[rep_movsd]

I have this crazy idea about how to make an intake tract whose diameter can be varied smoothly on the fly, without having anything get in the way.

Could this be useful in getting more intake velocity while still allowing a large intake?
 

[olhogrider]

Yamaha V-Boost on the V-Max was a clever way to do that. Small intake runners that fed each cylinder ensured good low rpm running. At higher rpm a butterfly would open, allowing each cylinder to breathe through two intake runners. Some newer engines vary the length of the intake to suit the situation.

[Vince]

     This is also done on the exhaust side for the same reasons. Two stroke engines can have mechanically driven guillotine or rotary door valves in the exhaust port. Some sport bikes have electrically controlled valves in the exhaust pipe itself.
     If your aim is pure power these devices are probably worthwhile. In normal use I don't believe the electro/mechanical complexity and cost are worth the trouble. These type of intake and exhaust devices can smooth out power delivery, but so can learning proper throttle technique. And while these devices also offer potential power gains, if you are not racing at Daytona it's kind of a moot point.
     

[rep_movsd]

I was thinking of a thick rubber hose stretched to form a pipe, and two metal rings on either end - they would have a number of rods going from one ring to the other with pivots and also glued to the rubber. When the rings are twisted relative to each other, the pipe gets a parabolic shape and narrows.

It's more of a though experiment than a practical thing.

[ace.cafe]

As a general matter, the intake manifold is not the ideal location to be establishing the critical cross-sectional area.

[rep_movsd]

Where would it be effective then?
Before the carb itself?

[ace.cafe]

Carb throat or valve throat, depending on the port design and application.
Some engines like pushrod V8 American engines are forced to have it at the pushrod pinch in the port, but that is forced by the OEM location of the pushrod tubes.