Re: Compression Ratio for the curious

["Vee Dubb" <Scirious@hotmail.com>]

No such thing as TMI, dude.  Good words.
----- Original Message -----
From: Brett Van Sprewenburg <brett@netacc.net>
To: <scirocco-l@scirocco.org>
Sent: Friday, May 25, 2001 12:21 AM
Subject: Compression Ratio for the curious


>
> 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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>

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