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Time to do compression check on 2 motors

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Old 01-21-2004, 11:03 AM
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Default Time to do compression check on 2 motors

Having compression check done on 2 motors on boat that I may be purchasing. The mercruiser mechanic that I have contacted charges $70 per hour for labor. How much time should it take to do a compression test on two 350 mag's in a boat 8'6 wide so there is plenty of room?
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Old 01-21-2004, 11:08 AM
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no more than 2 hrs..i would think
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Old 01-21-2004, 11:10 AM
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Outside plugs, about an hour,

Inside plugs, who knows, depends on how close the motors are to each other!!!! Shouldnt be too bad with small blocks.
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Old 01-21-2004, 11:21 AM
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Is he doing a leak-down comp check?
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Old 01-21-2004, 11:28 AM
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They had just recommended a compression test so I don't know the answer to that.
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Old 01-21-2004, 11:54 AM
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The compression check will only provide a fraction of information that the leak-down would. The leak-down almost provides too much info as it reveals problems that even the current owner wouldn't be aware of.
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Old 01-21-2004, 12:10 PM
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what is involved in doing a leakdown test? can this be done while the compression is being done? Cost???
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Old 01-21-2004, 12:43 PM
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Leak-down comp checks have been done on piston engine aircraft engines since they've been around, (and are required by law to be done annually). This procedure has been adopted into the marine industry to determine true engine health, and pin-point locations of problems.
A set pressure (usually 80 psi) is placed in each cylinder while it is at TDC, compression stroke through the spark-plug hole, and the resultant pressure is read on a second gauge on the manifold set.
In a perfect cylinder with great ring and valve sealing, the pressure would be 79-80.
Leakage air heard through the carb would be air leaking past the intake valve, air heard through the exhaust would be leakage past the exhaust valve, and air heard through the breather would be leakage past the rings.
If a tech is proficient at this check, it shouldn't take mcuh more than a few hours on a twin SBC given decent access, and will give you a lot better idea on the condition of the engines.
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Old 01-21-2004, 12:43 PM
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Yea, it can be done, not at the exact same time, but along with the compression check. In all honesty, one would normally do it in lue of.

For a leak down test, they hook up a air line to the spark plug hole and measure the amount of air that is leaking out of the cylinder. If an excessive amount of air is found to be leaking, they will go further and determine where the leakage is. They repeat this for every cylinder. This will give a brutally honest report on the engines condition and what is wrong. Try to be present for the test. You'll find it to be educational.
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Old 01-21-2004, 01:08 PM
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Just a question, I have never really undersood the leakdown vs. Compression test. Your explanation of where the air is leaking now explains that. BUT, wouldnt it only be necessary if a Cylinder is not holding compression. What is the difference between a cylinder holding 160# of compression on a comp test vs. 80# on a leakdown test? If it will hold the comp, isnt everything OK??? I am only asking because I just did a comp test, and everything was OK, what would I gain by doing a leakdown???
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