ACIS is a bit different than TVIS, but they are sort of similar.
ACIS is dealing with sound pressure waves to increase air into the motor, whereas TVIS is dealing with the inertia/speed of the intake air.
In inertial tuning, like TVIS, the diameter of the intake runner has an effect on the speed entering the combustion chamber (Bernoulli, venturi stuff, same amount of air entering a smaller area has to speed up). So smaller makes for faster intake air. But smaller has a limit to the amount of air that can go through it. So what TVIS does, is use a small runner to speed up the intake air (helps make torque), and opens up a second one at higher rpm, when it would start restricting power.
ACIS uses sound pressure waves for a similar purpose. It can actually create 'boost'. But the issue is how they do this. And it's also a bit mystical to me, so this may not be 100% on the ball, so bear with me.
Basically when the valves are suddenly opening and closing, the different pressure changes are making these 'pop' of pressure waves that move at the speed of sound up or down the intake (an exhaust) tract. Every time they meet a big pressure change, they 'pop' again and reverse direction. So they are bouncing back and forth down the intake or exhaust runners. This includes when one pulse from one cylinder goes up into the plenum, and the reaction pulse can be used by the other runners. So what ACIS is trying to do is time these pulses to 'stack up' and be creating positive pressure (boost) just as an intake valve is opening to cram in more air. However this is very rpm limited (this is what the short-runners=high rpm, long runners=low rpm comes from). How they time them and control them is by the lengths of the distance they travel before they reverse. What ACIS does is alters the length and/or volume that the sound pressure wave goes through, so that it takes more or less time, so that they can get it to optimize for more than one specific rpm range.
It gets even more complicated when valve overlap comes into effect, and the exhaust pressure waves actually fly up into the intake and can be used to enhance this even more. This is partly why super-high HP NA motors are peaky. Whereas the factory tunes these different effects to add torque/power in different areas to make a broad powerband. Performance tuning usually tunes them all for a specific, narrow rpm range that maximizes their effect on power.
Hope this helps. A bit.
