What a Restrictor Plate Does—and Why It Matters

A restrictor plate limits flow by narrowing a passage. In motorsport, it restricts the amount of air an engine can ingest, reducing horsepower and top speed to improve safety and competitive parity. More broadly, restrictor plates are used anywhere a precise cut in flow is needed—engines, turbo systems, and industrial piping included.

How a Restrictor Plate Works

At its core, a restrictor plate is a calibrated orifice: a thin, rigid plate with precisely sized holes that reduces the cross-sectional area available to a moving fluid (air or gas). This creates a pressure drop across the plate and caps mass flow. In an engine, less incoming air means less oxygen for combustion, so the control system (carburetor or ECU) meters in less fuel, resulting in lower torque and horsepower. The device doesn’t “slow the engine down” directly—it limits how much energy the engine can make by choking the intake path.

In Naturally Aspirated Engines

On carbureted or throttle-body fuel-injected engines, the plate is typically installed between the carb/throttle body and the intake manifold. By limiting the area through which air can pass, it reduces volumetric efficiency at higher throttle openings and engine speeds, flattening the power curve and lowering top-end performance. Drivers often report a softer throttle feel at high RPM, while low-RPM drivability can remain relatively normal.

In Turbocharged Engines

With turbocharged engines, a restrictor plate (or inlet restrictor) is often placed before the compressor. This caps the maximum mass airflow regardless of boost, effectively limiting peak power even if the turbo attempts to raise pressure. The turbo may work harder (higher shaft speed) to compensate, which can increase intake temperatures and shift compressor operation closer to surge or choke regions if not carefully calibrated.

Where You’ll See Them

Restrictor plates show up in several high-stakes settings where speed, safety, parity, or regulatory compliance are priorities. The following list outlines the most common contexts and why restrictor plates are used there.

• Stock car racing (historically): U.S. superspeedways used plates for decades to curb extreme speeds and reduce airborne crashes. Since 2019, NASCAR has transitioned from traditional restrictor plates to tapered spacers and specific aero rules to target set horsepower levels.
• International motorsport: FIA-regulated series (rally, GT, certain prototypes) use intake restrictors or boost limits to cap performance and maintain class parity. The exact diameter or boost map is set by technical regulations and may vary by class and event.
• Club racing and spec series: Plates or spacers standardize performance across different engine builds, helping contain costs and equalize competition.
• Industrial piping and HVAC: Orifice plates limit or measure flow, creating predictable pressure drops for control and instrumentation (e.g., in gas lines, process plants, or balancing airflows in ducts).
• Detuned road or learner-legal vehicles: Some markets limit vehicle output for licensing or insurance classes using intake restrictors, throttle stops, or ECU maps; the term “restrictor plate” is sometimes used informally for these hardware limits.

Taken together, these use cases highlight the restrictor plate’s role as a precise, enforceable way to manage flow and, by extension, performance, safety, and compliance.

What It Changes on Track (and Under the Hood)

For racers and engineers, the practical effects of a restrictor plate extend beyond a simple power cut. The following points capture the most significant changes in behavior, setup, and race dynamics.

• Lower peak horsepower and top speed: The intended outcome—cars can’t accelerate or run as fast, especially in high-speed sections.
• Altered torque curve: Peak torque drops and shifts; engines may feel more “choked” at high RPM, with drivability changes noticeable exiting corners.
• Closer pack racing: With less power, speed differentials shrink, tightening the field; while this can create exciting drafting battles, it can also raise the likelihood of multi-car incidents.
• Cooling and thermal loads: Engines and turbos may run differently under sustained restriction; calibration and cooling become critical to avoid detonation or heat soak.
• Setup and gearing: Teams often adjust gearing, aero, and fueling/ignition strategies to optimize within the restricted power envelope.

These effects mean a restrictor plate doesn’t just make a car slower; it reshapes race craft, engineering trade-offs, and even safety considerations.

Benefits and Trade-offs

Sanctioning bodies and engineers choose restrictors because they’re predictable and enforceable, but the approach brings pros and cons. The next list summarizes the key upsides and downsides.

• Pros: Improves safety by capping extremes of speed; enables parity and cost control; easy to inspect and regulate; relatively low-cost hardware solution.
• Cons: Can promote pack racing and “accordion” crashes; may reduce “natural” differentiation among cars; places additional thermal and mechanical stress on some components; can change the character of racing at certain tracks.

In practice, rule-makers balance these factors against alternatives like fuel-flow meters, boost limits, weight changes, or aerodynamic adjustments.

Restrictor Plate vs. Tapered Spacer

Though both limit airflow, a tapered spacer shapes the flow more progressively through a conical passage, often improving mixture quality and reducing turbulence compared with a flat-plate orifice. Since 2019, NASCAR has moved from traditional restrictor plates to tapered spacers combined with specific aerodynamic packages to target defined horsepower levels (for example, roughly 510 hp on superspeedways and higher at most other tracks with the current rules). Many other series use either air restrictors, boost limits, or balance-of-performance measures depending on class objectives.

Common Misconceptions

Because “restrictor plate” became a catch-all in racing culture, several misconceptions persist. The following list addresses the most frequent ones.

• “It slows the engine directly.” Not exactly—the engine still spins freely; the restrictor limits how much power it can make by reducing oxygen mass flow.
• “Boost can overcome a restrictor.” A properly enforced inlet restrictor caps mass flow even if the turbo raises pressure; the limit moves elsewhere (temperature, compressor map, shaft speed).
• “They’re still used everywhere in NASCAR.” Traditional plates are no longer the standard; modern NASCAR uses tapered spacers and aero rules to manage speeds.
• “Removing a restrictor is a harmless mod.” In sanctioned racing it’s illegal; on road vehicles it can violate emissions, noise, or licensing laws and may damage the engine if calibrations aren’t updated.

Understanding what a restrictor does—and does not do—helps set realistic expectations about performance and legality.

Practical Considerations and Safety

Anyone installing or tuning around a restrictor must be precise. The list below outlines essential best practices for compliance and reliability.

• Follow the rulebook: Diameter, geometry, placement, and inspection procedures are usually specified down to fractions of a millimeter.
• Retune appropriately: Adjust fueling, ignition, and boost control to avoid lean conditions, knock, or turbo overspeed.
• Monitor temperatures and pressures: Use sensors (EGT, IAT, MAP) to ensure the engine operates within safe limits.
• Inspect regularly: Plates can warp, clog, or erode; fasteners and gaskets must be secure to prevent leaks.

Diligent setup and monitoring ensure the restrictor achieves its regulatory goal without compromising reliability or safety.

Summary

A restrictor plate is a calibrated flow limiter. In racing, it curbs engine power by restricting intake airflow, reducing top speeds to enhance safety and parity. Outside motorsport, similar orifice plates control or measure flow in industrial systems. While effective and easy to police, restrictors reshape vehicle behavior and race dynamics, and modern series increasingly blend them with tapered spacers, boost limits, and aero rules to fine-tune performance targets.

What do restrictor plates do?

A restrictor plate or air restrictor is a device installed at the intake of an engine to limit its power.

Does NASCAR still run restrictor plates?

No, NASCAR’s top Cup Series no longer uses traditional restrictor plates; the tapered spacer replaced them in 2019 to control engine power and speeds at larger tracks, but restrictor plates have been brought back for the 2024 Xfinity Series at Indy and Michigan to encourage pack racing. The change in the Cup Series was made to eliminate the need for a physical plate and create a similar type of racing to what the tapered spacers already enabled at other tracks. 
History of the Change

• The End of an Era: The 2019 Daytona 500 was the last race for restrictor plates in the Cup Series. 
• The Tapered Spacer Solution: NASCAR transitioned to a tapered spacer, which restricts airflow to the engine, to achieve the same goal of reducing horsepower and limiting speeds. 
• Different Tapered Spacers: The tapered spacer package was further developed, with different sizes used depending on track length to maintain desired speeds and safety levels. 

The Purpose

• Safety: Opens in new tabRestrictor plates and tapered spacers are safety features to manage the high speeds at the fastest ovals, preventing cars from reaching dangerous speeds that the tracks and safety equipment aren’t designed for. 
• Racing Style: Opens in new tabThe goal is to maintain a tightly woven pack of cars for a more exciting racing style, rather than allowing cars to run at extremely high speeds in large gaps. 

How do restrictor plates affect speed?

Restrictor plates are placed between the carburetor and the intake manifold to reduce the flow of air and fuel into the engine’s combustion chamber, thus reducing horsepower and speed.

How fast can a NASCAR car go without a restrictor plate?

Without a restrictor plate, a modern NASCAR car’s top speed would likely exceed 210 mph, but historically, tests have shown speeds well over 230 mph, with Rusty Wallace reaching over 240 mph in a 2004 test at Talladega. While engines could produce this power, such high speeds are deemed unsafe due to increased aerodynamic instability and tire wear, which is why restrictor plates and their successor, the tapered spacer, are used to limit power and ensure driver safety on race tracks.
 
Historical Test Data 

• 2004 Test: Rusty Wallace reached top speeds over 240 mph and a single-lap average of 221 mph during a test at Talladega with an unrestricted motor.
• Aerodynamic Issues: At these extreme speeds, Wallace’s car experienced front-end lift and became aerodynamically unstable, even ripping rubber off the tires, demonstrating the safety concerns associated with such speeds.

Reasons for Restrictor Plates/Tapered Spacers

• Driver Safety: The primary reason for restrictor plates and tapered spacers is driver and fan safety. 
• Reducing Aerodynamic Instability: By reducing power, these devices prevent the cars from becoming too fast and unstable, which could lead to dangerous situations. 
• Controlling Speed: They limit the amount of air going into the engine, thereby reducing the engine’s power output and maximum speed. 

Modern NASCAR Cars

• NASCAR now uses tapered spacers instead of restrictor plates, but they still serve the same purpose of limiting engine power. 
• The amount of reduction in power and speed varies depending on the type of track, with larger spacers used on larger tracks. 
• The intention is to keep speeds at a manageable and safe level for racing.