How a differential pressure test is done

[boscoe99]

This applies to an airplane motor but the principle for an outboard motor is the same. This test is also erroneously referred to as a "leakdown" test in the marine world.

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Lots of other tips that DIY'rs might find useful. Enjoy

[rodbolt17]

no
its erronesously known as a differential test in the oxygen deprived world.
sheet I have been doing leakdown tests on race car and auto stuff since about 1975.

ALL it does.
simply freaking ALL.
\
you introduce a KNOWN pressure into a THEORETICALLY sealed cylinder.

then the OTHER gauge monitors how much MAKE UP air it takes to MAINTAIN that pressure in that SEALED cyl.

this aint freaking rocket science and the leakdown OR differential test has been about since about the 1920"s.
simply goes back to, suck
squeeze
bang
blow.
lose ANY 1 of them and performance suxs.