Part Four: Compression

[Termitebuffet]

You can use the word "gooder" here Randy.

"Whispers" I didn't think they'd know what I mint dot dot dot [emoji2]

 

[Mastermind]

"Whispers" I didn't think they'd know what I mint dot dot dot [emoji2]

I see......and you may be right.

 

[Stump Shot]

You can use the word "gooder" here Randy.

See, you're catching on...

 

[huskihl]

So far, I like compression. It's the one thing I can do that I know for absolute certainty will add power to a stock saw.
Everything else? Not so much...well, maybe

 

[Termitebuffet]

So far, I like compression. It's the one thing I can do that I know for absolute certainty will add power to a stock saw.
Everything else? Not so much...well, maybe

Sticker....sticker = mor powar.....lol [emoji2]

 

[huskihl]

Sticker....sticker = mor powar.....lol [emoji2]

These are on most of mine

 

[Termitebuffet]

These are on most of mine

View attachment 68940

Nice ....on the above pic I sent extra stickers ,back with the saw more power on trucks too....[emoji2]

 

[49er]

I like about 180 to 190 PSI compression. Good performance, it allows the saw to live and most importantly it is easy to live with.

I had a saw in the 240 range and after making a long cut with a 32 inch bar berried the saw kept running after the switch was pushed off. I had to pull the choke out to kill it. I used another saw the rest of the day. When I got home and looked things over I found the the nut had come of the band on the rubber manifold. Nothing was damaged internally. I was running Av-gas and it still ran on detonation.

 

[jake wells]

low compression can make power in two cycles
my 3.5 hp lawn boy only has 90 psi compression and that is spec.

 

[Jimmy in NC]

Compression is good ,extreme compression is extremely good ....imho [emoji200]

Making your buddy hit himself in the face trying to start extreme compression is extremely funny... especially if the brandy new AM elasto-start breaks on pull three.


Stihl components aren't made for Husky power..

 

[Termitebuffet]

Making your buddy hit himself in the face trying to start extreme compression is extremely funny... especially if the brandy new AM elasto-start breaks on pull three.


Stihl components aren't made for Husky power..

He he he , I can neither confirm nor deny said events ....lmao

 

[Stump Shot]

Has anyone tried to use an L - shaped top piston ring?

 

[XP_Slinger]

Has anyone tried to use an L - shaped top piston ring?

Never seen one. What applications would one normally find that type of ring? What's the benefit?

 

[smokey7]

Seen alot of them in old outboards and some old tractors. I think it had something to do with surface area for the gases to push behind to seal the bore. Alot of new pistons come with standard rings. If you want nostalgia you must order nos parts.

 

[Nutball]

Yes Sir, the whole trick to the game be it saws, 250r's or Top Fuel Dragsters is to run as low of octane as the engine can run without detonation to make the most power. You can always run higher octane than the minimum required and it won't hurt a thing, but due to the nature of higher octane fuels that have higher flashpoints, slower burn rates, and remain stable to higher compression ratios, you will actually lose a little bit of power and performance. Go the other way around and dump 87 oct into an engine that requires 110 oct minimum and things will get nasty for your engine and your wallet in a hurry. I love hearing people tell me how they tried some 110 oct in their stock (fill in the blank) that requires 87 oct minimum and they could "feel the difference". LOL!

The car I drive now doesn't call for any special octane rating, but it's clickly/sizzly on 87, and doesn't make those subtle sounds on 93 while gaining a few MPG and having noticeably more torque. I still tend to just run it on 87 as it's cheaper.

Could high octane gas be advantageous for getting high torque at low rpms since there's more time to burn fuel? Might also burn cleaner than just running low octane rich?

Good post Doc. In addition to heating the charge closer to combustion temp, higher compression also gives you a harder push on the piston. Which gives the crank more inertia to keep rollin once the burn is completed. The end result being a very real increase in torque. Extreme example would be Diesel engines. Ridiculously high compression with torque typically double its HP output but a low rpm range. Not trying to side track the thread with rattlers, just an example. I think this is a given that most understand but figured I'd throw it in here. Finding the right amount of compression for a given saw is like everything else in this series, we have to experiment to find that balance.

More compression should also speed up the burn rate correct? Because the density of fuel/air molecules is higher, closer together, so the flame travels faster. Theoretically adding surface area such as somewhat large grooves to the combustion chamber can increase the burn rate too because the groves will cause turbulence helping to keep the fuel & air well mixed and throwing the flame ahead faster. See the methane in sewer explosion episode of Mythbusters to get an idea of where my idea comes from. Though adding surface area would not be ideal for a chainsaw engine.

Rule of thumb for making power. High compression for gas motors and low compression for diesels.
Play with a few diesels back in the day. Used to lower comp a bit so you could get more fuel without
increasing cylinder pressures. Also play with cam timing. Especially on the 855 Cummins with cam
operated injectors. Slow the cam down which is retarding. That lowered cylinder pressures a bit and
then you can add more fuel. More fuel, more air, more power. Had some mean running Cummins
back in the day.

Something I was wondering recently, how do you get the most power with the most durability? Low compression and lots of fuel, or would high compression and less fuel do the same? Of course there is a max pressure number on the parts that you don't want to exceed as it will cause certain failure, though that precise number could get complicated considering materials tend to withstand more pressure if the duration is shorter since there is less time to squeeze or stretch materials far enough that they fail. So really short peak pressures might just pass as a shockwave, but then there's fatigue to consider from all that shock beating.

To use high compression and experience extreme peak pressures around TDC or to use low compression and compensate with more fuel to have a high overall average pressure? More compression should have a higher average pressure over the whole power stroke anyway, so would low compression and more fuel just be a waste of fuel? I guess one should aim for the most compression till detonation becomes a problem, then go with more fuel and air from there?

Also I wonder at what point do you guys experience a lot of failures? Can you say XXXpsi is about the limit to get at least a year out of a work saw, or XXXpsi is about the limit to get 20 races before blowing the engine crank or bearings?

 

[deye223]

Some saws just gotta be drop started.

Hey Randy I don't know if you remember porting this saw for me (MS460) you gave it 225psi .
And I love using it , it has not missed a beat I think you did in 2012 2013 .

 

[rattler]

 

[Mastermind]

Hey Randy I don't know if you remember porting this saw for me (MS460) you gave it 225psi .
And I love using it , it has not missed a beat I think you did in 2012 2013 .

I remember. I did a 261 for you too.

 

[deye223]

I remember. I did a 261 for you too.

Yes sir and I was smiling on Sunday I love using those saws that 460 will kick a 660s butt even with a 32"b&c .


After reading this thread I was wondering seeing that 261 was the 1st one you did did you advance the timing and if not do yo thing it would be worth doing it .......

Cheers D

 

[Mastermind]

Yes sir and I was smiling on Sunday I love using those saws that 460 will kick a 660s butt even with a 32"b&c .
View attachment 181539

After reading this thread I was wondering seeing that 261 was the 1st one you did did you advance the timing and if not do yo thing it would be worth doing it .......

Cheers D

I did not. And yes....it needs it.