ITB is short for Individual Throttle Bodies.

Where most engines have one throttle valve that suplies the entire engine with air, true performance engines may have a single throttle valve for each individual cylinder!

They are not all individually opened, you only have one throttle pedal, and all cylinders have to work just as hard as the others, so they all need the same amount of air.

When you have read the article about the intake manifold, you might already know what disadvantages that disign has, and what advantages ITB's have.

When every cylinder has it's own throttle valve, the need for a collector and runners are gone. And when the collector and the runners are gone, so is the messy airflow of the air going into the engine. This will result in a better flow into the cylinders along the total revband of the engine.

In the picture above, you can see the ITB's on a Toyota 4A-GE 20V engine. As you can see, the intake runnes can be kept very short and they can be mounted under a much better angle, relative to the engine head (On 'conventional' 4A-GE 16V engines without the ITB's, the runners have a slight curve in them to create sufficient runner lenght).

In this picture, you can still see the black 'airbox', covering the intakes of the ITB's. This is used on this engine, because the ITB's still use a common airfilter.

But that can be done differently and way more attractive and effective!

Another advantage ITB's have, is that you can use so called 'velocity stacks'. When you use these, you van ditch the airbox and tune the intakes a bit by extending them a bit. These extensions have a mouth, shaped like a trumpet. There is a reason that they are shaped like this, othger than just looking very pretty. In contrast to what a lot of people may think, most of the air sucked in through these velocity stacks doesn't come in through it's center. Most of the air is sucked in along the walls of the intake. Because of the trumpet shaped intake, the air is accelerated, leading to better and higher airflow into the engine.

The picture above might illustrate how velocity stacks can improve the airflow into the engine.

And similar to a conventional intake manifold, you can vary the lenght of the intake to tune the engine and create gains at certain RPM's.

Shrter stacks will provide a bit more power at the top end of the rev range. Shorter stacks will provide a bit more power in the mid ranges. So it's up to your own prefrences and use which size stack to mount.

From what I've heard from friends who run these engines with stacks, and read on the web, the Toyota 4A-GE 20V engine benefits most from stacks with a overal lenght of 75mm.