Chainsaw Porting Theory

[riverrat2]

One of the most intriguing saws I've owned was a DN 154SE. Wish I still had it. Made me wonder, that one did....

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!!!!

 

[wigglesworth]

Still have one MS440 he 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....

 

[Dieselshawn]

I wish diesel pickups were as easy as chainsaws to make power. Put higher flowing injectors in, advance timing, now we have to upgrade the clutch or automatic trans to start. I have grenaded enough parts already in my truck.

But I still want more power... next is turbos, even larger injectors.... when does it end? I smell smoke coming from my wallet.....

 

[Brush Ape]

Between work and mowing last year, I just needed some time off.

Wiggs that's no excuse. I mow every week.

 

[Miller Mod Saws]

Do you guys set quads higher or lower than duals? I seem to keep them around the same height as the duals,just wondering what you guys have found.

For instance do you change your transfer numbers much between a 394 to a 395?

Not sure if this has been answered just now getting to read this thread. I run 17° blowdown on a 394 and 20° on a 395. IIRC should go look at my notes. Lol.

 

[Miller Mod Saws]

Probably the offline version. [emoji41]

The 661 has the exhaust side of the main opening first. I tried it in a 7900.

How did it work on the 7900. Been thinking of doing the same.

 

[jacob j.]

One of the most intriguing saws I've owned was a DN 154SE. Wish I still had it. Made me wonder, that one did....

I got inside a DN 272 a few years ago to rebuild it for a faller near here. It was an excellent runner and had the stock muffler (the faller was working state ground and couldn't have a modified muffler).

It had a much different carburetor though- I think one off a 394.

 

[mdavlee]

How did it work on the 7900. Been thinking of doing the same.

It seemed a little better than flat.

 

[Miller Mod Saws]

Finally got this far read. Excellent discussion.
You will find no greater asset then this website and the combined knowledge of the many!

 

[Deets066]

Finally got this far read. Excellent discussion.
You will find no greater asset then this website and the combined knowledge of the many!

Amazing how much knowledge has been passed around about porting in the last month alone.

 

[drf256]

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.

 

[treesmith]

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.

Do you plug the decomps?

 

[drf256]

Do you plug the decomps?

I have on some builds. Left on others.

Decomps become useless after around 180 psi of cranking compression. They just pop closed after the first engine cycle. They can help with the inertia of getting your pull going.

At least that's been my experience.

 

[Al Smith]

Well now that's a point to ponder .The question is can you really get detonation,diesel like ignition on a two cycle?If so other than use some means to crank it other than a rope how could you spin it up ?

I know it gets talked about a lot but I for one have not been convinced it really is a problem.Then again theories and opinions vary widely over a lot of things.

Then again it might be possible if the mixture was extremely lean and the trapped compression was really high.
On modern auto engines they use knock sensors to combat pre ignition .They moniter this 20 times a second and adjust the fuel injector to a richer state if a knock is detected.
Now this I do know because I've had it happen .If you have a saw running ballz to the wall and have a teeny little tear in the boot which goes undetected and suddenly that tear opens up the engine will fart and hiccup to beat the band .Sounds like it will blow up any second .That's just simpley from going extremely lean from an air leak .

 

[drf256]

You can certainly get detonation in a 2T motor.

Fuel in every engine ignites itself. With the proper air fuel ratio, when a charge is brought to a certain temperature, the carbon bonds break and combine with oxygen. Compression increases the heat in the charge to get it closer to that temp. The spark simply increases the temp and the chain reaction occurs.

The key is to get it to occur at the right time and symmetrically. If you have sharp edges, hot spots or areas of less charge movement-there is a greater chance of heat in that area causing pre ignition of the charge before the spark plug fires.

Squish bands create velocity and swirl in the mixture. More swirl means more homogenous charge. Less setup for detonation with the same comp pressure.

Heat=energy. So if you're making more power, you're making more heat. Energy can never be destroyed, it just changes forms. In an engine, we are converting thermal energy to kinetic energy.

 

[Al Smith]

Agreed for the most part .However the mention of carbon hot spots ,lean mixture etc adds to the equation and is not auto/diesel ignition.It is ignition though .

Now reexamining this thing in another light would it be possible with high enough compression and low ignition temperatures of the fuel if under those conditions rather than preignition the mixture became explosive rather than just burn fast?

 

[drf256]

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.

 

[drf256]

On modern auto engines they use knock sensors to combat pre ignition .They moniter this 20 times a second and adjust the fuel injector to a richer state if a knock is detected.

I think the knock sensors actually trigger the computer to retard the ignition timing. I'm not sure if it makes the computer richen the mix.

 

[Al Smith]

Depends on how you define explosive and rapid combustion .--long drawn out copy and paste but it defines the phenominum in a nut shell ---
Engine Basics: Detonation and Pre-Ignition

Written by Allen W. Cline

Reprinted from Issue 54 of CONTACT! Magazine, published in January, 2000

All high output engines are prone to destructive tendencies as a result of over boost, misfueling, mis-tuning and inadequate cooling. The engine community pushes ever nearer to the limits of power output. As they often learn cylinder chamber combustion processes can quickly gravitate to engine failure. This article defines two types of engine failures, detonation and pre-ignition, that are as insidious in nature to users as they are hard to recognize and detect. This discussion is intended only as a primer about these combustion processes since whole books have been devoted to the subject.

First, let us review normal combustion. It is the burning of a fuel and air mixture charge in the combustion chamber. It should burn in a steady, even fashion across the chamber, originating at the spark plug and progressing across the chamber in a three dimensional fashion. Similar to a pebble in a glass smooth pond with the ripples spreading out, the flame front should progress in an orderly fashion. The burn moves all the way across the chamber and , quenches (cools) against the walls and the piston crown. The burn should be complete with no remaining fuel-air mixture. Note that the mixture does not "explode" but burns in an orderly fashion.

 

[drf256]

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.