Quelle Keith Calver und Motor's Auto Engines and Electrical Systems c.1970 (nix Guttenberg)
• If all of the readings are within 10 to 15% of each other, everything is OK. Healthy Minis usually run around 150 PSI give or take -- meters or compression may vary a bit, the main thing you're looking for its consistency
Compression testing and ratiosA
compression gauge tells you what pressure it sees in the combustion chamber when the engine is being driven by the starter motor.
The first problem is that compression gauges are not precision devices nor are they intended to be. They are only good for relative measurements.
A good way to test a compression gauge is to measure the same cylinder several times over a period of time and see how the measurements change.
Second, a cold engine, driven by a starter motor isn't a great approximation of an operating engine. The cam can have large effects on the numbers as can the cranking speed. This is why using a compression gauge to estimate compression ratio is likely to be inaccurate.
However, one table of CR and pressures comes with the following warning: "Just to satisfy the reader's curiosity, below is a table showing the approximate relationship between compression ratio and compression pressure at cranking speeds. However, even if a table such as this could be trusted, there is the question of gauge accuracy." ()
Ratio Pressure psi6.5 110
7.0 120
7.5 130
8.0 140
9.0 150
10.0 160
Kompression – Auskunft von Keith Calver (Mai 2011)
The actual compression ratio for this model (SPI 53 PS) I believe is 9.4:1. If you mean what the compression test gauge readings should be - there is no hard and fast figure as it depends very much on the condition of the engine, in particluar the piston rings and valves to valve seats seal.
It also depends on how the test is done. The correct way to do it is with the engine at proper operating temperature with the throttle wide open.
Variations in this will cause variations in the readings gained. However, they should all be within 5psi of each other. More than 10 psi different and there are problems looming. KC