wrong, tell me why. That way I can learn.

I'm right there with you Al. Loving the atmosphere of this place. Respectful.....that's what it is.

So I've come to learn that "4 stroking" occurs when the mix is too rich to fire. So that combustion cycle goes unburned, and then it mixes with some exhaust days and ignites on the next cycle. If it takes 2 cycles, then it's "6 stroking".

I'm having a hard time understanding why an engine goes from 4 stroking with no load to 2 stroking under load.

You lay the chain into wood, so now you have resistance.

Does the motor suddenly have more chamber heat to burn the mix better?

What's actually happening?

I have no idea.......but, I have other stuff I wanna figure out that takes center stage.

 

[Magic_Man]

@Mastermind I was thinking about compression and speed the other day. In a single cylinder saw the piston is fighting compression in both directions. Do the race guys sacrifice some compression on both ends to free up the movement of the piston ? I'm sure there would be a balancing point to be found between power and rpm.

 

[Mastermind]

@Mastermind I was thinking about compression and speed the other day. In a single cylinder saw the piston is fighting compression in both directions. Do the race guys sacrifice some compression on both ends to free up the movement of the piston ? I'm sure there would be a balancing point to be found between power and rpm.

Most race saws run less compression than I do in a work saw. Two different worlds though.....

 

[drf256]

Most race saws run less compression than I do in a work saw. Two different worlds though.....

Why?

 

[Al Smith]

To a point perhaps .Remember piped saws are usually pretty high ported and the static compression might appear to be low .However under dynamic conditions at resonance it will be rather high .Now how in the world a person could measure dynamic I have no idea.

 

[Mastermind]

Why?

Read Mr Smith's post just below yours. He nailed it......both for a piped saw, and a muffler saw on alky or nitro.

Dynamic compression and static compression are two completely different things.

If you can keep both primary, and secondary compression low, the engine will spin much freer.

Make the power from flow rather than compression, and it will hit the wood at say......17,000 rpm?

 

[Mastermind]

Joey (Stihlbro) and I fired up the ol pipe saw Friday.....it was a much better sounding saw with this new top end on it.

Lower static compression, fingers, bridges, smaller uppers, lightened piston, better port matching.....

Four stroking hard at 14,600....

 

[mdavlee]

I'm trying to have lower compression in some saws than others depending on what its goal is. Some at 165-170 are faster than those with 220.

 

[Keith Gandy]

It never really explodes. It burns. But it's semantics.

Yes, compress it hot enough and it will burn without a spark plug, just like a diesel.

The spark just adds a kernel of high temp to start the reaction.

So the closer you get to that critical temp of auto ignition, and the most charge you have to burn, you'll get the most heat and most gas expansion to drive the piston down.

Ive seen that! A 460 started running 1/2 throttle on its own and I hit the kill swich, then pulled the plug wire! Still running!! Had to choke her to kill it

 

[drf256]

Read Mr Smith's post just below yours. He nailed it......both for a piped saw, and a muffler saw on alky or nitro.

Dynamic compression and static compression are two completely different things.

If you can keep both primary, and secondary compression low, the engine will spin much freer.

Make the power from flow rather than compression, and it will hit the wood at say......17,000 rpm?

Besides a pipe, or alternate fuel, why would lower static compression work better?

In what instances would lower compression be desirable for performance?

If one exists, whats the break even point for compression increase vs. parasitic loss.

 

[MustangMike]

Two things, and just my opinion:

1) 4 stroking cleans up under load because the RPMs drop, and the charge has more time to burn.

2) Race saws do not need the same compression because the ports (etc) have been modified to bring in more fuel, just like you run lower compression in a SC car engine, the additional fuel in the chamber has the same effect as increased compression.

Great Thread!

 

[Motorhead]

Ive seen that! A 460 started running 1/2 throttle on its own and I hit the kill swich, then pulled the plug wire! Still running!! Had to choke her to kill it

Carbon build up will do that sometimes.

 

[Stihlbro]

I've been thinking about this thread for the last couple of days. I can't seem to get my words to come together right. But here goes, porting theories are kinda the processes used to enhance the performance of a chainsaw.

Most on this site can get the easy gains with muffler mods and deleting a base gasket. Those two thing alone can net a increase in power. Properly tuning a saw and understanding it is a must. Knowing what a sharp chain is really the first theory/process that should be learned first. I'll never forget that I went for the power before the chain in the beginning. My first muffler mod, I actually thought the saw was stronger to later learn I was just making noise.

Compression.......ah hem!!! All I can say is that it becomes part of the drive ability of the saw once changed. It creates power and often is a direct reflect on what style of porting used. Your geographical location can affect how much compression your saw will run best on. It's a fact no matter what I get here, it's always more in Kentucky. Lol. Remember the stock saw was originally built to be sold all over the world. It's a safe zone for power, starting and longevity. But has that not changed in some of the more recent new saws?

Back to the processes used, ultimately modifying a chainsaw, we are altering port timing to get better cylinder fill to obtain more power. We are undoing the restriction imposed from manufacturers to meet epa guidelines. The more fresh charge in the combustion process the more power. To many differences from saw to saw, and really it comes down to the personal preference of the operator or you are at the limit by what your chain will allow. Power gains are derived by experimenting and finding what works on a said saw. The first improvements are typically easy. The last few ounces of power is what is hard. To often the thought of getting the maximum power is the theory, but the reality of getting your expectations is the answer.

I relate modifying chainsaw to vehicles a lot, actually too much. But it's how I can relate. Dont we all not want a truck that can pull like a Mack, drive like a Cadillac but yet get mileage like a Geo? Do we not want the performance of sports car, with the room to carry a family of six? Does it make sense to have the fast 1/8 mile time to go to the store for a gallon milk?


Porting process are around what gives the saw the expectations you want. Numbers used are references. Sharing those numbers are how we relate to the gains to what is found that works.

 

[Keith Gandy]

Carbon build up will do that sometimes.

New built saw but it was running about 230 on compression with and the piston was close to the plug as I had cut too much from the squish and base. Alil freeport goin on too

 

[Mastermind]

Besides a pipe, or alternate fuel, why would lower static compression work better?

You know I like a higher compression ratio than some builders do. But experience has taught me that there are no one size fits all recipes.

For instance in a milling saw....long cuts, and extreme heat. This is one example where excessive compression would be counterproductive.

In what instances would lower compression be desirable for performance?

In a saw that rpm is preferable to torque.

If one exists, whats the break even point for compression increase vs. parasitic loss.

That's the million dollar question.

Smaller bores seem to "enjoy" more compression than bigger bores......

But I still don't have any solid answers. I'm ok with saying I don't know.

 

[Motorhead]

New built saw but it was running about 230 on compression with and the piston was close to the plug as I had cut too much from the squish and base. Alil freeport goin on too

No room for carbon in that one.

 

[Stihlbro]

High compression/parasitic loss--take said saw and make multiple cuts back to back and hear the saw slow down with more cuts is the loss.


Take a saw with high compression and relieve some compression and the saw frees up and cuts faster......parasitic loss.

 

[drf256]

High compression/parasitic loss--take said saw and make multiple cuts back to back and hear the saw slow down with more cuts is the loss.


Take a saw with high compression and relieve some compression and the saw frees up and cuts faster......parasitic loss.

So you're saying that higher compression is a negative in most work saws?

 

[jmssaws]

I have tried making cuts back to back to see if i could get it to slow down but it didn't, probably not enough compression for it to work.

 

[jmssaws]

So you're saying that higher compression is a negative in most work saws?

A worksaw is a broad title,I think a worksaw is what you take to work,simple as that.
Will a high compression saw last forever cutting timber? No but neither will a stock one,I worked 6 days falling with a 066 that's 240lb of compression and I didn't think it would last but it did just fine.

I look at it like this, if your worried about how long your saw is going to last maybe you should leave it stock.

I haven't seen parasitic yet but I've only been to 240,maybe Scott has.