Part Four: Compression

[Mastermind]

I have exactly zero experience with E85.

 

[mdavlee]

Saws will idle good with higher compression. I prefer about 175-190 for a good running saw. If torque is all the owner wants then I go higher or high elevation. I busted a starter on a 385 at my house that was over 220. It was an even 200 at 4500'.

 

[Moparmyway]

Would that be roughly about the diameter of the piston? More? less?

Just a little more than the piston diameter

 

[mdavlee]

Just a little more than the piston diameter

.05mm under or less

 

[Lee H]

Back in the day say early 80's Sachs Dolmar used dished pistons in there
50 and 60 cc saws. Not sure about the larger saws but those 50 to 60cc's
saws were great runners. They had power in the time.

My other thought on high compression is how will the upper and lower
rod bearings hold out. Seen lowers go on several saws over the years
and always thought is was over rev and lack of oil or crap oil. But with
the high compression you must be putting more of a load on them on
the up stroke.

 

[Deets066]

How about when You should or should not use a pop up. Pros n cons

Pop ups have their place, if you have a jug that is timed very well and you just want a compression increase then you should use a pop up.

If you want to lower the ex. As Randy said, then you want to cut the squish

 

[Dub11]

Is there going to be a " Part 5 how to save up cash to pay a porter" section?

 

[dustinwilt68]

Well Wayne, I find that trading is better for me.lol. but I like to barter and if booth parties end up happy, that's what counts.

 

[mdavlee]

Is there going to be a " Part 5 how to save up cash to pay a porter" section?

Step 1. Buy stihls in boxes
Step 2. Fix stihls
Step 3. Sell stihls
[emoji12]

 

[Dub11]

Step 1. Buy stihls in boxes
Step 2. Fix stihls
Step 3. Sell stihls
[emoji12]

Hell thats not panning out to good around hear ether lol.

 

[mdavlee]

Hell thats not panning out to good around hear ether lol.

Have to flip on Craigslist. This is a small market and we all want them dirt cheap.

 

[Dub11]

Have to flip on Craigslist. This is a small market and we all want them dirt cheap.

I put this one straight on cl. But I guess no one in Ne Kansas needs a saw right now.

 

[mdavlee]

I put this one straight on cl. But I guess no one in Ne Kansas needs a saw right now.

Saws are slow moving this time of year

 

[dustinwilt68]

Maybe try some Hooskies, people might finally be catching on.lmao

 

[drf256]

Compressing air (or charge in our case here) simply converts kinetic energy into thermal energy.

If you have an air compressor you know that the air gets warm when you compress it and cold when you let it expand. It's the theory of how refrigeration works.

So all compression is doing is adding heat (energy) to the charge so that it will combust faster and more efficiently. You want it at the point where it's nearly ready to burn, the spark then adds the drop of energy that tips it over into the reaction occurring.

The balance is what we want to know. When do we start to use more energy to heat the charge than the charge will create upon expansion? That's anyone's guess.

I believe that saws that cool more efficiently can take advantage of more compression than ones that don't. Ones that run hotter can make the same power with less compression. Smaller bore saws tend to have more cooling surface area vs. larger bore saws. I believe that is why they like more compression. It's also expends less energy to compress a smaller charge than larger ones.

We do need to cool the saw for other reasons though. We don't want the piston to melt, etc...

A husky 262 runs great with 230 psi. I prefer 210 on a Stihl 036. Both 48mm bores and the same stroke. The 262 has much more cooling area. An 036 should beat its pants off if the rest of the equation was the same, but the 262 has better port design. So again, it's balance that matters. But compression is part of it.

One last thing Compression does, it sells saws. Post a similar saw, one with 210 psi and one with 170. The 210 will sell faster, the bigger is better rule. Total BS.

 

[XP_Slinger]

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.

 

[Johnmn]

I put this one straight on cl. But I guess no one in Ne Kansas needs a saw right now.

What are you trying to sell?

 

[Dub11]

What are you trying to sell?

Just a 290 nothing special. I thought it was the baddest saw ever and only revealed it on special occasions like a samurai sword! Then I found this site and its for sale.

 

[Stihl working Hard]

Says the guy that doesn't own a compression tester...

LMFAO

 

[mdavlee]

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.

The highest power diesel engines are around 12.5-13:1 with crazy boost. Too much compression and a smaller chamber can lose power