compression average

[Dolmar_Tech_Mgr]

I know when you have low compression and the engine gets hot the saw will stall so i was thinking you loss compression as it gets warmer. But that is just my thinking.

 

[glens]

I'm thinking that's more a result of the combustion gases reaching places they normally wouldn't (thus excessively heating them and/or scrubbing oil film), because of the low compression.  I believe in an otherwise healthy engine, the compression will read lower cold than hot.

 

[glens]

Not entirely pertinent to the forum-style engines, but a good read can be had here for those who find clicking a link easier than going to their bookshelf.  Also, check their main menu page link.  The first article on horsepower/torque was pretty good; I haven't yet looked at any others, but they should all be good reading.

Glen

 

[ehp]

myself I donot use the 10-1 or any of that kind of numbers, I want what the motor has in real compression,
reason = this is a 2 stroke not a 4 stroke .
it has an ext. port and tranfser ports in the jug not valves.
if i make a motor with a ext. port that is 105 degrees with opening along with a transfer port of 70 degrees of opening it will not even be close to the compression of a motor with 74 degrees of opening on the ext, and 56 degrees of opening on transfers , this is if the 2 motors are the same engine with the same squish, so if they are the same 10-1 ratio and the second motor has 130 lbs of compression , the first motor would not run because it would be alot lower on true compression but bump that motor up to 175 lbs or so and watch it run then

 

[spacemule]

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.

Are these ratios figured using a full piston stroke or merely the effective ratio from the closing of the intake to tdc?

 

[DanMan1]

Not entirely pertinent to the forum-style engines, but a good read can be had here for
Glen

Not that I disagree with the article. I just don't see the need for the author to put 'Copyright Whaa-Whaa' after every 4 stroke basics, High school shop class level, information known by a million people already, paragraph.

Should we put Copyright after every good paragraph on this forum also?

 

[Crofter]

The way they are working the math it would have to be the corrected ratio from the top of the exhaust port.

 

[Al Smith]

Makes good sense

In relation to a 4 cycle,specificaly a Harley 74,this makes good sense.Although I never made one myself,the hopped up 74's,with a Shifton cam,and big valves,actually kicked over,easier that a stock engine in spite of the fact that these engines had higher comp.ratios.[ this example,goes back decades ] I don't know the specs on these cams but I would imagine the overlap of the valve timing would produce the same effect,as was given about the port timing on a 2 cycle.Clear as mud,eh.

 

[Lakeside53]

I've noticed nobody has mentioned that it's VERY important to get the correct compression gauge for small engines. There are two types- those with a schafer valve (like a tire valve) in the end that screws into the plug socket, and those without. If you use the type without the valve at the hose tip, you'll always get an abnormally low reading as the volume of the tube feeding the gauge is significant with respect to the volume of the cylinder. I got caught by a gauge that showed me 130lbs compression. After tearing down the motor and finding it in great shape, I borrowed another guage and it read 170lb. Took me a while, but I figured out that there was nothing wrong with either guage - just the wrong type. My local NAPA store sells the WRONG type for small engines.

I read compression on all Stihl engines before repair just to get a ballpark indicator of condition, but you can get mislead. New from stihl (036, MS200 and others) I see 170-180. A very good used engine will show 160-190lb, sometimes more (I have an 034 in right now with 192lb!). Below 120, they get harder to start and really don' t run well.

If there is excessive carbon buildup on the cylinder or piston, the compression can be 10lb higher, and this isn't necessarily good! I see this all the time where a bad carb has been rebuilt, but the piston/cylinder hasn't been decoked.

I had an 036 with 175lb that ran pretty good, but sounded a little bad (knocking) while idling (another Ebay "special"). Inspection of the bore though the exhaust port showed it to be excellent. The piston looked a little scratched on the intake side, but nothing unusual. I stripped it down. The cylinder bore BELOW the intake side was worn completely though the nikasil coating and about 1/2 mm into the aluminum casting. The "knocking" was from piston slap. I figure this was caused by running with a bad air cleaner (or none at all).

Some non-scientific observations: Compression has a lot to do with max horsepower. It's harder to see on saws as there are so many variables, but easy on blowers like BR400/420 as the conditions are constant. With tubes installed, good gaskets/seals, stock muffler, and rebuilt carb adjusted to max rpm on the rich side, then backed off 100rpm, those with compression of 145-175lb will typically pull 7200-7500 rpm (the fan wheel absorbs all the power so there isn't a high "no-load" rpm like a saw); compression of 135 or less will not pull more than 6200 rpm, and at 120 they really have problems running at all. It may not sound like a lot, but 1000prm at full power is significant.

 

[Crofter]

Lakeside: interesting; the fit of the piston skirt has a lot of effect on the base compression and carburetion but virtually none on cylinder compression.

 

[Lewis Brander]

An old rule of thumb:

Hello to all. The above threads are great. Really gets you to thinking. But I usually use the old rule of thumb (applies to old 4 cycle engines, but works for me on older 2 cycles, especially the reed valve types). I was told a long time ago if you grab the flywheel and spin it (by hand) against compression, and it spins back, you have enough compression to start the engine. I've seen saws that have 90 psi compression, when checked with a gauge, that start and run just fine for me. My old Disston saw will start (cold) on the second pull, after filling up the gas tank (after previously being ran dry) and letting her sit a few minutes. She'll usually start on the first pull, after she is ran for a few minutes. Usually if there is a good hot spark and the spark timing is right, minimum compression and the carb is adjusted right, I don't have much problem in starting and running these old saws. I'm no engineer or into the blower or racing thing as far as cars or saws go, but do try to use a bit of common sence when working on saws. I'm like my saws, low tech. JMO. Take care. Lewis.

 

[Dolmar_Tech_Mgr]

Are these ratios figured using a full piston stroke or merely the effective ratio from the closing of the intake to tdc?
__________________
Not real sure where those ratios came from there were published in the owners manuals years ago.

 

[ehp]

Dolmar man, if you were asking about the ratio's I gave , they are the degrees of open port, has nothing to do with TDC or BDC