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Restrictor plate
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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.
w top classes like Formula One limit only the displacement and air intake mouth dimension.
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Encyclopedia
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.
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 (or limit boost pressure in turbo engines).
Rallying
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. 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
NASCAR's Sprint Cup and Nationwide Series currently uses restrictor plates at Daytona International Speedway and Talladega Superspeedway. NASCAR routinely states that the Sprint Cup restrictor plate reduces engine power from approximately 750 hp to approximately 430 hp.
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. An undesired effect, however, is that all drivers tend to form very large "packs" of cars that run closely (there may only be one second separating the entire field at times) together for the majority of the race (it is especially difficult to break the packs at Talladega). These large packs reduce air resistance which allows the cars to run faster and makes drafting easier. These restrictions are supposedly in the interest of driver and fan safety, although many members of both of these groups feel that the closeness of cars and their inability to achieve separation may actually make the racing at these tracks more dangerous, as there are often massive and frightening multi-car pileups during races. Such a crash is dubbed "The Big One" by drivers and fans. At Daytona and Talladega, most races are marred by at least one occurrence of such a crash as cars rarely become separated. Talladega is 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, compounding the chances of a mistake by a driver, as well as the fact that handling is less of a factor there.
Reason for restrictor plates
There have been three reasons that NASCAR used restrictor plates in its history.
The first use came in 1971 as part of NASCAR's plans to reduce the size of engines from 427 cubic inches (7.0 L) to 358 cubic inches (5.9 L). In order to allow teams with smaller budgets to race the larger engines, NASCAR made mandatory the use of a restrictor plate to be placed on larger engines to equalize performance with smaller engines. (Note that 100 cubic inches equals 1.63871 L.) The transition ended in 1974, when NASCAR banned the larger engines, and went to the 358 cubic inch engine (a compression limit would be implemented in 1996). This was 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 terrifying crash of Bobby Allison at the 1987 Winston 500 at Talladega Superspeedway. Allison's Buick LeSabre blew a tire going into the tri-oval and flew tail-first into catch fencing early in the event, injuring spectators (although not actually entering into the grandstands). After a summer where the two subsequent superspeedway races were run with aids to prevent cars from flying, and smaller carburetors (390 cubic feet per minute instead of 750 cubic feet per minute) 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 Series began racing at Talladega, the plates were implemented.
In 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 are not used for Craftsman Truck Series trucks. Rather, air intake, aerodynamic, and, eventually, a tapered carburetor spacer were implemented for those races. Combined with the aerodynamic disadvantage of the trucks, this allows NASCAR to avoid the use of such equipment for the trucks.
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. The Nationwide Series still uses a restrictor plate and tapered spacer at the two tracks. The Trucks do not use any additional restriction except for the air intake restriction 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 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 Cup race, it was used just once at the 2000 Dura-Lube/Kmart 300, allowing Jeff Burton to dominate by leading all 300 laps in the ensuing race. Due to the lack of passing and the addition of an automatic kill switch in the case of a stuck throttle, 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, especially with the numerous deaths of star drivers in the history of the Whelen Modified Tour, yet no driver has died in the WMT at NHMS. The track has since been changed with soft walls and progressive banking to improve racing and safety.
Rusty Wallace tested a car at Talladega Superspeedway without a restrictor plate in 2004, reaching a top speed of in the backstretch and a one-lap average of . Wallace subsequently described the experience as "out of control".
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