Low End vs High End torque

[Knuto]

Thank you

Why don't road cars use this system?

Do they use variable valve timing systems instead of this?, could both systems be used together?

I can't think of any car using continously variable length intake, but a lot of them use a two step manifold:
Variable length intake manifold - Wikipedia, the free encyclopedia
4.6 Intake Manifolds

A 2-stage manifold can also be combined with a resonator
AutoZine Technical School - Engine

I'm not exactly sure why the continous system isn't used on road cars. Telescopic runners may be difficult to make economic when they are curved? Straight runners would maybe to long?
I don't know. Maybe the 2-stage is sufficient.

Exhaust tuning also gives more power. People have tried making variable length exhaust, but the high temperature and length makes it very hard to make a telescope mechanism. Either it leaks or it seizes.... IIRC

my 2 cents