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A restrictor plate or air restrictor is a device installed at the intake of an engine to limit its power. This kind of system is occasionally used in road vehicles (e.g., motorcycles) for insurance purposes, but mainly in automobile racing, to limit top speed and thus increase safety, to provide equal level of competition, and to lower costs; insurance purposes have also factored in for motorsports.
Racing series 
A few top classes like Formula One limit only the displacement and air intake mouth dimension. However, in 2006 air restrictors (as well as rev limiters) were used by Scuderia Toro Rosso to facilitate the transition to a new engine formula.
Many other racing series use additional air restrictors.
- Formula 3, 2000cc, 215 hp
- Deutsche Tourenwagen Masters, 4000cc, 470 hp
- FIA GT Championship (now FIA GT1 World Championship) and other series using FIA GT regulations
- Le Mans Prototypes used in American Le Mans Series and Le Mans Series have restrictors based on precalculated tables depending on the type and size of the engine and fuel
After Group B cars were outlawed from rallying because they were too powerful (rumored to have reached 600 hp), too fast and too dangerous, the FISA decided that rally cars should not have more than 300 hp (220 kW). For a while no special restrictions were needed for that (e.g. the Group A Lancia Delta HF 4WD had about 250 hp in 1987). But with development in the 1990s, Group A cars were rumored to have reached 405 hp or more. So the FIA mandated restrictors for supercharged and turbocharged engines in all categories (World Rally Car, Group A and Group N).
This means that the rally version of a car like the Mitsubishi Lancer Evolution can have less power than the street version (the "280" hp Evo VII was believed to have more than 300 hp, and in some markets the FQ-320, FQ-340, FQ-360, FQ-400 versions were sold, with the number representing the total horsepower).
It also means that the torque and power curves of the engine are unusual. The engine produces peak torque and almost maximum power at a relatively low RPM, and from there to the rev limiter the torque drops and the power does not increase much.
In 1995 Team Toyota Europe used an illegal device to bypass the restrictor (allowing an estimated extra 50 hp). Due to this the team lost their results in the 1995 season and was banned from rallying until the end of 1996.
NASCAR's Sprint Cup and Nationwide Series have used restrictor plates at Daytona International Speedway and Talladega Superspeedway since 1988. They are still in use with the 2012 season introduction of McLaren Electronics engine control unit providing electronic engine management for all NASCAR races and cars with carburetors finally being phased out.
The device limits the power output of the motor, hence slowing both the acceleration and the overall top speed obtainable on the tracks where the cars are so equipped. NASCAR routinely stated that the Sprint Cup restrictor plate reduces engine power from approximately 750 hp to approximately 430 hp.
A major effect, however, is that all drivers tend to form very large "packs" of cars that run closely together for the majority of the race. In these packs drivers often run three abreast across the track, consequently there may be as little as one second separating the entire field. These large packs reduce air resistance which allows the cars to run faster and makes drafting easier.
The restrictions are in the interest of driver and fan safety because higher speeds are closer to out-of-control than the 190 MPH range used for Daytona and Talladega; the severity of crashes at higher speeds is also much greater, shown by telemetry readings of wrecks such as Jerry Nadeau at Richmond and Michael McDowell at Texas that were far higher than registered on restrictor plate tracks. Drivers such as Rusty Wallace have also cited data showing that the sport's roof flaps cannot keep cars on the ground above 204 MPH.
At Daytona and Talladega, most races are marred by at least one multi-car crash as cars rarely become separated. Talladega has been considered the more likely track for these instances to occur as the track is wide enough to have three to four distinct lines of racing. With the new pavement at Daytona, three-wide racing is now possible and multi-car wrecks could become more common. The 2011 Daytona 500 saw a record number of cautions including an early 17-car pile-up. These wrecks tend to be singled out for criticism despite multicar crashes at other tracks and the generally greater severity of impact on non-restricted tracks. In addition, the packs were far smaller in 1988 through 1990 until more teams mastered the nuances of this kind of racing and improved their cars (and drivers) accordingly.
The 2011 Sprint Cup season was the last complete Cup season with carbureted engines. NASCAR announced that it would change to fuel injection for the 2012 season. The injection system used by NASCAR is a different system from that used in IndyCar Racing and other motorsports series and restrictor plates are usable with the NASCAR system; they were used for January 2012 testing at Daytona. The current restrictor plates are bolted beneath a throttle body that sits in the same place as the former carburetors. In October 2012, a major pileup happened at Talladega, in the last lap of the race. Most blame these restrictor plates.
Reason for restrictor plates 
The first use came in 1970 in response to escalating speeds. Following testing and input from drivers such as David Pearson, Bobby Isaac, and Bobby Allison, NASCAR mandated the use of a restrictor plate. Smaller engines, in the 358 cubic inch range, were exempt from the plates; the first car to race with a small block engine was Dick Brooks at the 1971 Daytona 500, where he ran a 1969 Dodge Daytona with a 305 CID engine. The transition period lasted until 1974. As the early 1970s use of restrictor plates was considered a transitional process, and as not every car used restrictor plates, this is not what most fans call "restrictor plate racing."
The second use came following the crash of Bobby Allison at the 1987 Winston 500 at Talladega Superspeedway. 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. While the car did not enter the grandstands it ripped out nearly 100 feet of fencing and flying debris injured several spectators. After a summer where the two subsequent superspeedway races were run with smaller carburetors (390 cubic feet per minute (cfm) instead of 830 cfm) proved to be inadequate to sufficiently slow the cars, NASCAR imposed restrictor plates again, this time at the two fastest circuits, both superspeedways: Daytona for all NASCAR-sanctioned races and Talladega for Cup races. The Automobile Racing Club of America also enforced restrictor plates at their events at the two tracks. In 1992, when the Busch Grand National series began racing at Talladega, the plates were implemented, in keeping with their use at Daytona .
On some tracks, NASCAR's concerns with speeds because of power-to-weight ratios result in restrictor plates at other tracks. The Goody's Dash Series (known now as the ISCARS series with its new ownership) used restrictor plates at Bristol during at least the last years of the series' existence when the cars were using six-cylinder engines (compared to the traditional four cylinder engines), in addition to their Daytona races.
However, restrictor plates initially were not used for Camping World Truck Series trucks. Rather, aerodynamic, air intake reduction through the use of a 390 cfm carburetor, and eventually a tapered carburetor spacer were implemented for those races. Combined with the aerodynamic disadvantage of the trucks, this allowed NASCAR to avoid the use of such equipment for the trucks until 2008. In 2008, the Nationwide Series and Truck Series began implementation of tapered spacers in the engines to restrict power compared to Sprint Cup cars at all 35 (NNS) and 25 (NCTS) races. Both these NASCAR series now use a restrictor plate and tapered spacer at the two tracks.
The third use came in 2000. Following fatal crashes of Adam Petty and Kenny Irwin, Jr. at the New Hampshire International Speedway during the May Busch Series and July Winston Cup Series races, NASCAR adopted Modified rules featuring a one-inch (2.54 cm) restrictor plate to slow the cars headed towards the tight turns as part of a series of reforms to alleviate stuck throttle problems which were alleged to have caused both fatal crashes. For the Winston Cup race, it was used just once at the 2000 Dura-Lube/Kmart 300. Jeff Burton led all 300 laps in the ensuing race, despite a 23-car two-abreast battle in the first ten laps, a dramatic charge past 22 cars in 100 laps by John Andretti (who finished seventh), and two charges by Bobby Labonte in the final 50 laps where he took the lead but Burton beat him back to the stripe. 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. However, the Modifieds still use a restrictor plate because the speeds are too great for that class of racecar without them. The track has since been changed with soft walls to improve racing safety. Restrictor plates remain a permanent fixture to the Modifieds and the racing has often broken 20 official lead changes for 100–125 laps of competition.
Rusty Wallace tested a car at Talladega Superspeedway without a restrictor plate in 2004, reaching a top speed of 228 mph (367 km/h) in the backstretch and a one-lap average of 221 mph (356 km/h). While admitting excitement at the achievement, Wallace also conceded, "There's no way we could be out there racing at those speeds... it would be insane to think we could have a pack of cars out there doing that."
The competitive quality of restrictor plate racing 
A frequent criticism of restrictor plates is the enormousness 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 was on the 200th lap of a race extended by the Safety Car rule). 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. The 2000 New Hampshire race was condemned because Jeff Burton led wire to wire; the plates were singled out as impeding ability to pass, a criticism contradicted by the use of restrictor plates in a Busch North support race the day before where the lead changed seven times in 100 laps and by the highly competitive nature of restrictor plated Modified races; as noted above the 300 also saw a 23-car battle for third in the first ten laps and a burst by 22 cars from John Andretti.
The criticism stems from reduction in throttle response brought by the restriction. The reduction in throttle response, however, has never been shown to have impeded ability to pass; the criticism was shot down in the first "modern" plate race, the 1988 Daytona 500, as the lead changed 25 times officially and saw several bursts where the lead changed several times a lap and also several bursts of sustained side-by-side racing, notably in the final 50 laps between Bobby Allison, Darrell Waltrip, Neil Bonnett, and Buddy Baker.
Said Waltrip before the race, "I feel, as a driver, now I can do more than I could before (the plates). Now, instead of a car just blasting by me with a burst of speed and a lot of horsepower, he's got to think his way, he got to DRIVE his way around me."
In the 1971-4 period of restrictor plate racing Daytona and Talladega broke 40 official lead changes six times, while Michigan International Speedway, also restricted, broke 35 official lead changes in both of its 1971 races.
- IMSA/ALMS Competitor Bulletin #10-01: Technical Specifications, 2010
- Aumann, Mark. "NASCAR takes 'really big step' with fuel injection". NASCAR. Retrieved 30 January 2012.
- Demmons, Doug (October 23, 2011). "NASCAR's change to fuel injection won't mean the end of restrictor plates". The Birmingham News. Retrieved February 16, 2013.
- "Top 10 NASCAR Crashes That Changed Racing (With Videos!)", Popular Mechanics (retrieved 10 Aug 2010)
- NASCAR.com – Rusty Wallace hits 228 mph in Talladega trial – June 10, 2004