The throttle body is another potential restriction in the air stream. Again, you should use a water manometer to measure the air flow at various RPMs before and after the throttle body. Generally, the throttle bore should be the same diameter as the air intake duct. If it is not, you may need a larger throttle body. Sometimes the stock throttle body has enough wall thickness to allow you to machine it to a larger bore, eliminating the need for a compatible idle air control. Most throttle bodies will allow at least a 2 mm increase in the throttle bore while most will allow an increase of up to 5 mm, and even 7 mm.
Increasing the throttle bore would not require a reprogramming of the engine control unit (ECU) but you would need to replace the throttle plate with a larger one when you increase the bore on the throttle body. Here you have two options: either have a throttle plate machined with the correct radius and angles around the edges, or bore the throttle body to a size that will allow you to fit a larger throttle plate from another engine. In most case, the latter will be the easiest and the most cost effective. Another option that will be even more cost effective would be to fit a larger throttle body from another engine. However, this option is complicated by the need for a compatible idle air control.
Throttle Plate Requirements
Another thing to consider on the throttle body is the smoothness of the throttle bore. Ridges, lips and rough surfaces in the throttle bore will disrupt air flow; therefore you should smooth them out be grinding away lips and ridges as you would when porting the cylinder head. You can accomplish this by using a die grinder. However, if the surface is rough, you may need to machine the bore and fit a slightly larger throttle plate. It would thus be better to take the condition of the throttle body into consideration when opting for a larger throttle body as it may make more sense to bore your existing throttle body and fit the throttle plate from a larger throttle body.
You should also slip stream the throttle spindle and the fixing screws but take care not to weaken the throttle spindle too much.
Multiple throttle bodies
When you fit a larger throttle body, you compromise a bit of drivability. This is because a small movement of the accelerator will lead to a larger increase in air flow through the larger throttle body. This could make the car jerky in traffic, and could lead to wheel spin on wet roads. A solution is to fit a multi-plate throttle body with two throttle plates. In this set up, only one throttle plate operates at low RPM while the second throttle plate only comes into play under heavy acceleration. This is accomplished by having the second throttle plate only opening when the first throttle plate is about 40% open.
Another solution is to have one throttle body for each cylinder. This would provide the best throttle control as the throttle bodies are close to the cylinder head. It will also have one throttle plate for each cylinder, which greatly improves airflow into the cylinder, enhancing power and provides the best top end power.