compression with a compression tester

i have some troubles wrapping my brain around this.

when an engine has lets say a compression ratio of 10 to 1 does that means that the compression at TDC is 10 bar if the cylinder pressure is 1 bar (absolute) at BDC? for example when i connect a compression tester in a optimal world it would read 10 bar, or am i confusing things now?

The compression ratio is the volume of the cylinder at TDC divided by the volume of the cylinder at BDC. If the valves were closed with 1 bar of pressure at BDC, you would indeed measure 10 bar of pressure at TDC assuming no leaks through the valves / rings. These are things we can measure so they are often used to characterize the capability of the engine. Sometimes this is called the STATIC compression ratio.

Now when you use a compression tester you are measuring the DYNAMIC compression ratio of the engine. To get the most accurate reading, you need to remove all the other spark plugs, and have the throttle wide open. Because of the dynamic nature of air flow into the cylinder and the valve timing, you should capture the ambient pressure near the bottom of the stroke. And due to the sealing of the valves and rings, you may not measure the expected pressure as the peak. It's possible to measure more or less depending upon the cranking speed.

What the compression test does tell you is if there is any significant difference between cylinders if you check them all.

Dean, you are missing one important factor in your example which is heat. If you'll quickly compress air 10 times from 1 bar not only will it become under 10 bar pressure but it will also get hotter than initial temperature was as per thermodynamics law (because of the work being done to the air by compressing it). Because of that additional heat the pressure will rise some more. Usually compression pressure is 1.3-1.4 times (in Bar) higher than compression ratio providing the piston rings and valves are sealing combustion chamber good enough. If you'll let cylinder to cool down over time the pressure will get very close to compression ratio.

The compression ratio is the theoretical relationship between the swept volume and the unswept volume.

In practice there are several variables, some mentioned above, but a big one is the closing point of the intake valve. The actual compression of the charge starts approximately from the point the valve actually closes.

Because of these variables, manufacturers will normally give a nominal range that might be expected from an engine in good condition, that's warm, and that has a good battery to crank the engine over reasonably quickly, and this may be quite different from what one might expect from the nominal compression ratio.

Where a compression tester comes in very handy, and it's the main reason I use one, is to compare the cylinders of an engine to each other. They should normally be within at least 10PSI of each other, and if one, or more, is significantly higher, or more commonly lower, than the others there is some sort of problem with the engine.

As the check takes just a few minutes, it can be useful to do a check each time spark plugs are replaced, or as part of a normal service, because by recording the values potential problems may be identified before they get too serious.

thanks for the answers guy's this cleared up a lot.