Compression question

[Workshop]

For me, a compression gauge reading is nothing more than a test to see how well the cylinder (piston, rings, gaskets and valves, in the case of a 4 stroke) is sealing. It's used to check the mechanical condition of the engine. Where real power is generated is how well the cylinder is filled with air and fuel. However, you still have to compress it to get power out of it. So, compression readings do come in to play in as much as a higher reading can indicate a better performing engine. Volumetric efficiency is where real power comes from. An engine is basically an air pump. How well the cylinder is filled at speed determines how much power comes from it.
I'm reminded of reading a few years ago about NASCAR engines. It was stated that the static compression was about 9 or 10 to one. But at eight thousand rpms, through volumetric efficiency, the compression was equivalent to about 14 or 15 to 1.
Just my thinking on the subject, but I'm no expert.

 

[Deets066]

For me, a compression gauge reading is nothing more than a test to see how well the cylinder (piston, rings, gaskets and valves, in the case of a 4 stroke) is sealing. It's used to check the mechanical condition of the engine. Where real power is generated is how well the cylinder is filled with air and fuel. However, you still have to compress it to get power out of it. So, compression readings do come in to play in as much as a higher reading can indicate a better performing engine. Volumetric efficiency is where real power comes from. An engine is basically an air pump. How well the cylinder is filled at speed determines how much power comes from it.
I'm reminded of reading a few years ago about NASCAR engines. It was stated that the static compression was about 9 or 10 to one. But at eight thousand rpms, through volumetric efficiency, the compression was equivalent to about 14 or 15 to 1.
Just my thinking on the subject, but I'm no expert.

If two engines had the exact same size chamber, piston crown height, squish, ex. Port height. Only then could you use compression reading to determine ring seal.

Lots of variables

 

[Workshop]

Yes, lots of variables. That's why I say it should be used as a test, a way of gathering information to determine what needs to be done to repair an engine. With modern production technology all engines are, theoretically, created equal. With that being said, why does one run better than another? Hence, variables.
That's why I say compression readings should be used as part of a diagnostic procedure.

 

[Deets066]

Compression readings can be used for diagnostic readings, I totally agree. But comp readings are used as a tool when determining what machining needs to be done as far as port work goes. Once again there are more variables.

But a compression testor has more that one use

 

[mdavlee]

Higher elevations need more compression to help the loss of air density. Some of those turn out to be 225-235 for me at 1200'. They end up 180-200 when they're up around 5k'. Other than that use or when selling someone asks for a reading.

 

[sawfun]

Higher elevations need more compression to help the loss of air density. Some of those turn out to be 225-235 for me at 1200'. They end up 180-200 when they're up around 5k'. Other than that use or when selling someone asks for a reading.

That sounds like nitro tuning. Combustion chamber volume vs air density. And volumetric efficiency IS the key. If you can get more fuel in and get it to burn you get more energy. If you take away space for this to happen via too little combustion chamber area or timing which does not allow for the best volumetric efficiency you get less power even though you may have great static compression. Plus you've created more resistance through the energy required to compress the air or mixture resulting in more heat. More heat can help make for a more complete burn but is also an energy loss in its own right. Sometimes more fuel, as in a rich setting, can cool an engine. The trick is ,through ignition timing, and ignition power ((hotter spark), being able to light the increased fuel volume to make use of the greater potential stored power. Different fuel types can make this tuning more challenging.

 

[wcorey]

I'd be interested to see how much the differential between the two would change with a little oil in the cylinders.