Manifold pressure :
Old "high-performance" sports cars in the 1930s had a manifold pressure gauge. I suspect few knew what part it should play in handling the throttle/gears, but they then disappeared in more recent times. I wonder why.
Actually my current car still has a manifold pressure probe, it is just not displayed but used by the automatic gearbox and all its electronic wizardry to determine when to change gear. If you think about it, the concept is somewhat similar to a constant-speed prop, except in five steps rather than truly constant.
And before you ask, yes I CAN do a manual gearbox on a car
