Dick,
You're forgetting one critical variable - valve duration. On the stock MGB,
the intake valve doesn't close until 56 degrees after bottom dead center, while
on the compression stroke. Therefore, while the piston is rising on the
compression stroke, you still have an open valve! That's why you cannot rely
upon the direct mathematical solution you have used to derive the geometrical
compression ratio.
A "wilder" cam will have greater duration (usually) and can actually show a
lesser reading on a compression test than when performed with a stock cam (all
other things being equal.)
Bill
Dick Dell wrote:
> At 11:01 AM 08/31/2000 -0500, you wrote:
> >Is there a calculation which you can use the compression test data to
> >calculate the compression ratio on your car?
> >
>
> Bill
>
> Theoretically, you can calculate the actual compression ratio by simply
> taking a compression test reading with a basic compression tester.
>
> Atmospheric pressure at sea level is about 14.7 lbs/sq inch. This is known
> as 'one atmosphere'. If you live in higher altitudes (like Colorado, for
> instance) your normal atmospheric pressure might be 0.85 atmospheres or
> about 12.5 lbs/sq in. If you life in Colorado and you do a compression test
> and it shows a cylinder pressure of 125 lbs, then you have a compression
> ratin of 10:1. If you live close to sea level and you have a 10:1
> compression ratio then your compression reading would be 147 lbs.
>
> If atmospheric pressure at your location is 13.5 lbs/sq in, and your
> compression test reads 115 lbs, then your compression ratio is about 8.5:1
> (which is 115 divided by 13.5) I have found this to be a pretty accurate
> means of finding your true compression ratio.
>
> If you need to find out what the true atmospheric pressure is at your
> location call your local airport weather service. They routinely give that
> information to pilots who need to check their gauges.
>
> I'm not a scientist, and my process might not be correct, but it's worked
> for me! If I'm wrong, someone please correct me.
>
> Dick Dell
> Raleigh NC
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