Post by colinjay on Dec 10, 2007 16:25:05 GMT -5
In another post, Mick mentioned wanting to check the actual compression ratio of his engine, and having to wait until he gets the time to remove the head from the engine to do so. It is possible, and I believe more accurate to do a CR check with the engine assembled. Here is how I have done it previously;
1. with the spark plug removed, turn the engine over to TDC on compression.
2. lay the bike (or engine if out of the frame) over to the right until the spark plug hole is vertical.
3. using a burette or a large syringe, fill the combustion chamber with a light viscocity oil (I have used 10w for oil in the past) until it reaches the bottom of the spark plug hole, noting how much oil was required. This give you a fairly accurate measurement of the actual combustion chamber volume. Note this will be different the volume you get if you have just turned a head upside down and filled it, as it takes into account the displacement of the piston crown.
4. now for the math, add the combustion chamber volume you have measured to the swept volume of the cylinder, this give the total uncompressed volume. Divide the uncompressed volume by the combustion chamber volume, this give the compression ratio.
An example; results for my modified engine when checked a few years ago.
Combustion chamber volume (planed head + 11:1 Hi comp piston) = 44cc
Swept volume (0.5mm OS piston) = 505.17cc
Uncompressed volume = 549.17cc
Uncompressed volume / combustion chamber volume (549.17/44) = 12.48
Therefore Compression ratio = 12.48:1
Note: getting the oil that you pour into the combustion chamber out is a challenge and will be messy. I have tried two methods;
1. with the spark plug hole blocked, remove the exhaust and kick the engine over and just let the oil go everywhere (very messy)
2. lay the bike onto its left hand side and let the oil run out the plug hole. (a bit less messy)
Either way you will get a lot of smoke the first time you star the engine.
CJ
1. with the spark plug removed, turn the engine over to TDC on compression.
2. lay the bike (or engine if out of the frame) over to the right until the spark plug hole is vertical.
3. using a burette or a large syringe, fill the combustion chamber with a light viscocity oil (I have used 10w for oil in the past) until it reaches the bottom of the spark plug hole, noting how much oil was required. This give you a fairly accurate measurement of the actual combustion chamber volume. Note this will be different the volume you get if you have just turned a head upside down and filled it, as it takes into account the displacement of the piston crown.
4. now for the math, add the combustion chamber volume you have measured to the swept volume of the cylinder, this give the total uncompressed volume. Divide the uncompressed volume by the combustion chamber volume, this give the compression ratio.
An example; results for my modified engine when checked a few years ago.
Combustion chamber volume (planed head + 11:1 Hi comp piston) = 44cc
Swept volume (0.5mm OS piston) = 505.17cc
Uncompressed volume = 549.17cc
Uncompressed volume / combustion chamber volume (549.17/44) = 12.48
Therefore Compression ratio = 12.48:1
Note: getting the oil that you pour into the combustion chamber out is a challenge and will be messy. I have tried two methods;
1. with the spark plug hole blocked, remove the exhaust and kick the engine over and just let the oil go everywhere (very messy)
2. lay the bike onto its left hand side and let the oil run out the plug hole. (a bit less messy)
Either way you will get a lot of smoke the first time you star the engine.
CJ