"I have two questions.  What is the function of a restrictor plate? My first thought was that it controled the speed of the car.  Many years ago, when a person bought a new car from a dealership, there was what they called a govenor install for a break in period.  it restricted the cars speed to 50 mph if I remember correctlly.  Then when X amount of miles were put on the engine, it was removed by the dealership."

I'm by no means an engineer, just a fan of NASCAR like everyone else.

That having been said, the example you gave pretty much explains the end of result of what a restrictor plate does.

In slightly more technical terms...

Understand that all combustion engines run on a mixture of air and fuel. The more air is allowed into the combustion chamber, the higher the horsepower. Well, a restrictor plate is a plate that is attached into the air intake chamber of the engine. As you can guess, this restricts the amount of air that's allowed into the combustion chamber and thus, lowers the horsepower which, in turn, restricts the maximum speed that the car can travel at.

  "The second question is, What is the funtion of the oil plate cover?  There was an issue a few weeks back of the oil plate on Carl Edwards car coming loose or being off.  I saw on the program what an oil plate was, I just want to know it's function?  In my minds eye, a loose oil plate cover would mean oil would be flying everywhere. Gearhead Mom"

I'll defer to Stock Car Science for a complete explanation.