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Originally Posted by Brian Carlton
I would be somewhat concerned with a tester that used the injector openings.
The tester would need to exactly displace the volume of the injector in order to get accurate readings. If the tester allows any air space, where there would normally be the injector, the results will be artificially low.
This also applies to the glow plugs. The glow plug takes up space. If the tester does not take up the same space, the results will not be accurate.
But, the glow plug is much smaller than the injector.  |
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..