lets talk compression

[314epw]

I've been told compression should be 150 to 160 PSI.Is this correct?Is this hot or cold?I got a new guage and I am confused.New 359 is at 140 cold.362xp is at 120 cold.I put 5 drops of oil in and compression went to 135 PSI.dropped to 120 PSI and stayed there for over 12 hours.Saw runs great. what if anything am I missing?
Next question is on sprockets.7 teeth or 8 teeth?Is it that little less spacing on the sprocket(8T) that will give you more torque in the cut?:bang:
Ed

 

[computeruser]

On sprockets:

http://madsens1.com/sproktun.htm

 

[retoocs555]

I've been told compression should be 150 to 160 PSI.Is this correct?Is this hot or cold?I got a new guage and I am confused.New 359 is at 140 cold.362xp is at 120 cold.I put 5 drops of oil in and compression went to 135 PSI.dropped to 120 PSI and stayed there for over 12 hours.Saw runs great. what if anything am I missing?
Next question is on sprockets.7 teeth or 8 teeth?Is it that little less spacing on the sprocket(8T) that will give you more torque in the cut?:bang:
Ed

I like to see compression range from a minimum of 110psi on up to 160-170 in some brand new saws. 'Normal' I would say is +/- 130psi. I've had saws run on as low as 75psi but they stalled right out if you put them into any wood.

 

[no longer collecting]

compression.

This is a very controvertial subject. Some will say that the gauge is not accurate on chainsaw because of the small stroke and different lenght of rubber pipe from the gauge to the plug tread, other will argue on the amount of rings on the piston and many more arguments. One thing is that, no matter what you use..... in my opinion when you make a compression test you need 10 seconds of starter pull to bring the compression to it's maximum value. Internal combustion engine 2 or 4 strokes,will require a minimum compression of 90 lbs to perform....ideal is 120. Your new chainsaw is tighter so it is normal that the compression is increased....There is nothing wrong with a saw having 140 and another one having 120.....The engineering design plays a significant role getting those results. for your second question changing the sprocket will affect the rpm,torke and performance depending on the type of wood you cut. It is like fine tuning your machine.

 

[Cut4fun]

I have a 1983 partner 5000 + that goes 160 psi cold. The rest that are stock have been going 140 - 150 cold. The modded 066 and 7900 are alot higher psi cold.

 

[Mr.]

The Matco tool guy replaced a compression tester that went to 220 for a friend.

One pull, cold. The needle wouldn't leave 30 when I, I mean they, handed it back.

Fred

Now use the 350# racing unit.

 

[Paul B]

The pressure on the gauge is determined by the compression ratio. The higher the compression ratio the higher the reading. Of course if you have a blower on that puppy it will read higher. A compression ratio of 10:1 will read 147 psi at sea level hence 10 X 14.7 (atmospheric pressure). Now of course worn ring and scratched cylinder walls all effect the reading because of leakage.

 

[Lakeside53]

Very few saws from the last 15 years work well below 130, and most won't start at 110 to 115..

"mid life" is around 145-150...

New 160-175.. after a short run in period.

And yes, you need a gauge that does not take the hose volume into account - i.e. has a valve at the tip. I've seen (and bought a few cheap) many saws that supposedly had "90-120 compression", and when tested with the correct Gauge, are actually in the 150 range..

Do a search - this has been discussed/argued many times in the past.

 

[Mr.]

Watch out for the Oscar Wilde quotes Andy. One got me banned. LOL

Fred

 

[Sparky_NY]

If I am reading you right, you put oil in and the compression lowered? Something rotten in Denmark there for sure, surprised nobody commented. The theory of the oil is to help seal leaky rings, the compression should rise somewhat, the worse the rings, the more the rise. It would never lower the compression however. Something wrong with the setup or technique OR the compression gauge does not repeat accurately.

I've been told compression should be 150 to 160 PSI.Is this correct?Is this hot or cold?I got a new guage and I am confused.New 359 is at 140 cold.362xp is at 120 cold.I put 5 drops of oil in and compression went to 135 PSI.dropped to 120 PSI and stayed there for over 12 hours.Saw runs great. what if anything am I missing?
Next question is on sprockets.7 teeth or 8 teeth?Is it that little less spacing on the sprocket(8T) that will give you more torque in the cut?:bang:
Ed

 

[Lakeside53]

Watch out for the Oscar Wilde quotes Andy. One got me banned. LOL

Fred

Which one?

 

[Big Woody]

Go to autozone and buy an Actron compression gauge for about $30. It has the valve stem in the tip where it threads into the plug hole. I just had to replace the stem since the one that came installed had failed. It comes with a few spares and they are identical to the ones used in auto tires.

This gauge works good. The compression on the Makita 6401 Home Depot rental saw I just picked up yesterday for $205 tested out at 165 psi after around 6 or seven pulls. The 6401 I bought about 1.5 years ago from HD tested out at 165 also.

My (bought well used) 3120, 090 and Jonsered 2094 all run about 140 psi cold and they cut really well. However I am sure they would be even more impressive with higher compression.

 

[retoocs555]

If a saw runs and cuts through wood with a sharp chain then the compression is good enough. It doesn't matter what the actual reading is.

Anybody who wants to get rid of their saw because it has 129psi compression can send it to me. It's junk anyway.

A compression gauge can be a good money making tool for a lot of shops. The senario I often hear about is a customer will bring in a poorly performing saw, the tech at the counter will put a compression gauge straight on it before taking it in for repair. Get a sub 130psi compression reading and say the saw is junk. Buy a new one or replace the piston and rings. Without any further investigation of what the actual problem might be. These are usually the shops/dealers that only service late model stihl or husky saws. And turn away anybody for repairs with other brands or any saw more than 10-15 years old.

If you are concerned with having top performance, yes, of course compression is very important, the more the better. But if you just want to cut up some fallen trees or firewood or whatever, if your saw works it works regardless of how much compression it has.