actual meaning of "low compression"

[swimmin' for shore]

I'm not a mechanic. I'm getting better, though. I'm getting a conflicting understanding of the term "low compression". I understand how to check it, and very basically what causes it...What does it do to you, though? Does your motor run more weakly? Some say that it doesn't matter, as long as each cylinder is within 10% of the other. Is this true? I feel like 50 psi on cylinders that were set up for 125 or so is bad, but is it? Do I avoid all motors with low compression? What are the most common causes? If there's another place that I should be reading, please redirect me. Thanks.

 

[Joe Reeves]

Re: actual meaning of "low compression"

Simply put.... The fuel air mixture must be compressed (compression)) in order to develop power when ignited.<br /><br />Gas in a deep pail, add a spark, you get a poof.<br /><br />Gas in a deep pail,compress with a sealed plunger, add a spark, it'll blow your head off!<br /><br />Best bet is to check with your local library for a book on the basics of the internal combustion engine.

 

[JB]

Re: actual meaning of "low compression"

Compression readings vary so much from guage to guage and from variations in technique that it is nearly impossible to give you a number for "good" compression, SFS.<br /><br />We assume that when you do a comp. test you do all of the cylinders the same. That way we can tell you that if one reads more than 10% less than the other(s) it has "low" compression. As long as all read nearly the same we usually call the compression okay.<br /><br />An engine might give you 125psi with one guage and technique and 50psi with another guage and technique. It is still a good engine if the readings you actually get are within the 10% range.

 

[swimmin' for shore]

Re: actual meaning of "low compression"

So, even though my 18 hp fastwin may have come from the factor with 125 psi on each cylinder, if they now read at 50 psi each, the engine will run about the same?

 

[G DANE]

Re: actual meaning of "low compression"

No. <br />Usually in a two strokes lifetime, an overheating, blown gasket, leanrunning cylinder, cooked rings or what do I know, will hurt a cylinder. This usually happens long before it wears out. However, now and they motors show up that just had their maintenance and ran untill they wore down. If you have a gauge, and can test it on several different motors, and can see compression is low, lets say 50 on a motor, it will have severe starting issues, if you manage to get it going at all. What JB says, is there is huge difference between gauges and cold/warm, battery condition, open/closed throttle, dry or oily cylinders. Fogging oil in cylinders can give higher readings on a worn or damaged cylinder. Not all designs provide exactly same cooling to all cylinders, which will cause unequal wear and uneven compression, as motor passes thru normal lifetime.

 

[djzyla1980]

Re: actual meaning of "low compression"

I believe it's going to take a bit into account... But Compression is basically the same for any motor... auto,boat, ect...<br /><br />When the piston moves in the cylinder it is sealed by the rings and the head gasket. Compressing the gas and sealing the force of ignited fuel is it's purpose. When the plug ignites the fuel it forces the piston downward. (or sideways)If there was no compression the piston would not move by the force of ignited fuel. Instead the force that once pushed the piston back exscapes out. <br /><br />During normal and abnormal wear things can happen within the cylinder to reduce comp. Included is broken or worn rings, warped cylinder wall, hole in wall or piston, bad head gasket and etc. These and many others will allow the air fuel mix and or the force of this ignited mix to exit the cylinder, making the cylinder less effiecient. When the cylinders wear normally the compression readings will be within the range 10%. If abnormal wear has occurred then one or more cylinder reading(s) will be out of that range. Also if reading were ok yesterday but something happened and today they weren't it would be cause for concern.<br /><br />If you take a motor that has has been ran for 30 years... you can expect lower numbers. Sometimes these lower numbers are ok... and are normal. I do however think theres a point were the numbers are too low... what are they?? don't know... But if I did a reading and the motor reads in range of 50... but still runs good I wouldn't be concerned but if that same motor reading 50 range runs like crap it's prolly time for a rebuild. <br /><br />In reality... a compression test is good... but a leak down test is better (tells you how fast the cylinder looses compression) <br /><br />This is my best way to describe it... Outboards are pretty new to me but auto's aren't. To sum it up... <br />Compression=How good the cylinder is sealed=effiecency from that cylinder <br /><br /><br />If theres a better way to explain someone please step in.....

 

[rodbolt]

Re: actual meaning of "low compression"

Dave<br /> ya got it<br /> its an articulated rod internal combustion engine. the cycle is suck squeeze bang and blow. they all work that way, period.<br /> without getting overly techinical there are only a few ways to make power. squeezing a known volume of air and fuel more before ignighting it is one. compression/combustion pressures is a funtion of the cylinder sealing capeability. on a two stroke leakaage past the rings not only allows a compression loss but allows hot corrosive combustion gasses to blast past the rings, this has 2 immediate effects. one is to eat at the rings,ring lands and piston skirts and the second is to contaminate the incoming air fuel mix. in order to burn completly the air to fuel ratio is critical,its about 14,7 to 1. over rich gives poor burn,low combustion temps and excessive deposit build up. to lean gives excessive combustion temps and a tendancy to have the mix go off in an uncontroled explosion, also know as detonation.<br /> on most two stroke outboards the lubrication is also part of the fuel mix. as the combustion temps climb due to a lean condition your also reducing the amount of avalible lubrication. the piston rings also serve a function of piston cooling. they must be able to transfer heat to the cooler parts of the cyl wall. the incoming air/fuel charge also helps cool the piston.<br />thats why its so rare to see a two stroke with a cyl ridge, long before they wear out most suffer a catastrophic over heat or a lean out.<br /> my advice to all new techs is to understand the basic mechanical principles of an articulated rod internal combustion engine. without it your sunk. you dont have to get into rod/stroke ratios and pistons speeds and some of the various other arcane things but you must understand how and why the basic mechanical part works. the cooling, ignition,induction and exhaust systems are just sub systems. eliminate each one as a culprit for a poor running engine and it works well again.<br /> on most older motors the metalurgy and machineing technology was rather spotty. so 3 ring pistons and thick cyl sleeves were normal. motors tended to forgive more due to the margins built in. todays modern engines run very close to the failure margins. let a system degrade,not quit just degrade, then ask max power at full load and failure is a fact. a perfect example was the big bore high ring piston/cyl design of some of the jonny rude v4/v6. wasnt a design flaw was a customer neglect issue. if the subsystems were meticulously maintained they rarely failed, let it get just a little lean or just a bit hot and the failure was rapid and catastrophic.on a 4 stroke the combustion chamber actually has a cooling cycle, a 2 stroke does not and are therefore much more susceptible to cooling system degradation failures.<br /> 99% of all the failures I see are customer induced failures, like the guy that says his ten year old water pump "suddenly failed" and lunched a piston. I tell them it was not sudden it took 9 years. if you look at how badly most marine products are treated and the environment they run in its actually amazing they do as well as they do.<br /> wish I had a dollar for every tale of woe that started with, well it wouldnt run quite right but we ran it anyway.

 

[swimmin' for shore]

Re: actual meaning of "low compression"

You guys are Gods. Anyone can still feel free to add to this discussion, of course. I appreciate the information that's been gleaned thus far. <br /><br />rodbolt, an answer to your comments-I've been hearing some of those stories. The last guy that I picked up a motor from said that he couldn't figure out why it was smoking. I said, "Did you check to see if the water was pumping?" He said he hadn't, but that he'd gone ahead and run it into the boat ramp anyway. He didn't know what he'd done, but he knew it was not going to run right again for him. I took it apart. Of course, the impellor was in a dozen pieces. The mainifold gasket was shot. The compression, though, is 50 psi. I fixed it and ran it. It runs like a champ. I wonder, though, if I've made a mistake with this motor. I don't mind rebuilding what can be rebuilt, if necessary, but I don't want to continue making these kinds of mistakes with what I buy. It gets costly and time consuming. <br />Dave, if you don't mind the question, how do you do this leak test? It sounds obvious, but I'd rather hear it from you before giving it a shot. And what's a good result vs. a bad result with that?

 

[djzyla1980]

Re: actual meaning of "low compression"

A leak down test is simply putting a sealed cylinder (exhaust and intakes closed (at or near TDC) Under compression and watching how long it takes for the pressure to drop. A cylinder is not 100% sealed but what you look for is the time it takes to drop. Lets pull some numbers outta the air... if you do a leak down test and 3/4 cylinders leak down to 0 in lets say 20 seconds. But the 4th leaks down in 5 there is more air exscaping that cylinder at alot faster rate then the other three. Although when reading comp from turning over the piston, movement it reads in range like the others(more then likely one that is in range but at the low end) This is caused when the movement of the parts allows it to read higher. I will do this process (in autos)when all Comp tests look good but I still have an issue. Wether it's blowby or etc. <br /><br />Although it is functioning correctly with the standard test a leak down states that there is an abnormal leak. Could be head gasket rings etc. Same as the standard low compression reading But sometimes just turning a engine over reading the numbers isn't accurate enough to say theres a prob. <br /><br />These engines run at 6-1000 rpm's at idle. If the compression leaks out at a faster rate (as stated) you will loose the efficiency of that cylinder.