In message m, Martin
Underwood writes

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?

Why were high-compression-ratio petrol engines and 5-Star petrol phased
out? I can remember in the 1970s, many makes of car had a
top-of-the-range model with a high-compression engine.
A butterfly valve controls the flow of air in a petrol engine to keep
the air fuel mixture correct, thus causing a depression on the inlet
side. This is often used to power servo brakes.
I believe 5 star was phased out because of it's high lead content.
--
Clive