You might not notice it in warm weather if it's missing, but your car also came stock with a warm air intake, it's a flexible piece of air duct that goes from the exhaust manifold to the air intake, they sometimes go missing because they get tattered and the car seems to run ok without it... for a while, but make sure it's there and the engine will see a constant supply of 100F air.
It's part number 2 in the drawing, if it is missing auto part stores can get it if you ask for flexible heat duct.