Compression Test V/S Leakage Test

wrechin2

New member
<font color="#000000">Just wanted to post a test explaining the differences between the 2 test and give some examples.........</font></p>

<font color="#ff0033">COMPRESSION TEST</font>......<font color="#000000">What a compression test does is tells you how much pressure that piston/rings can make moving from the bottom of the cylinder to the top. The problem with a compression test is that it is not a real accurate way to determine the health of a engine as it usually has to have a very serious issue to show up on a compression test. I have a insert out of a manual that states if a engine has equal compression and is hard to start or runs poorly, to check for mechanical issues. So even OMC (Johnson/Evinrude) understands this and puts this in their service manuals. All engines work off the same principles. I have dealt with several engines that have passed a compression test and failed a leakage test. </font></p>

<font color="#000000">I mainly use a compression testto help determine what octane fuel that a engine requires. The higher the compression, the more heat is produced in the combustion chamber. This can cause the temperature in the combustion chamber to ignite the fuel instead of the spark plug (detonation). Causing engine damage. The higher the octane, the slower the fuel burns at the same compression ratio. So to get the most out of the fuel, you need to use the correct octane for your compressionas too high of octane will cause a 2 stroke to actually loose power. More on this later!</font></p>

<font color="#ff3300">LEAKAGE TEST</font><font color="#000000">..... First you must understand what happens in a cylinder. The piston begins moving to the top of the cylindercompressing the air/fuel charge making it more flammable.Just before the piston reaches thevery top of the cylinder, spark will occur. (thisknow as timing for example 23* Before Top Dead Center)Just as the piston reaches the top and starts on it's way back down the air/fuelthat has already startedto burn will finish burning creatinga large amount of pressure pushing on the piston thus rotating the crankshaft, drive shaft, and then the prop.So what a leakage test does is check the seal of the piston rings and gasket. Remember whencombustion is happening, there is a large amount of pressure pushing on the piston. If you have a poor seal, then some of this pressure will go elsewhere and the engine will have a power loss. This leakage is also known as blow by. Now that you understand what is going on you can see if you run 93 octane (slower burning) in a engine that needs 87 octane (faster burning) it will mess up the combustion process and cause a power loss. </font></p>

<font color="#000000">What a leakage test does, is induces air into the cylinder as the engine is being held from moving and this tests the seal of the rings and head gasket. What I do is remove all the spark plugs and using ascrewdriver I will rotate the piston to the top and then down 1/2" to 3/4" past theTDC (top dead center)and mark my screwdriver with tapeso all are tested at the same height. I do this as a carbon ring will form at the top of the cylinder and give you a false reading. I will include Ditchdoc666's engine as a prime example of this. I willthen hold the engine with either a pair of vice grips on the flywheel pushing against the starter or using a breaker bar. The leakage tester has two 100 PSI gauges. One tells mehow much air I am inducing into the cylinder, I use 100 PSI as it is easier to determine %, and the other tells me how much the cylinder I am testing is holding. IF it is holding 90 PSI that is 10%, 80 PSI= 20%, 70 PSI= 30% and so on. So if the cylinder is 10% or less it is a healthy cylinder any more than that it is starting to coke or beginning to have issues. If it has 50% or 60% you have some bad issues. </font></p>

<font color="#000000">I will include one that we found a coked ring on the 2nd ring. What is coking? Carbon is a by product of combustion. So cokingis carbon that builds up around the rings and causes them to stick. In a automotive applications (4 strokes) the rings will rotate and will keep a lot of the carbon from sticking the rings in the ring lands. On a 2 stroke, they have pins that keep the rings from rotating. Because of this I see a lot of rings that will be stuck in the ring groove and willcause high leakage. That is why you need to regularlyrunchemicals like sea foam or Berryman's or anything similar to help prevent it.I will now include some examples..... </font></p>

<font color="#000000">I hope this helps!!!!!!!! </font></p>

<font color="#000000">Ditchdoc666's Engine. The bottom ring was so stuck I had to use pliers and break itto remove it!!!!</font></p>

http://www.chattanoogafishingforum.com/forums/thread-view.asp?tid=27877&posts=27&mid=214730&highlight=150+&highlightmode=1&action=search</p>

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EricM

New member
Great info James, lets us all understand a little more about our engines and how to care for them. emoThumbsup
 

wrechin2

New member
No problem. I have typed this many times and figure it was time to just post it.
 

wrechin2

New member
Bubbakat - 3/6/2011 5:28 PM James if you don't mind I would like to save that and post it some where that its needed also.
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No problem. emoToast </p>
 

jim_pat

New member
James

As usual great post. In the beginning you said you mostly use the compression test for octane usage. At what compression should concider higher octane, and how do take into concideration the load factor ie, boat weight and cargo etc? emoScratch
 

wrechin2

New member
A GOOD RULE OF THUMB.....145 PSI with a bass boat type load use 93 octane with a loop charged engine. Some other factor come into play such as sleeve thickness. A 2L mercury can get away with more compression due to it having a very thick sleeve. It is the ability of heat transfer. You can also get away with more compression if you are spinning a small pitch prop. Almost like comparing walking on flat ground versus climing a mountain. On the OMC you are wanting to built, it will be very hard to get into the "high compression" range without extensive head work. That is why so many people use the 2.7L heads on 3L. I hope this helps!
 

slimjim

New member
Thanks for this timely info. This week a local dealer was servicing my 1995 Merc 150 HP EFI and did a compression test. 3 cylinders were at 120, one at 117, one at 115, and one at 100. The engine starts first time every time and runs great. The dealer knows I would like to have a newer boat. Was he correct in telling me I'm sitting on a time bomb or was he scaring me into trading boats? Thanks for your advice.
 
Good Post James
Thank you
 

wrechin2

New member
Slim, </p>

As you have read there is a difference. There are a few reasons that a cylinder will have lower compression. (1) it can have scoring in the cylinder wall just like the pistons pictured, (2) the rings could have a lot of carbon on them causing them not to move freely and not seal against the cylinder wall but depending on how bad they are carboned, you can sometimes reverse it with cleaners such as seafoam but one as high as ditchdoc's a rebuild was the only way, and (3) the cylinder could have became egg shaped and this is causing the rings not to be able to seal all the way around. (4)head gasket starting to blow.Reasons 2 and 3 will just usually result in a power loss only but is recommended that you do address it.Reason 4 will cause reason 5</p>

Some rare reasons are (5) Bent connecting rod, (6) rod or wrist pin bearings failing, (7) modified (ground on) combustion chamberfromapreviously blown engine, (8) crooked deck to name a few. </p>

Volfans,</p>

Most V-6 engines I charge $35 to do a compression and leakage test on. I highly recommend it when looking at a boat to purchase. I have rebuilt many engines that people have just purchased. </p>

Aretheybiting, </p>

You are welcome.</p>
 

djsmarinelectronics

Active member
I have been told by 3 marine dealers that you can only run a leakage test on a four stroke engine, is that true? Or did they just not have time to fool with it?
 

wrechin2

New member
LOL!!! That is not true. I do and have done leakage test for years on 2 stroke. That would be like saying only a 4 stoke makes compression. On a leakage test the engine is held and then air is induced into the cylinder and it measures the seal of the rings and head gasket by how much is escaping (AKA blowby). On a GOOD cylinder you can measure anywhere from just past the exhaust port to TDC and it will pass by being 10% or less. Did you read Ditchdocs thread? It was all about a leakage test and how we found a stuck ring by doing one on his engine.

http://chattanoogafishingforum.com/...&highlight=150+&highlightmode=1&action=search
 
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