Limited Potency

At Stock Car Racing, we hear a lot of buzz about superspeedways that require restrictor plates, such as Daytona, Talladega, and now Loudon. It’s not unusual for us to receive letters with questions about them. To help everyone grasp the basics about restrictor plates, we spoke with longtime Chief Engine Builder Jerry Vess at Andy Petree Racing in East Flat Rock, North Carolina. Vess gave us a thorough understanding of restrictor plates by answering the five most commonly asked questions. Here is what we learned.

In the beginning.

Restrictor plates were used in the early ’70s to slow down the Chrysler Hemis, but they were only used for a few years. As a result of Bill Elliott’s 212.809-mph record qualifying lap during the ’87 season, and to prevent speeds from increasing, the following year modern-day restrictor plates were implemented.

What is a restrictor plate?

A restrictor plate is a thin piece of metal with four holes in the center. The size of the four holes varied during the 2000 Winston Cup season, starting with four 29/32-inch holes. Prior to the DieHard 500 at Talladega in April 2000, the holes were reduced to 7/8 inch, and in September they increased to 1 inch before the Loudon race. But with the 1-inch holes, changes to the car’s body also were made to control the speed.

Where is the restrictor plate located?

It is mounted between the carburetor, which supplies air and fuel to the engine, and the intake manifold.

Why are restrictor plates used?

Restrictor plates are used to slow down and regulate the speed of race cars. Cars are easier to control and safer at slower speeds.

What is the purpose of a restrictor plate?

The holes in the restrictor plate are considerably smaller than on the bottom of the carburetor and top of the intake manifold. These small holes limit the amount of air and fuel that flow into the engine. As a result, the engine is tricked into thinking there’s a smaller carburetor than what is actually being used. This reduces the engine’s power and the car’s speed.

How does a restrictor plate affect the engine?

The engine’s horsepower is reduced. A typical Winston Cup engine produces about 750 hp. When a 1-inch restrictor plate is added, typically one third of the power is taken away, which drops the output to about 500 hp. “This makes the Winston Cup cars run like Busch cars; the engine’s power level is actually a little lower,” says Vess.

Not only does a restrictor plate reduce the engine’s power, but it also hinders its ability to reach the non-plate engine’s 9,000 rpm, lowering it to 7,000 rpm. This creates a challenge to engine builders—because the non-plate engine’s maximum power reaches above 7,000 rpm, the engine has to be built solely for restrictor-plate use. To unlock the maximum horsepower and torque from a restrictor-plate engine below 7,000 rpm, a lot of time and money is spent to test different variations of heads, cams, and intake manifolds to develop the best combination to keep the flow of air constant. Power loss from friction and weight becomes much more severe than in a non-plate engine. Every part of the engine is examined to find ways to reduce weight and friction.

Sterling Example

Sterling Marlin’s name may not quickly come to mind when thinking of drivers who can get the most out of restrictor plate races. But listen up—he’s fourth among active drivers in restrictor-plate wins, with five victories. And he’s one of just three active drivers to have won at least twice at each of the two regular restrictor plate tracks, Daytona and Talladega.

Marlin knows the stats well, but he’s not out bragging.

“Being called ‘one of the best restrictor plate drivers’ sounds a lot like being called one of the best bumper-car drivers in the history of the county fair. It’s hard to tell exactly whether you’re being complimented or not,” Marlin says.

“The prettiest pig in the pigpen is still a pig. But if you’re going to be a hog, you want to be the best hog out there. If we’re going to run restrictor plates, I want to be the best at it I can be.”

The big tracks are nice, Marlin says, but he’d rather be bumping and grinding on a short track somewhere. Still, he knows that in the eyes of many fans, drivers have not truly arrived until they’ve scored a victory at one of the giant tracks.

Marlin considers tracks like Daytona and Talladega to just be part of his job. If that’s the case, then this guy would be up for a serious promotion if he could post similar stats at other tracks.

“I don’t know what it is. I just feel comfortable on those tracks. I think that’s half the battle,” Marlin says. “A lot of guys sit there and wonder when the big one is going to happen, and they worry about the walls and other cars and all of that kind of stuff. Me, I just sit there and drive, do my best, and go as hard as I can.”