Compression question

[Chainsaw Jim]

So I'm trying to wrap part of my head around this whole compression thing that seems to have a few differing opinions and or reported results.
I took two 5 cube saws and comp checked both to compare something. One maxes out at 240psi the other maxes out at 150psi. The 240psi saw consistently reached 90psi on every first pull while the 150psi saw consistently reached 95psi on every first pull. Same length pull rope and it seemed like each of the two turned over an equal number of times per pull.
Does this mean they have the same running compression?

 

[Deets066]

Good question Jim, I'm sure someone will provide an answer that I can't understand

 

[Deets066]

I'm sure when spinning 15,000 rpm it all changes from what you can pull over by hand

 

[jb-chainsaws]

Yeah this whole thing hurts my head a little so I try not to think about it too often, however I'm interested to see what the more knowledgable folks on here come up with

 

[Outback]

The 90/95 is just the air pressure your able to generate in the area between the schrader valve in the end of the hose on the cylinder and the gauge end of said hose. Think of it like air compressor, compressor tank. The next pulse from the "compressor" cylinder, fills the "tank" hose a little more each time as each subsequent pulse has to pump against the 90 psi, or more, already in the hose held behind the schrader valve.

As for your 90/95, that's probably just how hard/fast you pulled on each saw for your 5 points difference. Your 5 psi might be intake restriction related too. Are you testing between the same piston/stroke sized saws? Shouldn't have any relation to dynamic, running, compression.

 

[czar800]

 

[Chainsaw Jim]

The 90/95 is just the air pressure your able to generate in the area between the schrader valve in the end of the hose on the cylinder and the gauge end of said hose. Think of it like air compressor, compressor tank. The next pulse from the "compressor" cylinder, fills the "tank" hose a little more each time as each subsequent pulse has to pump against the 90 psi, or more, already in the hose held behind the schrader valve.

As for your 90/95, that's probably just how hard/fast you pulled on each saw for your 5 points difference. Your 5 psi might be intake restriction related too. Are you testing between the same piston/stroke sized saws? Shouldn't have any relation to dynamic, running, compression.

I used saws with different strokes and bore sizes on each end of the 5 cube range. I gave each about twenty or so one pull readings with open throttles.
This should eliminate any results that would make it conflicting due to the same exact volume being tested.

 

[plcnut]

I am not a saw mechanic, but I would think that a high reading after many pulls is going to reflect that you have a high quality ring seal. A higher reading after 1 pull is going to reflect that you have a lower exhaust.
Does this thought process line up with what you are seeing?

 

[drf256]

Interesting question.

Too many variables to cull anything scientific from your observation.

My guess would be that the lower compression saw is easier to pull over, so you got more speed and filled the dead space in the compression tester tube faster. Even though you only reached 90-95# in the tester dead space, the saws were still pulling over at their respective compressions and venting a small amount of that through the tiny shrader valve in the tester.

I don't believe you could make any assumptions about dynamic compression from your observations.

Just my $.02

 

[Fruecrue]

On the first pull, at 0 psig, the volume of the cylinder, above the exhaust port is the volume to be compressed. The variables would be clearance volume (that's the terminology used in a refrigeration compressor anyway) of the head above the piston at TDC and the volume of your compression tester.

If original displacement volume differs at all, initial gage reading would also differ.


The difference in clearance volume, determined by many factors, including squish and combustion chamber shape, is what ultimately gives you the final compression #.

After several pulls, when the tester (with check valve) is already up to pressure and no longer needs to be filled, your compression becomes more a accurate representation of each stroke.

The saw with lower final compression would tend to roll more than its high comp. friend. more strokes, higher gage reading on first pull.

This is how I sees it anyway, but I've been known to make chit up as I go.

 

[paragonbuilder]

I am not a saw mechanic, but I would think that a high reading after many pulls is going to reflect that you have a high quality ring seal. A higher reading after 1 pull is going to reflect that you have a lower exhaust.
Does this thought process line up with what you are seeing?

I think this makes the most sense.
[emoji106]

 

[Stump Shot]

Interesting subject! I would agree with the too many variables to really nail this down to one thing or another here. I would also be curious though, as to how the two saws in question ran against each other head to head. Personally, when I landed here I was very surprised as to how much emphasis was placed on static compression as a performance indicator, having only used compression ratio as a guide in all my years prior. Thinking about it you could do the same test on a V8 engine and not get the same results on every cylinder that should be exactly the same. Another thing to ponder, an old school 2-stroke racing mechanic told me that he was only interested in how much could be made in the first roll over(pull). For myself I only drag out my compression tester to see if there is enough to run. When changing the ratio and I know everything is mechanically sound, I let the chips fall where they may, and don't even care to know. Surely this is an unpopular thing for me to say here, but it's just how I go about things, doesn't mean that the way others go about them are any better or worse, we'll probably not live long enough to decide who's right/wrong anyway.

This thread does however make me wonder if CC'ing the combustion cambers of these little motors would bear fruit and lead us to a ratio that would be preferred over others, so one did not go beyond a tipping point in power made. I'm afraid this too would be very difficult to nail down. I guess if it were easy, it would not be so darn interesting. SS

 

[czar800]

I am not a saw mechanic, but I would think that a high reading after many pulls is going to reflect that you have a high quality ring seal. A higher reading after 1 pull is going to reflect that you have a lower exhaust.
Does this thought process line up with what you are seeing?

I wonder if one cylinder has a little more oil on the rings then the other?

 

[Chainsaw Jim]

I wonder if one cylinder has a little more oil on the rings then the other?

No, I did the best I could to be even between the two during the event. I didn't spin one faster or harder than the other. I actually pulled both of those ropes so many times I pretty much eliminated myself as a variable.

 

[Chainsaw Jim]

One variable I can start with is shortening the compression tester hose. If the volume is small enough on the tester there should be a more accurate one pull readout.

 

[plcnut]

A short piece of 1/8" sch 160 pipe may work well. There should be a minimal amount of air to compress.

 

[sawfun]

Static vs dynamic compression. I knew a guy with a stock 12:1 427 L88 Chevy who's cranking compression was 90 lbs vs 150 for my 454. However that L88 would spank my Chevy as the cam in 4 strokes, which is akin to port timing in two strokes, allowed for less cranking pressure due to its overlap profile. It sure didn't seem to effect it's running compression though. I've read that some of the saw racers use less static compression in their saws. Likely the energy lost in compressing vs gains found elseware come into play.

 

[Brush Ape]

Good question Jim, I'm sure someone will provide an answer that I can't understand

Haha. Geewhiz that's some good stuff Keith. Golldarn that was funny.

 

[Deets066]

So what if you do all that and come up with the same results?

Are you goin to stop increasing compression?

 

[Deets066]

Haha. Geewhiz that's some good stuff Keith. Golldarn that was funny.

Damn Keith, where you been hiding