What does Overdrive do and I have a knocking?

[dbcel]

First i was wandering what Overdrive does for your car. Is better on or off? Second I have a 91 240sx. For the past month it had a light knocking then all of a sudden it got loud and my car almost stalled 3-4 times and the oil light comes on but i just changed the oil. Its so bad that i can't even drive it anymore. It mainly happens when i rev the engine and let off the gas. And it smells bad also.

[DAEDALUS]

Overdrive is just a tall gear...I think the definition is that it moves the final drive ratio below 1:1. Improves gas mileage at least.

As for your knock, that sounds really bad. Sounds like rod knock to me. One of your bearings might be going, which would explain your low oil pressure. Get a real gauge on there and measure the pressure. I bet it's below 10psi hot. A good shop should be able to diagnose it. Might be time for a swap.

[leper421]

First i was wandering what Overdrive does for your car. Is better on or off?

If you are refering to the overdrive button on an AT shifter, it tells the transmission whether or not to use 4th gear. Turning it off when in 4th will force the transmission to downshift. Its only practical use is to get more power when cruising above 50mph if you want to pass someone or just want a burst of power.