Re: Compression testing, hot or cold?
The compression test we do on airplanes is called a Differential Compression Test, where "X" number of lbs. of air pressure(usually 80 psi) is pumped into the cylinder with the piston at approximately TDC and it is compared to the air pressure the cylinder is holding. Usually you want to see a cylinder hold at least 75% of the psi put in. Any less than that and you have problems. The tool is a relatively inexpensive tool that has two gauges, one for pressure in and one for pressure held. More so than how much pressure the cylinder holds, the more important thing is that all the cylinders be relatively close in compression. The cylinder compressions are usually logged as being 70 over 80 or 76 over 80 with 80 being the pressure in and the other number being the amount held. This is a very good way to detect valve or ring problems. I always do airplane cylinder compression checks with the engine cold. A hot engine's pistons & rings usually expand more than the cylinder itself causing higher readings. I would do the compression check on an outboard cold also.
The compression test we do on airplanes is called a Differential Compression Test, where "X" number of lbs. of air pressure(usually 80 psi) is pumped into the cylinder with the piston at approximately TDC and it is compared to the air pressure the cylinder is holding. Usually you want to see a cylinder hold at least 75% of the psi put in. Any less than that and you have problems. The tool is a relatively inexpensive tool that has two gauges, one for pressure in and one for pressure held. More so than how much pressure the cylinder holds, the more important thing is that all the cylinders be relatively close in compression. The cylinder compressions are usually logged as being 70 over 80 or 76 over 80 with 80 being the pressure in and the other number being the amount held. This is a very good way to detect valve or ring problems. I always do airplane cylinder compression checks with the engine cold. A hot engine's pistons & rings usually expand more than the cylinder itself causing higher readings. I would do the compression check on an outboard cold also.
You're right, a leak down test is a slang term for cylinder differential test. Same thing, but different from a static compression test.<br /><br />Some things folks here might find interesting....<br /><br />Leak down tests for 2-stroke outboards and 4-stroke outboards are performed different using different tools for each. <br /><br />2-stroke leak down tests are done at lower pressures, say 10 psi max, where 4-stroke leak down tests are higher, say 100 psi max. You don't want to blow out seals and gaskets so check the book for what the manufacture of the engine suggests. Thus the 4-stroke tools require compressed air, while the 2-stroke tools can be as simple as a homemade blood pressure pump bulb (that's the type I use).<br /><br />In doing a 2-stroke leak down test, you need to seal off the intake and exhaust. You will be pressurizing the engine crankcase and don't want air leaking out the exhaust or the carb. They make rubber plugs for this, but I use pieces of innertube rubber between the exhaust manifold plate and the engine, and a tapered rubber bung with a fitting (for the bulb hose) that I stuff and clamp in the carb boot. If there is no carb boot sometimes I make a flat plate with a gasket that bolts up. The 4-stroke doesn't require blocking these openings off, because they have their own exhaust and intake valves. You just have to time the engine (TDC) so that these valves are closed prior to pressurizing.<br /><br />For a 2-stroke I like to pump my bulb up to 8-10 psi. A tight motor should hold this pressure for about 10 minutes. If the pressure starts to drop I spray a soapy solution of water all around the motor to find a leak. Sometimes it's the pto or mag seal or the head or base gasket. As skydad posted, with the 4-stroke you're looking for a comparison in the pressure going into the cylinder vs. what the cylinder is holding. This is why the 4-stroke tool requires two gauges and the 2-stroke tool only one. On a 4-stroke you can actually hear air leakage out the exhaust pipe, out the intake, or out the oil filler. Hearing air out the exhaust means a bad exhaust valve. Air out the intake means a bad intake valve. Hearing air out the oil filler means you're losing compression through the rings. Bubbles in the coolant means possible cracked cylinder or block.<br /><br />Oh, yea....I always to a warm compression check first, before the leak down test. Kinda fun stuff to do. <br /><br />Skydad, Lycoming aircraft requires the cylinder differential test to be done with a hot engine. "Operate engine until normal cylinder temperature and oil temperature is obtained. Proceed with test as soon as possible." But I'm not sure what you're working on.<br /><br />I think their theory (and mine) is that things do expand when hot, and in a good engine compression should rise significantly under hotter conditions. But a cracked piston, broken ring, or cracked cylinder wall will not expand as readily (because of the fracture). Thus the compression will not come up proportionately to the other good cylinder when warmed. Cold, they may all look good. But heated, things happen. I almost think of it in opposite....what isn't expanding and sealing, and why isnt' the compression rising?<br /><br /> 
