> The ONLY way
> to accurately determine compression ratio is to measure cylinder and
> chamber volumes. Here is the formula. Use it and you will never have
> to worry. Compression Ratio (CR) = V1 + V2 divided by V2. V1 is the
> volume of the cylinder with the piston at bottom dead center (bdc). V2
> is the volume of space above the piston at top dead center (tdc). By
> using this formula, you don't have to worry about piston shape.
Ted, I either disagree, or I don't understand what you're saying. The
formula is right, but to get the right answer, V1 has to be the swept
volume, and V2 has to be the unswept volume. If for example you have dished
pistons (not sure why you would on a Triumph, but just for example), the
dead space in the 'dish' has to be included in V2, but not in V1.
As an extreme example, my 1970 Audi 100LS had the combustion chamber
entirely cast into the top of the piston, the head surface was flat ! Your
measurement method would give it an infinite compression ratio. (It was
actually about 10.2:1, but the funky inverted hemisphere combustion chamber
shape let it run without knocking on regular pump gas.)
Likewise, for domed pistons that come above the block surface, the volume of
the dome must be subtracted from the measured V2. So, the shape of the
piston must always be taken into consideration when measuring CR. Only when
it's flat, and comes level with the block surface at TDC (which is the case
for most stock Triumphs), can it be ignored.
If you do have a funky piston shape, one way to measure it is to compare the
measured cylinder volume at BDC with the computed swept volume; the
difference gets added or subtracted to V2. Perhaps this is what you meant
to say ?
Randall
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