Compression Discussion

[Magic_Man]

I would like to discuss compression in 2 stroke engines without insulting anyone or anybody getting red-assed(my word for butt hurt) . My background is mostly in 4 stroke engines. As I am learning more about 2 strokes and how they work I find it interesting that so many put so much value in cranking compression test pressure readings. It seems there are so many variables in a 2 stroke that would contribute to cranking pressure variables.

Things like ring and cylinder condition, port timings, and even case compression all come into play. Am I also right in thinking that with the case pressures and scavenging effect that the cranking pressures would actually increase with rpm ?

What made me thing about this is that when we used to build v8 engines we would calculate the static compression at let's say 10:1. Once a cam was installed actual cranking compression would be less depending on intake and exhaust openings. In actuality the higher the performance cam(more agressive intake and exhaust timing), the lower cranking compression would be. It seems this would be the same in the 2 stroke, more agressive timings would lead to lower cranking pressure numbers.

 

[Stihlbro]

2 stroke porting can be related to selecting cam shaft for 4 stroke.

Compression makes power.

Compression builds heat.

Porting affects/effects compression.

On a 2 stroke, it is once the exhaust port close, it has trapped what it can then continues to compress. The higher the exhaust the less trapped volume.

 

[TJ the Chainsaw Mechanic]

What he said.

 

[Magic_Man]

2 stroke porting can be related to selecting cam shaft for 4 stroke.

Compression makes power.

Compression builds heat.

Porting affects/effects compression.

On a 2 stroke, it is once the exhaust port close, it has trapped what it can then continues to compress. The higher the exhaust the less trapped volume.

That's what I thought , so how does case compression come in to play ? Is there a gain in cranking compression there or does all the pressure get blown out the exhaust?

 

[Stihlbro]

One more though, being you compared this to a automotive engine. Modifying a engine for a drag car is not a ideal for a dump truck.
Modify a saw to go fast is not the best choice milling.
I can relate modifying a 2 stroke to what I call driveability.
Automotive world,compression is limited to what "gas your wallet can affort."
Chainsaw two stroke compression can be determined on what your plan is for the saw.

Case compression the way I understand it, once the intake closes and the pressure that is built up in crankcase before the transfers open. But.....case volume is a direct affect, which every saw is different. It's really a balancing act to get performance gains. You gain in one area but loose in another.

 

[Magic_Man]

One more though, being you compared this to a automotive engine. Modifying a engine for a drag car is not a ideal for a dump truck.
Modify a saw to go fast is not the best choice milling.
I can relate modifying a 2 stroke to what I call driveability.
Automotive world,compression is limited to what "gas your wallet can affort."
Chainsaw two stroke compression can be determined on what your plan is for the saw.

Case compression the way I understand it, once the intake closes and the pressure that is built up in crankcase before the transfers open. But.....case volume is a direct affect, which every saw is different. It's really a balancing act to get performance gains. You gain in one area but loose in another.

That I understand, that's why I said the higher performance cam actually reduces cranking compression. A stout RV or towing cam in that engine may generate let's say 225 psi where a drag cam may only crank 180 psi. So yes I under stand that correlation in that the higher psi numbers most likely are somewhat relative to high torque timing numbers.

Case compression is a new idea to me obviously. I under stand what it is and kind of how it works in very simple form. I'm just not sure how or even if case compression affects combustion compression, and if it does, do case compression and cranking compression numbers rise with rpm due to sharper port cutoff times and scavenging effects ?

 

[Mag Craft]

There is also the combustion chamber itself to take into consideration. One of the things that builders do is to cut the top of the combustion chamber to make it more flat and also to decrease the size of the chamber. There is a point though if the chamber becomes to flat and compression to high that pre ignition can occur at the very edges of the chamber. So like most things there is a limit.

 

[ABarrick]

I'm not a 2-Stroke guru but in my opinion, static compression has more to do with the design of the combustion chamber and health of the engine components. If the rings are making a good seal and you make a change to alter the combustion chamber size, the compression is going to change accordingly. Smaller squish/chamber should yield a higher compression reading. This does not guarantee a higher running compression. Your running compression takes into account other factors like scavenging and cylinder filling which are a result of port timing and area, cylinder design etc. Take a 346xp for example. I have one with roughly 180psi compression and had another one with 215psi compression. The saw with less compression was/is a much better running saw, thus I still have it and sold the other.

 

[Stihlbro]

I'm not a 2-Stroke guru but in my opinion, static compression has more to do with the design of the combustion chamber and health of the engine components. If the rings are making a good seal and you make a change to alter the combustion chamber size, the compression is going to change accordingly. Smaller squish/chamber should yield a higher compression reading. This does not guarantee a higher running compression. Your running compression takes into account other factors like scavenging and cylinder filling which are a result of port timing and area, cylinder design etc. Take a 346xp for example. I have one with roughly 180psi compression and had another one with 215psi compression. The saw with less compression was/is a much better running saw, thus I still have it and sold the other.

Excellent post!

 

[drf256]

That's what I thought , so how does case compression come in to play ? Is there a gain in cranking compression there or does all the pressure get blown out the exhaust?

There's no 100% rule for all engines, that's the problem.

More case compression shouldnt affect measured static compression at starting rpm IMHO. It probably does affect dynamic compression though. That's more seat of your pants feel.

It's a bit different than 4 stroke cranking compression where intake closing is everything. You don't really get to bleed off high cranking compression in a 2 stroke like you can with a big cam.

 

[jmssaws]

Compression is a peice of the puzzle but you still need port harmony or your not going to make power.

I hear that people have seen 185lbs saws hitting the wall with compression and gaining when they loose some, it's not the compression making the problem.

2 saws with the same numbers but one with more compression can easily be out cut by the lesser saw I've seen it many times but it's not beating it because of less compression it's beating it because it has the right amount of compression for the numbers so it's in harmony.

As compression goes up the exhaust needs to go up with it or the saw feels "bound" . It's not a compression problem it's a timing problem.

 

[Al Smith]

Now here's where the cheese gets binding .In some instances it's benefical to bump the ignition timing an RCH .It's been said it can be detrimental on a high compression engine .

Then the subject of compression itself .Is this compression ratio or trapped compression ,similar but not exactly the same .

Take for example a Husqvarna 2100 .If there was ever a high comp saw engine in stock form that's one .If there was ever a hard saw to pull over to start it set the bar .I've never heard of anyone lowing the comp on one .

 

[jmssaws]

As much ignition and compression as your fuel will allow you to run will make the most power as long as you have your ducks in a row.

A 395xp is a high compression saw from the factory and is the king of stock saws,mine was 195 stock which is more than alot of ported "worksaws". It takes alot of compression to be detrimental to a saw.

 

[drf256]

Now here's where the cheese gets binding .In some instances it's benefical to bump the ignition timing an RCH .It's been said it can be detrimental on a high compression engine .

Then the subject of compression itself .Is this compression ratio or trapped compression ,similar but not exactly the same .

Take for example a Husqvarna 2100 .If there was ever a high comp saw engine in stock form that's one .If there was ever a hard saw to pull over to start it set the bar .I've never heard of anyone lowing the comp on one .

"Compression Ratio" is a misnomer on a 2 stroke. Swept volume and trapped volume play into it. On a 4 stroke, you have a cylinder displacement that compresses into a chamber displacement. So there is a ratio.

Its more "Compression pressure" on a 2 stroke.