compression average

I was testing a friends' saw the other day to see where its compression was at. What would be an average? 100 and over is good, under 100 is bad? Throttle open or closed? How many pulls on the starter? Just curious how it is done and what is some average numbers. This was touched on a while back but never really solidified in numbers and method. Thanks.

compression

On a cold saw 150 lbs is a good figure, especially for a small saw, the larger saws will will vary between 160 to over 200 lbs. depending on the make and model.

Four pulls is enough to determine compression, a good saw will jump right up to 75 to 100 lbs on average for the first pull, after the first pull the saw should climb steadly on the gauge only in smaller increments.

As far as the throttle open or closed, I wouldn't think it would make much difference, I could be wrong on this one. The throttle plate has to let air pass into the venturi for the saw at run at idle, so it shouldn't make a difference in the compression numbers, weather it is open or closed. Like I said I could be wrong on this one, I am not what you call an expert on the subject, I've heard of this before myself, would be interesting to hear what others of have to say about this.

Hope this helps you out

Larry

I have been a mechanic for over 20 years and have never seen 200 psi on a compression test on anything up to and including a 900 hp SuperStock HEMI ! The size of the engine does not dictate the max pressure that can be seen on a compression test. A 2 cycle engine has less than 1/2 the stroke to build compression. I would use the following numbers 115 psi and below is suspect, might run but there are problems. 115 to 160 psi ideal. 160 psi and above look for plugged exhaust or a scored piston, I had a Partner cut off saw show me 175 psi with a piston that was scored beyond belief.

compression

Where are these figures published?

Wayne, They are not that I know of. I am just answering a question based on observation.

I like to see 130 to 145 lbs compression on most saws the 7900 stock is higher than that.

I have seen lots of saws with over 200 lbs of compression ,

I think part of the discussion has to be around the guages themselves...I bet you would see three different readings with three different guages. I have always looked to have over 110 as a base line with my motorcycles. Some of the higher numbers with two strokes touched the 150-160 range but most good motors I have tested...with MY guage sit between 120-150psi. My saws are a bit less. All my Mac's & Homelites sit between 110-130 with my little mini Homelite "E-Z" with the highest compression with 132psi and my "one-pull start" McCulloch 797 lowest at 118psi. Interesting enough the McCulloch is the easiest starting saw I have and runs well but most importantly it IDLES well indicating it has sufficient compression. If I saw less than 110, I would expect the starting to be more difficult and less than 100 the saw or motorcycle needs the top end looked at. Again this is with MY guage!!! And what the real PSI is vs what my guage measures..I have no idea! So you sort of use the imperical data method with the guage you have and raw experience. (This is from 34 years of building & maintaining two strokes for MX motorcycles, Mercury outboards, and now saws....and this guage is almost 20 years old and still works enough to predict motor health based on..IMPERICAL data)

Another point is most racing motors I have seen & built that shoot for RPM's don't have the really high compression ratio numbers..the ones tuned for midrange and low range do. Up the compression ratio and you get more punch off the bottom at the expense of raw peak RPM numbers. I have to wonder if those 12,000 & higher RPM saws really have the 180-200psi numbers some say & if so why? I have seen a few really high compression racemotors (in the 180psi range) over the years and they all need over 100 octane race gas & super careful jetting & ignition timing to keep from pinging their guts out. Most saws run on pump gas. They have to for the world wide market where gas can get pretty sketchy. Maybe things different because they are so oversquare?. (Fourstrokes are another story competely...my VOR's just bury my guage!)

Comp. readings

I think they can some times be mis leading.I have one Mac sp 125 that I resleeved ,that I can pull over without use the comp release[less than 1 hr,on the engine run time],that cuts as well as my other one,which is quite peppy,to say the least.I've never taken the time,for a comp. check,it is usually pretty evedent,if there is a problem.An example is my 042 Stihl,which rapidly lost power,in a long cut.It seems some fool[ me ,duh],forgot to check the case gasket when doing a partial rebuild .Now,I get to "lick my calf over",so to speak. The ironic thing about all this,is the fact I have given this advice to other folks,and forget to do it myself.

compression

The only published figure I have ever seen for compression figures comes from the Chain Saw service manuel, 10th edition in the Homelite section for SXL-925 p. 183. Quote " For optimum performance of Model SXL-925, cylinder compression should be 155 - 185 psi (1069-1275kPa ) with engine at normal operating temperture". I assume that cold testing would translate into higher numbers than that.

I wish they would publish some figures on saws to have a standard to go by.

Larry

"160 psi and above look for plugged exhaust or a scored piston, I had a Partner cut off saw show me 175 psi with a piston that was scored beyond belief."
__________________

Maybe I'm still asleep. I just can't fathom how a scored piston raises compression. Could you please explain.

To be honest, I never use a compression gauge.

Once I worked on a old Green Machine/Shindaiwa trimmer, the
boss told me to rebuild the carb. Thing ran ok, idled, etc...
But sluggish and not a lot of power.
Took the muffler off, which is what I would have done in
the first place, and one of the rings was in two pieces.
Tore it down, and the other ring was paper thin in the middle.

Put a new set of rings in it, and it ran great.

I am glad I do not work for anyone who diagnoses the
problem for me anymore.
No point to my post really, I guess. Just bored, depressed
and pissed off! I only got socks and underwear! I even had
to stay sober!

Dan, Because there was so much metal transfer between the piston and cyliner there was actually LESS cylinder to wall clearance. The tight fit aloowed for high compression readings when engine speed was low as by pulling the starter rope. Somewhere in the Stihl manuals there is a line that reads something like "You should not assume that the piston and cylinder are ok if your tests show a good reading, the compression test will only verify a failed piston."

A comp test should always be done with a with the trigger held wide open. As for engines with over 200 psi of compression. I have seen many and I own a few. MY old cr250 was around 180 stock, my 7900 is 170 stock.

Fish said:

To be honest, I never use a compression gauge.

Once I worked on a old Green Machine/Shindaiwa trimmer, the
boss told me to rebuild the cart. Thing ran ok, idled, etc...
But sluggish and not a lot of power.
Took the muffler off, which is what I would have done in
the first place, and one of the rings was in two pieces.
Tore it down, and the other ring was paper thin in the middle.

Put a new set of rings in it, and it ran great.

I am glad I do not work for anyone who diagnoses the
problem for me anymore.
No point to my post really, I guess. Just bored, depressed
and pissed off! I only got socks and underwear! I even had
to stay sober!

Click to expand...

There is a lot that tests do not show. Tests are a good help, but without the brain engaged, there will be a lot of tests.

test

That is why you pick up the saw by the starter handle first thing. If it holds, then pull it over, should spin free. That will get you going in the right direction in the first minute.

Years ago Sachs-Dolmar listed the compression ratio in the owners manual small saws were 8-1 or fraction over 8. Larger saws were 10-1 in most cases and seems like I remember one that was 11-1. If I am correct to get the compression numbers you speak of you would mutiply the first number in the ratio times the number that represents atmospheric pressure which is 14.7 lbs. So a saw with a 10-1 ratio would be 147 lbs/sq/in. Most of my test show saws that are 10-1 when under 120 lbs may run but not very long as when the cylinder gets hot the saw loses some compression and stops running.

That being the case,as it is,my old Macs,at 6 to 1,would be around 90 psi.It would also indicate to me,if my thinking is correct,that with an iron liner,the thermal expansion would be less than the aluminum piston,ergo less losses,due to this factor,at higher cylinder temps.Hmm.

I sure hope my thinking of compression ratio and atmospheric pressure are correct. Or maybe i just made it up.

It's correct; at mean sea level.

I was under the impression that compression readings would be lower with a cold engine than with a hot one.

Glen

In my way of thinking,that would also be correct.