The engine is the beating heart of any stock car. Stock car engines are pushrod-driven V8s displacing no more than 358 cubic inches. The blocks are made of high alloy content iron, and support two-valve aluminum heads with combustion chambers so small they're practically nonexistent. All American stock car classes currently require that the engine run on a carburetor, mostly because electronic fuel injection is too difficult to regulate.
NASCAR engines generally produce between 750 and 800 horsepower, and commonly run at sustained rpms of between 9,300 and 9,600. That kind of engine speed is fairly astronomical for any V8, especially for those of the fairly large displacement that NASCAR allows.
Given their incredible power to weight ratio, your average NASCAR's performance in the quarter mile is nothing to write home about. According to Drag Times, one 2004-season Nextel cup Intrepid ran 11.9 seconds on the quarter mile at 137 mph. But don't take this to mean that stock cars are in any way slow. That 137-mph trap speed at the dragstrip is about 75 mph shy of the car's true top speed (around 210), which is a serious indicator that such stock cars simply aren't geared for acceleration. With quarter-mile-optimized gearing and drag slicks, such a car could easily see low 10- or high 9-second timeslips.
Stock cars rely heavily on aerodynamics. At 200-plus mph, even the most minuscule of changes to the car's body can result in a huge gain or loss in speed. Stock cars run as low to the ground as the rules allow (which reduces how much air they have to move and decreases body lift at speed), and use large, angled rear spoilers to increase downforce at high speed. Illustrating the fully integrated nature of stock car design, even the springs in the suspension play a role in aerodynamics. NASCARs tend to use springs with one or two very soft coils, which allows air pressure at speed to push the car closer to the ground.
Race speeds are actually quite a bit lower than they were during the aero-wars of the 1960s and 1970s. While 210-plus mph averages were once the norm at Talladega (the fastest track in NASCAR's arsenal), averages in 2010 run closer to 190. It's not that modern NASCARs can't go faster (Rusty Wallace hit over 228 mph during an unrestricted testing run in 2005); NASCAR's officials intentionally limit the cars' potential top speeds with restrictor plates (which limit how much air the engine can inhale) and gearing limitations. If top speeds attained in NASCAR would have continued to rise at the same rate that they did from 1955 (100 mph) to 1975 (217 mph), 2010's NASCARs would regularly see a little over 400 mph.
As defined by Webster's dictionary, "stock" means "kept regularly in stock." However, this descriptor has become something of a joke when describing modern stock cars; in most cases, the only parts on any modern stock car to come from an assembly line are the roof and trunk lid. Modern stock cars are designed to a set of regulations and are as purpose-built as sniper rifles.