Does anybody know how accurate DIY cylinder-compression gauges are?
Mine was showing around 200 psi on all my Metro cylinders a few weeks (about 500 miles) before the headgasket went.
I've just bought a Micra and it's also showing around 200 psi on all its cylinders as well.
Should I be worried?
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Errrr, how many bar is that?
Is it a Gunsons gauge? I use that and it's pretty good.
I just looked at my gauge which is coincidentally right next to my computer! 200psi is just under 14 Bar which is absolutely fine.
Your head gasket going 500 miles later is irrelevant, it has to go at sometime, before that time it was ok, after it was not ok, it could also have gone from a location which has nothing to do with your cylinders, and oil way or a waterway for example.
Relax your new car is fine.
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Pressure is compression ratio times atmospheric pressure, when done slowly, and ignoring valve leaks. Somewhat higher when done quickly because the gas (air) gets hot when compressed. Atmospheric pressure is about 14.7 lbs per square inch (though the value used for 1 bar is somewhat less at about 14 PSI).
Thus with 9:1 compression ratio (?) you would expect about 9 times 14.7, which is 132 psi, if the gas temperature did not rise (it will), known as iso-thermal. I am a bit rusty these days as to what to add for the temp rise -- known as adiabatic compression. Perhaps 50% is not too much.
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