NASCAR, the highest governing body and top level division for stock car racing in the United States, has used a range of different types of engine configurations and displacements since its inaugural season in 1949.

The 1949 Oldsmobile Rocket V-8, with a displacement of 303 cu in (5.0 L), is widely recognized as the first postwar modern overhead valve (OHV) engine to become available to the public.

However, the end of the Korean War in 1953 started an economic boom, and then car buyers immediately began demanding more powerful engines.

However, even without official factory support or the use of fuel injection, Buck Baker won in 1957 driving a small-block V-8 Chevrolet Bel Air.

In 1961, Ford introduced the "FE" (Ford-Edsel) 390 in a low drag Galaxie "Starliner", but 1960 and '61 championships were won by drivers in 409-powered Chevrolet Impalas.

It became apparent that manufacturers were willing to produce increasingly larger engines to remain competitive (Ford had developed a 483 they hoped to race).

However, modern technology has allowed power outputs near 900 horsepower (670 kW) in unrestricted form while retaining the conventional basic engine design.

[25][26][27] In fact, before NASCAR instituted the gear rule, Cup engines were capable of operating more than 10,000 rpm, and producing near 1,000 horsepower (750 kW).

Contemporary Cup engines run 9,800 rpm, 87.59 fps (26.95 m/s), at the road course events, on Pocono Raceway's long front stretch, and at Martinsville Speedway (a .526-mile short-track).

At the backbone 1.5- to 2.0-mile tri-oval tracks of NASCAR, the engines produce over 850 hp running 9,200-9,400 rpm for 500 miles, 600 mi for the Coca-Cola 600 Charlotte race.

[35][36][37][38] NASCAR will test a hybrid powertrain at the 2023 24 Heures du Mans in the Next Gen car with a smaller engine, per request of the Automobile Club de l'Ouest with a potential of a formula change in 2024.

Unrestricted, Sprint Cup cars produce over 750 horsepower (560 kW) and can run at speeds in excess of 200 mph.

However, an unexpected consequence of the introduction of the Car of Tomorrow and re-paving of Daytona and Talladega track surfaces has been the separation of the pack into two-car pairs.

Beginning in 1971, NASCAR rewrote the rules to effectively force the Ford and Chrysler specialty cars out of the competition by limiting them to 305c.i.

After a series of flips and dangerous crashes in the 1980s, NASCAR began requiring all cars to run a restrictor plate at Daytona and Talladega.

At these races, in addition to the restrictor plate, there are a variety of other technical rules and regulations to keep the cars stable and on the track.

At the high-speed superspeedways, track limits are marked by a double yellow line separating the apron from the racing surface.

The superspeedway track limits (often referred as the "yellow line rule") have been part of considerable criticism and controversies, such as when Regan Smith was stripped of second-place finish at the 2008 AMP Energy 500 following a last-lap pass attempt that went below the line[48][49] and controversies surrounding the finish of the 2020 YellaWood 500, with former drivers turned television coverage pundits Dale Jarrett and Dale Earnhardt Jr. (the latter being involved in a track limits controversy at the 2003 Aaron's 499) calling for the repeal of the rule.

The following race at Talladega that year would be run with a smaller carburetor, however, NASCAR mandated the use of the restrictor plate at the end of the season.

The restrictions are in the interest of driver and fan safety because speeds higher than the 190 mph range used for Daytona and Talladega risk cars turning over through sheer aerodynamic forces alone.

Drivers such as Rusty Wallace have cited data showing that the roof flaps used on the cars cannot keep them on the ground above 204 MPH.

[citation needed] The drawback to the use of the restrictor plates has been the increased size of packs of cars caused by the decreased power coupled with the drag the vehicles naturally produce.

The shape of the spacer helps a car funnel more air smoothly into the manifold, increasing fuel performance, while ensuring airflow is still restricted.

Allison's Buick LeSabre blew a tire going into the tri-oval at 200 mph (320 km/h), spun around and became airborne, flying tail-first into the catch fencing.

The use of restrictor plates, intended as an emergency measure pending a more permanent replacement, in any event, was discontinued at New Hampshire for the following race for Cup only.

Restrictor plates remain a permanent fixture on the Modifieds and the racing has often broken 20 official lead changes for 100–125 laps of competition.

A frequent criticism of restrictor plates is the enormous size of packs in the racing, with "Big One" wrecks as noted above singled out for condemnation despite the greater violence of "smaller" crashes on unrestricted tracks.

In restrictor plate racing the packs have brought about an often-enormous increase in positional passing; at Talladega Superspeedway the Sprint Cup cars have broken 40 official lead changes sixteen times from 1988 onward, including both 2010 Sprint Cup races at Talladega, which had 87 official lead changes in the regulation 188 laps.

(The 2010 Aaron's 499 had 88 lead changes, but the 88th – the race-winning pass by Kevin Harvick – was on the last lap of the third attempt at a green-white-checkered finish).

Daytona International Speedway has generally been less competitive because the age of the asphalt (the track was repaved in 1978 and again in 2010) has reduced grip for the cars and thus handling has impeded passing ability to a significant extent.