stevieb
ArboristSite Operative
I hear about compression all the time. Good compression etcc.. but what does it mean. How is it measured and with what. And how do you know what is good compression?
Compression? What do you mean???
- Thread starter stevieb
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BostonBull
Addicted to ArboristSite
There are numerous methods to testing compression.
You can pick the saw up by the starter handle for a unmeasurable test.
If it stays up or only falls one rev good compression, if it falls slowly probably getting weak, and if it falls instantly definitely needs some work. If it is seized up the saw will not pull over.
Or you can get a compression tester with the right adapter fitted to the end and run a compression test through the spark plug hole. You want a compression tester that the adapter screws into with NO hose between.
A new saw generally has 175lbs, a good used saw will run about 135-150, a saw in need of a rebuild of some extent will be below 130.
A good modded saw will have in excess of 200lbs!
I am sure others will chime in and add to what I have said as I only briefly covered the subject.
You can pick the saw up by the starter handle for a unmeasurable test.
If it stays up or only falls one rev good compression, if it falls slowly probably getting weak, and if it falls instantly definitely needs some work. If it is seized up the saw will not pull over.
Or you can get a compression tester with the right adapter fitted to the end and run a compression test through the spark plug hole. You want a compression tester that the adapter screws into with NO hose between.
A new saw generally has 175lbs, a good used saw will run about 135-150, a saw in need of a rebuild of some extent will be below 130.
A good modded saw will have in excess of 200lbs!
I am sure others will chime in and add to what I have said as I only briefly covered the subject.
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Lakeside53
Stihl Wrenching
A hose is fine (and much more convenient) on a compression tester - just make sure it has a schrader valve in the screw-in tip.
timberwolf
Addicted to ArboristSite
Over 200 PSI on a work saw is dancing with the devil. :rockn: Yes compression makes power. but getting up near or over 200 psi is running the safety margines pretty tight as far as octane and heat. Also with that much compression idle can get choppy and starting the thing can be a pain.
Certainly with high compression the expected engine life would be suffering too, not specifically that compression causes wear, but as more power is being made more load on internal surfaces is unavoidable, more heat is a given.
There is another angle (dynamic compression) that should be looked at, But it is hard to measure. As compression is only a measure at cranking speed (no fire) of how much air/fuel is trapped in the cylinder before it is compressed. That said an engine with small transfers and a restrictive carb or poor scavenging might easily run at 200 plus psi. This is a situation where at operating RPM charge density is low, maybe only 50%-60%. on the otherhand a saw with good porting, larger carb and good scavenging may be charging the cylinder to 80%, on pipe this could 125% or more.
Not sure how to say this clearly, but compressing a cylinder that is only half full of charge produces much less heat and actual compression than compressing a cylinder that is stuffed full of charge.
I found this out with the piped wild thing I messed with for a while, took it to 300 psi. I am sure the only reason in could take it was the the charging efficiency was so poor. Other saws with much beter charging (big transfers, carbs... showed signs of maxing out compression wise on down at 180-190psi.
Certainly with high compression the expected engine life would be suffering too, not specifically that compression causes wear, but as more power is being made more load on internal surfaces is unavoidable, more heat is a given.
There is another angle (dynamic compression) that should be looked at, But it is hard to measure. As compression is only a measure at cranking speed (no fire) of how much air/fuel is trapped in the cylinder before it is compressed. That said an engine with small transfers and a restrictive carb or poor scavenging might easily run at 200 plus psi. This is a situation where at operating RPM charge density is low, maybe only 50%-60%. on the otherhand a saw with good porting, larger carb and good scavenging may be charging the cylinder to 80%, on pipe this could 125% or more.
Not sure how to say this clearly, but compressing a cylinder that is only half full of charge produces much less heat and actual compression than compressing a cylinder that is stuffed full of charge.
I found this out with the piped wild thing I messed with for a while, took it to 300 psi. I am sure the only reason in could take it was the the charging efficiency was so poor. Other saws with much beter charging (big transfers, carbs... showed signs of maxing out compression wise on down at 180-190psi.
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stevieb
ArboristSite Operative
I'm rebuilding a couple of saws at the moment. Can you recomend a tester which want cost the earth 
stevieb
ArboristSite Operative
Also how can you moderate the compression???
GASoline71
Mr. Nice Guy
You can't "moderate" it... but you can make higher compression by removing a little bit of the bottom of the jug. Or by using a thinner base gasket.
Gary
Gary
Lakeside53
Stihl Wrenching
I'm rebuilding a couple of saws at the moment. Can you recomend a tester which want cost the earth
Baileys sells one.
simple explanation
Compression, or compression ratio....
Is the amount of "squeeze" the engine has designed into it.
Using sea level air pressure (14.7) you can say that a saw that has 12:1 ratio will have about 175 lbs of pressure. Maybe. There is one more thing to figure in. Efficiency. Most engines are less than 100% efficient. For the sake of arguement we can assume something around 80% on a 4 stroke. I am not sure on a two stroke.....
On a saw about the only thing that can leak compression is the piston/cylinder. This gets pretty interesting as it dilutes the intake charge. Maybe that is why some of my old saws didn't run too good! :deadhorse:
-pat
Compression, or compression ratio....
Is the amount of "squeeze" the engine has designed into it.
Using sea level air pressure (14.7) you can say that a saw that has 12:1 ratio will have about 175 lbs of pressure. Maybe. There is one more thing to figure in. Efficiency. Most engines are less than 100% efficient. For the sake of arguement we can assume something around 80% on a 4 stroke. I am not sure on a two stroke.....
On a saw about the only thing that can leak compression is the piston/cylinder. This gets pretty interesting as it dilutes the intake charge. Maybe that is why some of my old saws didn't run too good! :deadhorse:
-pat
sperho
ArboristSite Operative
With the Schrader valve at the wrong end... I have an Actron with the valve at the wrong end too. Argh.
Lakeside53
Stihl Wrenching
That's interesting.. I would have thought they'd know better!
djmercer1
ArboristSite Operative
for the sake of an arguement, why is it better to have the schrader valve nearer the spark plug hole?
mechanically, i cant see what difference that the aditional volume contained within the hose makes if it is before or after the valve.
Lakeside53
Stihl Wrenching
Guess you haven't read all the other threads on this topic...
Here's a simple HYPOTHETICAL example.
Compressed volume of cylinder = 5cc
"Compression" = 150lb
Add a compression gauge with a 5cc hose volume and no schrader valve at the tip.
"New" compression reading is only 75lb...
Many saws have been pulled apart because of "low compression" readings using the wrong type of gauge.
Here's a simple HYPOTHETICAL example.
Compressed volume of cylinder = 5cc
"Compression" = 150lb
Add a compression gauge with a 5cc hose volume and no schrader valve at the tip.
"New" compression reading is only 75lb...
Many saws have been pulled apart because of "low compression" readings using the wrong type of gauge.
ropensaddle
Feel Lucky
I have a lisle and it works fine it is for auto but seems to work on saws
djmercer1
ArboristSite Operative
i have seen similar arguments, but to my mind they are making an incorrect assumption. for the reasoning to be accurate, the pressure in the cylinder would have to return to 0psi each cycle of the engine. in the pressure tests ive seen/completed the pressure builds and drops throughout the test and trends towards a maximum. the reading from a compression test isnt the amount of pressure built up through a single cycle, its the maximum pressure an engine can contain.
shawn.m
ArboristSite Operative
I usually turn a engine over several times to find the highest peak pressure. The main thing that I look for is what is the average pressure (what you are really measuring) for that engine at the time you are measuring it. Then compare it to later readings.
I have three gauges and their is a range between the three, Actron rubber hose, Lisle- rubber hose, and a rubber tip Lisle.
I have three gauges and their is a range between the three, Actron rubber hose, Lisle- rubber hose, and a rubber tip Lisle.
Lakeside53
Stihl Wrenching
The pressure in the cylinder returns to zero (or really really close) each time - the exhaust port is open to the world.
The pressure as you read it "building" your gauge is built up by the valve at the gauge. In a perfect world, if the gauge and hose had NO volume, the reading would be instantaneous.
The correct compression gauge needs the valve at the tip, not at the gauge. None of this matters when the cylinder is very large in comparison to the hose volume - as in a car.
shawn.m
ArboristSite Operative
just pull the rope faster :hmm3grin2orange:
Lakeside53
Stihl Wrenching
If you have a gauge with a valve, you are reading peak pressure (in any one cylinder), not average. If it changes when you crank it - that's due to changing lubrication sealing the rings/piston (ignoring valves for this example).
