So, the question is : Why can I get a compression pressure of 520 psi from an engine with a limit of around 400psi from displacement vs volume vs c/r as previously stated?

First and second law of thermodynamics....or,
Development of the Adiabatic Condition.

First Law states that energy cannot be created or destroyed, rather, the amount of energy lost in a steady state process cannot be greater than the amount of energy gained.

Compression pressure is a measure of the effective ability of an engine to compress air, not it's sealing capacity or mechanical ability to perform as an engine....or the effective work it does.

Due to the pressure rise/duration, there is a rise in effective pressure due to compressive heat input.

Saying the engine will compress the air to any particular value due to it's internal dimensions has no value unless you compress the air at a speed that does not generate heat and also, and very important...absolutely, no air leaks!....at present, not an attainable condition.

Running a compression test is a measure of several cycles, heat generated pressure bearing no relation to swept volume and dependent on c/r + rpm speed = psi....or work.

A diesel engine in poor condition is often able to generate 400 psi in summer time and start right up, but in a Canadian winter is no more than a relatively dry boat anchor.

Also, intake valve closing timing is critical to psi generated. As is valve clearence and seat condition....to say nothing of ring land wear.

A more meaningful reading from any combustion engine is a Leak Down Test.


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I think I got part if it when you minus the 2cc it changes the comprssion ratio and I did not think that through.

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