In article m,
Martin Underwood wrote:
Why should the compression of a petrol engine vary with throttle
position? Surely compression ratio is determined simply by the ratio of
the volume of the cylinder with the piston at the top of its travel to
the volume with the piston at the bottom of its travel. Or am I missing
something very obvious?

It's the actual pressure caused by compression that's important, and a
throttle causes a partial vacuum - so compressing air at less than
atmospheric pressure gives a lower final pressure. Similarly, using a
supercharger etc increases 'atmospheric' pressure and the final compressed
pressure. If you could develop a supercharger which was super efficient,
you could negate the effect of the throttle.

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*It was all so different before everything changed.

Dave Plowman London SW 12
RIP Acorn