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TOPIC: Interesting compression info.

Interesting compression info. 1 year 10 months ago #142823

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I have my '77 140hp Evinrude up for sale, good running motor with t/t. I had a buyer inquire about the compression and I told him it was mid 70s on all four. He said he'd have to rebuild it and then the "negotiations" began, I wasn't going to sell the farm for one man's opin.
Tried to inform him that this was a factory modified motor for running on E-whatever gas. Even has the factory sticker on it stating that. The modification include lower compression, don't know why but here's an interesting discussion about that issue. For you guys info.
It's a cut and paste and then I'll put in a picture of the sticker that I have on my motor.

Low Compression Pistons.... Looking for a reason..
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‎12-03-2014 07:30

Hello E-Nation,

I'm new to E-Nation, but not new to the boating world. I'm looking for a non-bias answer to a question that I've heard asked a lot. I work on a lot of older Johnson/Evinrudes and the first thing I do with one is check the spark and compression. 90% of these engines are older then I am, so I don't know a lot of the little things about them. The first time I had, what I thought at the time, low compression numbers (80's and 90's) I told my boss that this engine has a ring issue. He told me that it was common for the loopers to have low comp numbers. So I took that to heart and didn't think twice about it. Lately I've been thinking about it more and more... Doing some reading on forums, I found someone that talked about the fuel changing and taking the lead out of it, John Rudes went to a lower compression to allow a lower octain fuel to be ran. That makes sense to me... Today my boss told me 2 things that I couldn't really wrap my head around. Let me know what you think, as I have no one else to ask...



First, your compression readings are going to be lower if you use a comp gauge that has a long hose then it would be if you had one that screwed right into the head without the hose. I'm not sure about that...doesn't seem correct.

Second, if you have about 90 psi while cranking, once the engine fires and you start moving that piston, the engine doesn't get all of the spent fuel out.... so you're left with your new charge, mixed with some spent charge that will bring the comp up to about 140 while running. What I was told in school, is that the engine as well as the piston are designed with scavenging in mind, to ensure that all of the exhaust has been removed from the cyl. In doing this, there is always a little amount of the new charge that gets sent out of the exhaust ports. Correct me if i'm wrong. I've been trying to find a good answer to the lower compression engines and have not found what I'm looking for. I just don't understand how a 150 can run so well on such low compression. Any info from some old school evinrude guys would be great..

Thank you
Compression numbers on a 2-stroke engine are dependent on the uppermost area of the exhaust ports. The piston does not start to compress the fuel-air mixture until the top piston ring closes off the exhaust.

As a rule of thumb, performance engines have high exhaust ports and thus at cranking rpm, a compression gauge may indicate a lower psi number than a regular recreational motor with lower exhaust ports. Most people are confused by 2-stroke compression results as they have been indoctrinated by automotive 4-stroke compression numbers as those engines start compressing as soon as the exhaust valve closes and the piston travels a greater distance in the cylinder to "squish" the mixture.
This also explains to me when I take the compression test (with a hose) it reads as stated. When I button it all up and try to turn it over by hand the compression is so hard it's difficult to get over the "hump". The brute fires right up. I've hade motors with low compression before and they were a bear to start.
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Interesting compression info. 1 year 10 months ago #142828

Cal,
Because of the size of the exhaust ports, there is a considerable amount of upward travel of the piston before they close up. So nothing is compressed until that point.
Now, remember in an operating engine you have several (4 or 6 in a V engine) dumping into the exhaust system. Add to that, exhaust back pressure from the water column and heat. All will have an effect actual compression.
As for the gauge & length of hose; the volume of the hose must be added to the volume of the cylinder at TDC & that will also effect the reading. This is regardless of 2 or 4 stroke.
I'm not an engineer, I just accept the fact OMC V engines in particular read low but start easy & run strong. I guess these variables are why OMC never published compression & just gave the within 10% standard.
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Interesting compression info. 1 year 10 months ago #142829

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Thank you. Always good info on this forum.

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Interesting compression info. 1 year 10 months ago #142830

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The 140 is a "hot" running motor, i.e. exactly as was said, it's ported for higher performance. So a garden-variety, Plane-Jane 85hp (which has the same bore and stroke as the 140) will likely have more cranking compression than the 140.

My good boating buddy had a 1350 (the earlier brother of the 140) and he said the compression was so high that he had trouble getting it cranked over. He put a new starter motor on it, bigger battery, and ran larger battery cables. It would start OK but still cranked slower than you'd expect. The octane requirement for these was very critical, as they would ping on poor gas. Even the gas left sitting in the fuel lines and carbs for extended periods could degrade and cause detonation issues if you didn't idle the engine for a while and use up that old gas.

OMC did put out thicker head gaskets to lower the compression. Quite sure there was a service bulletin about that, too.

The most important thing is not the ultimate psi number, but that the compression readings are even with each other. It's almost impossible for an engine to wear out such that every cylinder is worn low to the same value. At least extremely unlikely. I don't think I've ever seen one like that, in a problem engine there's always one or more cylinder with lower readings.

Also more important is how it runs. If it starts well, idles in gear smoothly, accelerates and runs well with the power you'd expect for the horsepower, no nasty mechanical noises, it's probably a good engine. Baseline compression is just one aspect of engine health.

A leakdown test is an even better indication of piston/ring/cylinder sealing and many times will detect a problem that you just can't see with a compression test.

Anyway, the guy is just probably trying to get you down on price. Tell him to come over and see the engine run, that should convince him!
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Interesting compression info. 1 year 5 months ago #143741

What you want to look for on a compression testers hose is a check valve in the fitting which screws into the head. If the hose doesn't have a check valve, you wont get a true reading but with a check valve, the length of the hose doesn't really matter as the compression pressure and the pressure in the hose will eventually equalize. Spinning the motor by rope will take a few extra pulls but it will happen.
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Interesting compression info. 1 year 5 months ago #143742

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Interesting continuation here. I did sell that special motor but the person who bought it could not comprehend the workings and timing. He whined and complained that I cheated him. When he bought the running motor I gave him all the factory information about the special timing and compression, The new rebuilt starter was installed and the motor ran perfect but it did suck gas horribly. He bought another new starter and had the same cranking problem....duh. Messed with the timing and screwed the motor up so bad he parted it out. He just couldn't wrap his head around the modifications. Shame he trashed the special baby. That honey had a working factory t/t, new carb rebuild, new fuel pump, plugs, complete impeller kit, wires etc.
That really soured me to say the least. If he shows at Tomahawk next summer I'll need some help restraining myself.

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Interesting compression info. 1 year 5 months ago #143743

Let me add something to the compression Ga./hose information. Just an FYi
Not all Shrader valves are the same. Depending on the internal spring pressure of the Shrader valve you will get different compression readings. I went through this process while searching for a replacement valve for my old MAC Tools Ga. Thankfully the other mechanic at the shop had the identical Ga. to mine. Had to do some research into it but eventually came up with the correct Shrader valve and compression readings once again matched between the two gauges.

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