Date:         Tue, 5 Jan 1999 11:24:16 -0600
Reply-To:     Blue Eyes <lvlearn@MCI2000.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Blue Eyes <lvlearn@MCI2000.COM>
Organization: Vexation Computer
Subject:      Re: Compression losses
Content-type: text/plain; charset=us-ascii

Will asked about how compression losses may occur.  Allow me
to take a stab at explaining this.

The compression ratio is the numerical comparison of the
volume contained by the cylinder plus the cylinder head when
the piston is all the way down at the bottom of its stroke
(this position is called "bottom dead center" on the
crankshaft rotation) compared to that same volume at the top
of its stroke, or "top dead center."

Ok, now consider where the trapped and squeezed gasses can
leak.  One route is past the piston rings if they don't seal
against the cylinder walls well enough.   Another place is
the parting line between the cylinder head and the block,
where the head gasket is supposed to seal them together.
Another place is out the spark plug hole, but that's got the
best seal of all these potential leaks.  And the last normal
place is past any valves that lead into of out of the
combustion area.  If you have a cracked head or block, or a
hole in the piston top, that's an abnormal class of leak.
I've experienced all three in overstressing components.  If
anyof these routes is leaking, you need to reseal it.

The valves are called "poppet valves."   They're shaped
rather like the common valve that pops up from the bottom of
typical bathroom sinks.   If it leaks, you can put some
grinding compound (wet scouring powder paste works fine for
sinks) between the seat and the valve and spin the valve
until they wear together.  But if the fluid leaked in one
area so long that it cut a groove in the seat or the valve
sealing contact area, you have to regrind or replace that
surface.   Once a leak starts, the fluid flow through the
leaking area erodes that breach faster than the rest of the
sealing surface until it cuts the parts so badly that just
"touching them up" with some minor regrinding won't work any
more.  Then they must be replaced.  So it's best to catch
valve leaks before the leaking flow does too much cutting.
Got the picture?
John