Compression testing and metric gauges

["John Fielding" <johnf@domain.elided>]

A few days ago I noticed someone asking about the conversion from bar to lb/sq in.  I
intended to reply right away but the Alfetta has been taking all my spare time as I was
trying to get it ready for a race meeting this weekend, now the meeting has been cancelled
so I can spend more time indoors reading Alfa Digests!

Strictly speaking the correct term for the pressure in imperial units is no longer psi
(pounds per square inch) but today the accepted abbreviation is lbf/in2.  Because the
pressure is measured in the amount of lbs, which is a unit of force, acting on a unit of
imperial area - the square inch.  So the correct abbreviation is lbf/in squared.

However, the psi will be with us for many years to come and jolly useful it is too.  But
the really nice thing about the metric unit of pressure - bar - is that you no longer need
to do calculations to work out what your compression ratio is!  The digestee said he
measured 8.2 bar on his gauge, well in that case the apparent compression ratio was 8.2
times atmospheric or 8.2:1 CR.  Neat !

A thing to remember with motors which have radical camshafts - high lift, long duration
etc - is that the motor does not work very well as an air pump at cranking speeds.  So the
apparent CR measured will be less than the actual CR when the motor is running at above
idle speed.  But the compression test is generally intended to pin point a difference in
cylinder pressures to diagnose head related faults etc.

Just thought you would like to know.

John
Durban
South Africa
Alfetta 1.8L turbo - not feeling too good today :(
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