I'm no expert, but here is a formula a friend passed on to me when I had the
same question.
Compression Ratio = (C - P + G + D + V) / (C - P + G + D)
where:
C = combustion chamber volume
P = piston dome volume
G = head gasket volume
D = deck height volume
V = cylinder block volume
Larry Hoy, Denver, CO. USA
1967 MGB Roadster, Vintage Racer
1969 MGB Roadster, Driver
1970 MGB Roadster, Fixer upper
http://home.cwix.com/~larryhoy@cwix.com
> -----Original Message-----
> From: owner-mgs@autox.team.net [mailto:owner-mgs@autox.team.net]On
> Behalf Of Neil Cotty
> Sent: Wednesday, December 16, 1998 1:53 AM
> To: mgs@autox.team.net
> Subject: Math
>
>
> Hi all,
>
> I know I'm a dumbo so don't bother telling me I am, even after this
> message ok! <G>
>
> I've been trying to learn a little automotive math to work out
> Compression Ratios for the purpose of knowing what to machine, how much
> to machine etc, what affect different pistons have on CR etc etc - 'what
> if' stuff. Anyway the point is, I can't work back to the quoted CR in
> the MG manual of 8.8.1.
>
> I must be missing something, or receiving something different to what
> I'm expecting - or more probably the data I have is not correct for the
> calculation.
>
> Bore: 3.16"
> Stroke: 3.5"
>
> >From that I calced Swept Volume to be: 449.82 cc's.
> Combustion Chamber Volume: 42.5cc
>
> SV + CCV / CCV = 11.58 CR
>
> Ok, what am I doing wrong? My spreadsheet I did up works from the test
> data in this blueprinting book I have. Is there something different
> about the data I'm supplying? I guess there must be.. I've ordered a
> real auto math book that I guess will answer this, but somebody want to
> explain anyways? :)) Is this a GCR vs CR thing? I have no idea what the
> difference is at this point!?! <G>
>
> Comments please! :) Total newbie - handle with care. <G>
>
> Thanks,
> Neil.
>
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