Compressin

[angelo c]

So a fellow member and I were chatting about a saw he was "diagnosing" and of course we discussed everything that could be wrong....after two hours of that we arrived at a "HMMM MM" moment. It was late and I was cleaning some pine sap covered saws...but the question was...

Is a "tach" a good guage of "compression" ....meaning if a saw has low compression(135lbs) will it rev higher or lower on the tach then a saw with say 188 lbs..my argument was the saw can't rev 14k with weak compression so the guage must be wrong/ bad.

Saw is a meteor piston in a cleaned up OEM Ms 360 not ported or muffed.

so my question is , is a tach a good backup to a compression guage, vice versa or neither ?

 

[Overclock]

"HMMM MM"

thanks that felt great

 

[CR888]

No...l do not think a tach is a good measure of compression but a compression gauge usually works pretty well. Or if you want the 'poor man's' gauge you just hold the saw by the starter handle/rope and let it drop. How slowly/quickly it falls can give a poor man with a brain a fair idea.

 

[NewToStihl]

So a fellow member and I were chatting about a saw he was "diagnosing" and of course we discussed everything that could be wrong....after two hours of that we arrived at a "HMMM MM" moment. It was late and I was cleaning some pine sap covered saws...but the question was...

Is a "tach" a good guage of "compression" ....meaning if a saw has low compression(135lbs) will it rev higher or lower on the tach then a saw with say 188 lbs..my argument was the saw can't rev 14k with weak compression so the guage must be wrong/ bad.

Saw is a meteor piston in a cleaned up OEM Ms 360 not ported or muffed.

so my question is , is a tach a good backup to a compression guage, vice versa or neither ?

This is an interesting question. I can't say I know a definitive answer, but I do have some thoughts/opinions. For starters I wouldn't think RPMs in any way would correlate to compression. Mostly because RPM can change very easily with a tweak on the H screw, fuel variations, elevation changes, etc. Perhaps if you tach'd a saw and then managed to score it or do something to change compression without any fuel or atmospheric changes and then tach'd it again you could find some relationship?

Also, you can look at the correlation between compression and RPM a couple of ways. I see your point in saying if it isn't making good compression (i.e. power) then it won't pull as high of RPM. However, you could also consider that with less compression it's meeting less resistance on the upstroke which may actually work towards increasing RPMs. Who knows....

 

[Red97]

I say no, if you have the same volume of fuel/air in the cylinder same port timing and ignitiontiming no load rpm should stay consistent. Now if you tach a saw under load I believe the lower compression saw would show.

 

[old-cat]

A tach has very little value for diagnosing chainsaws, not for tuning or anything.
The only use I would have is to see how many rpms in the cut after porting.

 

[Jason280]

Tach would be worthless in that regard, you'd be able to discern more simply by running and "listening" to the saw.

 

[fearofpavement]

Low compression saws will often be difficult to start, idle poorly and just run bad. However, once you get one wound out I don't think there is much correlation to compression and rpm.
My take on compression is that low compression indicates a problem but high (normal) compression does not mean there isn't a problem. I have had badly scored pistons give a good compression reading. (and recently found a bad running saw with good compression had a section of piston skirt broken off)

 

[NewToStihl]

My take on compression is that low compression indicates a problem but high (normal) compression does not mean there isn't a problem.

Amen to that! I'm sure a lot of folks here have similar stories, but I've seen it too. I used to keep a old beat up old MS290 just as a loaner. If you pulled the muffler you saw a truly horribly scored piston and frankly when I got the saw I was surprised it even turned over. Either way, it made just a hair under 150 psi and ran like a champ after I got it cleaned up and tuned correctly. I eventually sold it to a guy who was desperate for a low cost chainsaw to cut firewood with. I told him exactly what was going on with the saw internally and told him I didn't figure it would last long, but he didn't care - just needed something to cut with for a short time until he could afford a better saw. He's been cutting firewood with it for close to two years now with no problems!

I also have an 090 with a scored piston & cylinder. Not just transfer on the cylinder either; it has some plating damage. The saw makes 150 psi (which is right on the money for those big boys) and runs great. Knowing the condition of the p/c and also that the saw ran fine I wanted to do some experimenting. I pulled the cylinder and took a bunch of photos of the cylinder walls and piston (damage was on exhaust side due to too lean of a setting on the H screw when I got it). I then put it back together, tuned it correctly, and ran it for a few hours. After that compression was still right at 150 psi. I pulled the cylinder again and based on the 'before' photos there was no change at all to the piston or cylinder. It's a classic case of a saw you would never expect to have any problem at all unless you pulled the muffler...

 

[angelo c]

Good stuff so far....but like we came up with.....nothing concrete either way...

Where's one a dem ingineering guys when you need em ?

Paging Chris-pa!!!
Chris -pa ??????

We have a question in the 'chainsaw' isle...

 

[drf255]

Just my opinion, which is worth what you paid for it.

I don't think you can compare it at all. Static compression differs from dynamic compression.

When a saws turning 14,000 rpm, the amount of actual compression is dictated by a Huge amount of variables. Port flow, muffler flow, spark advance, etc...

In a normally aspirated motor, you're not gonna fill a cylinder 100% while it's running. How well it does, IIRC, is referred to as volumetric efficiency.

So I'd say a low compression saw, like 150 psi, with excellent port flow, a muffler mod, a well designed chamber for flame travel, would probably rev higher than a 200 psi saw with the opposite of the above. Sometimes mods that will make a saw have lower static compression, like raising the exhaust port ceiling, will translate into higher rpms.

So I say NO.

 

[angelo c]

Just my opinion, which is worth what you paid for it.

I don't think you can compare it at all. Static compression differs from dynamic compression.

When a saws turning 14,000 rpm, the amount of actual compression is dictated by a Huge amount of variables. Port flow, muffler flow, spark advance, etc...

In a normally aspirated motor, you're not gonna fill a cylinder 100% while it's running. How well it does, IIRC, is referred to as volumetric efficiency.

So I'd say a low compression saw, like 150 psi, with excellent port flow, a muffler mod, a well designed chamber for flame travel, would probably rev higher than a 200 psi saw with the opposite of the above. Sometimes mods that will make a saw have lower static compression, like raising the exhaust port ceiling, will translate into higher rpms.

So I say NO.

Bone stock OEM Cyl- Meteor Piston- no muff mod

 

[pantelis]

So a fellow member and I were chatting about a saw he was "diagnosing" and of course we discussed everything that could be wrong....after two hours of that we arrived at a "HMMM MM" moment. It was late and I was cleaning some pine sap covered saws...but the question was...

Is a "tach" a good guage of "compression" ....meaning if a saw has low compression(135lbs) will it rev higher or lower on the tach then a saw with say 188 lbs..my argument was the saw can't rev 14k with weak compression so the guage must be wrong/ bad.

Saw is a meteor piston in a cleaned up OEM Ms 360 not ported or muffed.

so my question is , is a tach a good backup to a compression guage, vice versa or neither ?

No, the tach not tell you about the compression, if the saw rev high or slow if the saw, then looking for leaks, or other problems. if you see the tach this moment give 2800 and after a sec giv 2400 and again 2800 this is problem. in a good machine the tach give 2800 and maybe have a little differense about 50 or 100.

 

[pantelis]

The diagnostics tools from husky and stihl understand the leaks , from how long take the saw to go from high to low, but i read the manuals and i speak again

 

[Chris-PA]

Paging Chris-pa!!!
Chris -pa ??????

LOL - I don't think I have enough experience to answer well, but I'll throw something against the wall anyway....

I think compression readings can be a useful diagnostic tool if you know what a particular saw (or series of saw) reads on a given gauge. Other than that I don't think low speed pressure readings are worth a hill of beans. I don't think the amount of pressure a cylinder can hold at low speeds when cranked by hand is well correlated to what it can do at high speed under combustion conditions.

I also don't think max no load WOT rpm is meaningful other than as a way to set mixture. If it changes it could be a sign of a problem, like any change, but it is likely to be due to many things (like carb problems) beyond compression.

That said, low compression will kill power quickly, so if the saw cannot hold rpm under load (like when it's leaned on), then cylinder pressure may be leaking away. But wouldn't think reduced compression would change no load max rpm in a predictable way - it just doesn't take much to rev no load.

 

[lone wolf]

Bone stock OEM Cyl- Meteor Piston- no muff mod

What are you trying to do there exactly?

 

[lone wolf]

What about a leak down test of the rings when the piston is near or at TDC?

 

[CR888]

AS members think about saws WAY too much!!lol. Thats OK....l understand that cad infection well but this thread is a little too funny!!

 

[Adirondackstihl]

No

 

[Bilge Rat LT 20]

We used a leak down test to check on motor health in race cars. A compression test will not show a problem in many cases.
The leak down will show you where a problem is, rings, valves, seals.

The compression test was used to make sure we had a legal motor, compression was limited by the rules.