Compression Ratio for the curious

[Brett Van Sprewenburg <brett@netacc.net>]

For the enternally curious, here's how to figure out your compression
ratio (from one of the Raven books):

	CD + PV + HV + HGV / PV + HV + HGV

CD = cylinder displacement (swept volume of cylinder)
HV = head volume
HGV = head gasket volume
PV = piston volume

CD = bore * bore * stroke * 0.0031416 / 4

To determine the piston volume, lower the piston in the cylinder and
smear a very light coat of white grease around the top of the cylinder.
Turn the crankshaft until the piston comes up to TDC for the piston
being checked (they're not all the same).  The white grease will seal
the rings against the cylinder wall, making it liquid tight.  Wipe
away the excess grease at the top.  Get your piece of plexiglass with
a hole in the center and a graduated burette.  Apply the white
grease to the surface of the head around the cylinder being measured.
Place the plexiglass over that cylinder, making sure the white grease
totally seals the plexiglass to surface.  Fill the burette with a
light oil and decant the oil through the hole in the plexiglass
until the cylinder is filled and there is no air bubbles beneath the
plexiglass.  Note the volume of oil in the burette and subtract
this reading from the volume it had in it when you started.  This is
the volume of the piston.

The same sort of thing must be done with the cylinder head, except
the valves must be sealed (with white grease) and closed.

Compressed head gasket volume can be figured:

bore * bore * thickness * 0.0031416 / 4

Thickness can be measured from the one you just took off, measure
in millimeters.

For example, a compression ratio increase from 10:1 to 11:1 will
net approximately a 1% increase in HP.

This is probably TMI, eh? ;-)

==Brett
 \/  '84 Scirocco (ITB racer 2B) | "Hot VW's, take two home. They're small"
\/\/ '88 Scirocco 16v (Show), '92 Passat 16v (Winter+) | - brett@netacc.net

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