Custom Ported Stock TB
A while back I posted images and a PowerPoint spreadsheet on a number of boards describing a (Contour/ZX2) Zetec throttle body that was bored, polished and had a 57.6 mm throttle plate installed. (Stock is 55.0 mm.)
Images of 57.6 mm Zetec Throttle Body
Powerpoint file describing bored throttle body
If you have a larger throttle plate of the appropriate size you can usually bore an existing throttle body to fit. However, making a new throttle plate is more complicated than just cutting a new plate from brass, aluminum or stainless steel sheet stock.
Most throttle plates are closed at an inclined angle, typically 7-10 degrees. If the throttle bore is round the throttle plate must be oval to correctly fit the bore. It also helps if the edges are beveled to come in close contact with the walls of the bore.
To correctly machine an oval throttle plate you need to make a fixture that holds the sheet metal at the correct angle. If you then machine the piece to the correct diameter (less clearance) the resulting plate will be oval. The short axis will equal the machining diameter and the long axis will be the machining diameter divided by the cosine of the incline angle.
For example, the Contour Zetec throttle body was bored to 57.56 mm. With a 0.04 mm clearance the short axis of the throttle plate was 57.52 mm. The incline angle is 7.8 degrees. When the plate was machined at 7.8 degrees the short axis measured close to spec at 57.53 mm and the long axis was 58.03 mm which is close to the calculated 58.06 mm.
For incline angles of 8-8.5 degrees making the long axis 1.01 times the short axis is a good approximation. However, if you try to use a round (instead of oval) plate in a 55-60 mm bore the gap at the long ends will be 0.55-0.60 mm. If the long axis is the correct dimension the throttle plate will jam across the the short axis. The result is too much air leakage, high idle rpm and the dreaded CEL.
I've done this mod for a number of years on Weber carburetors and Porsche throttle bodies. A small machine shop owned by a friend developed a fixture to hold the plates for machining and he has made a number of plates for me.
If anyone is interested, the images and the powerpoint file can be accessed through the links given above. You might be able to find a local shop to make the throttle plates.
The Powerpoint file describes machining the throttle shaft flat on both sides. Other methods,such as removing one side of the shaft or removing a center section could also be used with a custom throttle plate.
There may also be commecial sources for standard sized throttle plates, but I don't have any connections.
