A pop quiz on some engine tests

[flderoos@mmm.com]

>Date: Thu, 02 Apr 1998 10:51:16 -0500
>From: four.rings@MCIONE.com
>Subject: A pop quiz on some engine tests

>Now the question: how come that after doing the head work and lapping
>the valves the compression figures have dropped while the leak down >test
>shows a great improvement, meaning that the valves are lapped very well
>and that the rings seal very tight? Why is such contradiction? A damn
>mystery!

Igor,

Well, I see two things that would cause the decrease in cranking
compression and the decrease in leak down percentage.  First, the valve
grinding should improve (decrease) leakdown assuming it is the major
source of leakage.  With bad rings, it may or may not cause much of a
change in leak down percentage.  

Secondly, by grinding the valves, you have increased the effective
combustion chamber volume, thus lowered the compression ratio.  This,
however, will be a minor change.  The major source of the decreased
cranking compression would be from the removal of the carbon from the
pistons.  With carbon buildup, you have a higher compression ratio than
you would calculate from combustion chamber volume and cylinder volume.
In effect, you have domed pistons.  So with the removal of the carbon,
you are back to the original cylinder volume and consequently the lower
compression.  Carbon build up is one of the reasons that the high
performance engines of the sixties (no knock sensors to compensate by
varying timing) would run great for a while, then start knocking.  Heat
retention by the carbon also was a problem).

Good quiz,

Fred