The throttle plate is connected to the accelerator pedal either by a cable or electronically and allows more air into the cylinder when the pedal is pressed. This in result produces more power.
After an upgrade of the air filter and intake path, the next potential flow restriction is at the throttle plate(s). In most compressor applications a single body will be used before the blower inlet, and on turbo cars and naturally aspirated cars attached to the inlet plenum. On certain sports models multiple throttle bodies are used to provide more fine grain control of the air flow to each cylinder.
Single throttle body plate size
|Throttle plate (mm)||Nitrous - Max. HP||Blower* - Max. HP||Turbo - Max. HP|
Multiple throttle body plate size
|Throttle plate||Valves per cylinder||Priority|
|1.5-1.7 times the inlet valve diameter||4||High speed performance|
|1.2-1.4 times the inlet valve diameter||4||Low speed performance|
|1.4-1.6 times the inlet valve diameter||2||High speed performance|
|1-1.3 times the inlet valve diameter||2||Low speed performance|
If your stock throttle plate is too small look for a replacement at the breakers with a larger plate. Ensure that the idle air control and throttle potentiometer are compatible and are at the same location. Alternatively, if the stock throttle body wall thickness is sufficient, the body can be machined to accommodate a larger plate thereby avoiding any compatibility issue with another throttle body. Most will bore at least 2mm, and many will allow 5-7 mm. The target bore size should be based on the new throttle plate dimension, found from a breaker or machined.
Remember, the larger the plate the more fine-grain control is lost. Driving around in city with a very large single plate can be a jerky experience. A smaller plate allows for a more steady increase in power, while a larger plate gives a more steep increase in power. This can be solved by using an electronically controlled plate (by programming the ecu), multi throttle setup or a progressive throttle setup.
Any lips, ridges and edges should be ground down to increase flow as much as possible.