Ever wondered what your compression ratio is? without having to cc your head?
While researching another subject, I came across the Internal-Combustion
Engine section of "Mark's Standard Handbook for Mechanical Engineers, 8th
Edition". My curiosity was triggered when I noticed a paragraph on
Compression-Compression Ratio.
The paragraph revealed a formula for estimating Compression Pressure.
Quoting, it reads:
"The compression pressure may be estimated from the relation P = Ra E1.33 x
Pm, where Pm is the intake manifold pressure and Ra is the actual Compression
Ratio."
Due to the limits of ASCII text, I have to use an old notation method to
indicate the exponential part of the formula. What it translates to is: The
Compression Pressure is equal to the Compression Ratio raised to the 1.33
power multiplied times the intake manifold pressure.
We all know that the mainfold pressure and compression pressure can be
measured. Therefore, using a little mathmatical manipulation, we find that
the Compression Ratio is the 1.33 root of the (compression pressure divided by
the intake manifold pressure) or Ra = (P/Pm)1/E1.33
In order for the result to be useful, the units of both pressures should be
the same e.g. psi or kg/M2. As you perhaps know, manifold pressures are
usually measured in terms of inches of Mercury or the metric equivalent.
Therefore some units conversion may be necessary for the manifold pressure.
(Hint: multiply inches of mercury by 0.4912 to get psi)
Perhaps this will be of some value to the members, but mind you, this is
considered only an approximation.
If this has already been presented to the List, please forgive the
repetition.
It would be interesting to compare to values determined by the approximation
formula and the volumetric ratio method. I do not have the patience to
remove and cc my cylinder head however.
Anyone wanna try it and advise?
Neil Harmon
Stone Mountain, GA
|