Pete,
If you look at page 76 of the Mini Mania performance catalog link below
it gives you all the info you are looking for. But you do need to
measure the cc's of your head to get it accurate.
http://www.minimania.com/pdfile/mini14.pdf
With my Fiat motor boring it 1.2mm over size gave me 0.4 increase and
shaving the head 0.010 gave me another 0.4 increase. Keep in mind an
over size piston will increase you compression if the compression height
and piston head design are the same as stock. Also the compressed
thickness of the head gasket must be included in any calculations you
do. And if you want a real head ache cam timing plays a big factor in
what your actual cylinder pressure is versus your static compression
ratio and it all has to work together to avoid detonation and pre
ignition. The longer the duration of the cam the and the greater the
overlap the lower the cylinder pressure and also the lower the vacuum
thus the requirement for higher compression ratio it's all a dynamic
relationship.
Time for a math head ache.
Doug Hamilton
1960 Triumph TR3A
1963 Fiat 1200 Cabriolet ( now 1262 with 9.1:1 compression )
Date: Tue, 15 Jan 2002 13:30:28 -0800
From: Pete & Aprille Chadwell <pandachadwell@mac.com>
Subject: Cylinder head thickness vs. comp ratio
I'm curious about something. I'm wondering about figuring out
cylinder head thickness specs for certain 'target' compression ratio
figures.
Years ago I acquired a TR6 motor whose cylinder head had been heavily
shaved, but I didn't have any way to know what the compression ratio
was. So, using a little algebra (wouldn't my math teachers be
proud?) I figured out what the compression ratio for that head should
be based on its thickness as compared to Kastner's measurements for
10 or 10.5:1 and based on the stock thickness spec. According to my
figures, that cylinder head has a compression ratio of 11.8:1. Ouch!
No wonder the ring lands on the pistons had shattered!
That all seems convincing enough, but the trouble is I don't know if
the relationship between head thickness and compression ratio is
linear. If it's not, then I don't think my numbers are accurate.
Despite all that figuring, I really and truly belong in the home for
the mathematically challenged, so I can't guess as to whether that
relationship should or should not be linear. Anybody know if you can
extrapolate from known specs to get thicknesses for a given
compression ratio in this way? Assuming my arithmetic was correct
all those years ago, should my 11.8:1 figure be correct?
- --
Pete Chadwell
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