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Originally Posted by TonyFromWestOz
Surely the compression test is done over a few compression cycles of that cylinder. If the compression tester has a non-return valve, the discrepancy will be negligible.
I have never yet seen a compression tester, the volume of which was matched to the size of a spark plug on a SI engine. The gauge itself has a volume much greater than the spark plug displacement, then you need ot connect the valve to the combustion chamber.
If the internal volume of the compression tester increases the combustion chamber volume by less than 10% of the swept volume, and a non-return valve is fitted where it connects, the error in compression pressure will be less than 0.1% after 3 compressions and less than 0.01% after 4 compressions..
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Let's look at the volume that is prior to the non-return valve. Everything that is past the valve is not relevant, because multiple compression strokes will reduce any error in the gauge itself.
So, say the volume of the cylinder is 22 cc. with the glow plug in place. And, let's say the glow plug is taking up 3 cc. of displacement below the threads.
If you now install a tester that installs into the threads and does not extend down toward the top of the piston, the volume of the cylinder is now increased from 22 cc. to 25 cc. This would equate to a drop in pressure of 12%.
On an SI engine, say the volume of the cylinder is 66 cc. A spark plug, below the threads, has very little volume. There might be 1 cc. below the threads. So, a loss of 1 cc. of displacement due to the spark plug would only be 1.5%.
If the test is performed with the injector holes, the error has the potential of being significantly greater.