Part Four: Compression

[paragonbuilder]

Could you have a dished piston turned to drop the compression easily with no other changes to the jug or port timing?

Sure it could. How would that affect transfer flow?

.

I'm not sure, but I have a husky 350, with 100 psi, that has a dished piston, and with just a muffler mod, is impressive...
I keep asking myself why???
One day I will study it better...


Sent from my iPhone using Tapatalk

 

[Stump Shot]

Yes, I've CC'd chambers, but I can check the compression of a saw with good ring seal and know how big the combustion chamber is with a little math. Then with a little more I can figure out how much to take out of the squish to get within 5-10 psi of my target. It's all the ideal gas law, pv=nrt... and nrt is constant.



I shoot for 0.001" clearance. I have a few that are right on the money and sometimes I have to heat up the cylinder to get it to slip on the mandrel. I also drill and tap the center of each mandrel to hold sand paper on the end to increase the friction with the squishband, that way you don't need to put a lot of compression on the cylinder with the live center to take heavier cuts.


I usually run around 200-215 psi in most saws. I've even gone as far as to drill out decomp valves to make them easier for people to start.

Sounds pretty good, could you un - abbreviate - PV = NRT ?

 

[srcarr52]

Sounds pretty good, could you un - abbreviate - PV = NRT ?

Pressure * Volume = Number of molecules * R (universal gas constant) * Temp.
If nr is constant and if we assume the temperature is constant then:
P1*V1 = P2*V2

Something to note is that the pressures are in absolute. So P1 is atmospheric pressure, which is 14.7 psi at sea level. The reading from your compression gauge is actually psig (gauge pressure) which is referenced to the atmospheric pressure, so you'll need to add one atmosphere to the reading.

 

[Cobby08]

Compression as in compression shorts?

 

[Mastermind]

Compression as in compression shorts?

Don't make fun of my "Man Panties".

 

[Stump Shot]

Pressure * Volume = Number of molecules * R (universal gas constant) * Temp.
If nr is constant and if we assume the temperature is constant then:
P1*V1 = P2*V2

Something to note is that the pressures are in absolute. So P1 is atmospheric pressure, which is 14.7 psi at sea level. The reading from your compression gauge is actually psig (gauge pressure) which is referenced to the atmospheric pressure, so you'll need to add one atmosphere to the reading.

Thanks, will be putting that in my notes.

 

[t4driller]

My brain is starting to hurt... but I like it!!

Sent from my SM-G930V using Tapatalk

 

[Stump Shot]

I'm not sure, but I have a husky 350, with 100 psi, that has a dished piston, and with just a muffler mod, is impressive...
I keep asking myself why???
One day I will study it better...


Sent from my iPhone using Tapatalk

Some two stroke racing engines were set up like that for snowmobiles years ago. The less resistance approach.

 

[Dub11]

Most people that talk to me about compression are worried about being able to start their saws, if I increase compression, there had better be a release on the cylinder, or most folks flat out don't want it. So, one saw that I built, did not have a release, I actually decreased compression, with the result of a lower RPM than factory spec. After the saw had been tried out and the resulting discussion, I offered to put a little more pep back into it. The new owner adamantly refused citing "while it does go slow, it's like a tractor motor and won't slow down when it's in a log, and it starts real easy". He was happy as a clam in mud, so just when you think that you know a thing, go figure...

If I have too stand on the saw with both feet and use both hands to start that what I want lol

 

[Definitive Dave]

compression is evil
carry on

 

[t4driller]

compression is evil
carry on

Compression and too much timing advance is evil..

Sent from my SM-G930V using Tapatalk

 

[Chainsaw Jim]

Compression ratio lessens when squish is tightened. There should be a sweet spot.

I don't hear much about octane levels in relation to compression. That seems very political when bought up when it's actually science.

 

[Four Paws]

In my experience detonation has to do with combustion chamber design and ignition timing.

I personally don't worry about measured compression an awful lot. I like a saw that has torque and has good manners. I like to see 185 psi or more. I would say my "ideal" value on my gauge is about 205 psi at my elevation. I find that the saws make good power there and still have good manners.

There is something to be said about a saw that will spool quick and rev free in your hands - they sound different and feel different - when you run one, you will know. It takes everything working together to yield the desired result.

 

[Chainsaw Jim]

In my experience detonation has to do with combustion chamber design and ignition timing.

I'll add poor quality fuel and maybe some water to that.

 

[Al Smith]

Now think about a stock flat top piston .Now think about cutting about 1/4-3/8" off the outside about 20 thou deep .Would that raise the compression ----No ---What it would do is change the flame propagation over the top of the piston .

As for me I have no idea how much compression most of my saws have,never tested them .However I have a D-handle on several if that means anything .

 

[Backtroller]

There is something to be said about a saw that will spool quick and rev free in your hands - they sound different and feel different - when you run one, you will know. It takes everything working together to yield the desired result.

This is what I'm chasing when I get a saw done. Never experienced it until I received my first ported saw, my MOFO 036.

 

[Hedgerow]

Compression:

I like to have enough.

But no more.

 

[Al Smith]

For tooling. How snug of a fit are the mandrels for the lathe. Do you turn them down so they are a snug fit or a little loose.


Sent from my SM-J320W8 using Tapatalk

I can't find the picture but I use a home made arbor in a collet chuck .1" piece of cold roll threaded 3/4" 10 with a screwed on arbor made of hard nylon .They fit tight enough I have to tap them on with a small mallet . The cylinder runs true . The arbor is made to such a length it sets fully seated about 1/4" or so inside the cylinder and I cut the chamfer on the lathe

 

[Deets066]

190-215 psi is where my saws usually end up. Seems to work well.

I'll pass on the decomps, I plug em all

 

[T.Roller]

190-215 psi is where my saws usually end up. Seems to work well.

I'll pass on the decomps, I plug em all

Yeah I didn't use the decomp on my 395 one time after Randy ported it and ended up replacing parts so I'll stick with decomps on large cc high compression saws.