Date:         Wed, 6 Jan 1999 10:33:38 -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

Mark wrote:
"One thing often not considered on four stroke engines, the
above it the
way to figure uncorrected compression ratios. The proper
'corrected'
compression ratio would take into consideration how late the
valve closes
as the piston is rising on compression. The longer duration
the cam, the
lower the corrected compression ratio."


I must respectfully disagree with this statement.  I don't
recall in the
decades I've read about this any in depth discussion THAT
FAILED TO
TAKE INTO ACCOUNT cylinder filling dynamics,  partial
contamination
between charges, scavenging and many other factors.  When I
think of
measuring these things, I'm reminded of the complexities
measuring
loudspeaker systems impedance  that may have crossovers,
electrostatic high frequency units and common dynamic low
frequency
units enclosed in reactive enclosures which alter low
frequency damping
factor feedback.  The complexities of estimating an engine's
true filling
compared to the nominal 100% ratio is just as messy and
difficult.  I
recognize that these complexities exist and find them
fascinating.

But my intentionally simplified discussion of nominal
compression ratio
was not intended to address how valve overlap, tuned intake
port effects,
dirty paper filters vs. K&N foam filters, exhaust system
restrictions and
resonances, rpm loading dynamics and so much more that most
of this
Vanagon subscriber base would find uninteresting.

I only wanted to explain how compression leaks may occur to
someone
unfamiliar with the basics.  In that context, using the
nominal compression
ratio seemed just right to me.  For someone trying to learn
the basics, a
simplified explanation still seems best to me.  My omission
was elective,
intentional, and I still think, appropriate for the purpose.

John