How And Why Do Race Cars Have Their Power Restricted?

In motor racing, the winner is decided when the fastest driver in the fastest car crosses the finish line. At least, that’s what it seems like on the face of it, but behind the scenes there’s often been a lot of careful regulation of performance to ensure that no one car outshines the rest.

The ultimate goal for competitors in racing is to win, and therefore they’ll do whatever it takes to be on the podium’s top spot after the chequered flag drops. The problem is, the organisers need to make money, and they do that by getting sponsors. To get sponsors, they need people to be watching, and if one car runs away from everyone else then no one will tune in. People like excitement and unpredictable results, so organisers engineer the cars to keep the racing close.

GT racing series employ this tactic, since the field consists of everything from smaller sports cars like the Porsche 911, to bona fide supercars like the Lamborghini Huracan. If performance levels were left unchecked, there wouldn’t be much competition - this would mean that even if people still tuned in, there would be no incentive for the slower manufacturers to stick around as the whipping boys, leaving the field to be made up of one or two models. By keeping a varied field, you ensure a variety of spectators resulting in higher viewing figures, and by keeping the racing exciting you’ll keep those viewers coming back for more.

Unsurprisingly, it’s not as simple as it sounds. The most obvious thing to do would be to look at power-to-weight figures, and either limit power or add weight to balance performance. The problem is, each car handles each circuit differently; some cars are better for top speed and have an advantage on high speed tracks, while others corner better or accelerate faster, making them better suited to tight tracks. Then there’s the differences between naturally-aspirated engines and forced induction, fuel economy figures for different engine sizes and types, and the aerodynamic efficiency of each car.

This is the Balance of Performance chart for the GT3 race weekend at Macau in 2015. As you can see, there’s a lot more to it than simply strangling an engine. Performance at previous race weekends is taken into account to get an idea of how quick a car is, and a committee places upper and lower limits on a number of areas to bring everyone into line. Each car also has a different minimum weight, with the ‘Committee [Kg]’ column referring to how much extra ballast a car must carry.

You can also see the diameter of the restrictions placed on the intake, with high horsepower cars like the McLaren 650S having a 36mm restriction, while the Porsche 911 GT3 R (which has a lot less horsepower) gets a much larger, and therefore less obstructive, 65mm diameter restriction on the intake. The 650S and Bentley Continental GT3 are both turbocharged, so restrictions are placed on the amount of boost pressure they can employ. Some series also put restrictions on the size of the fuel tank or the amount of fuel that can be used by each car during a race.

Predictably, this isn’t always easy to implement. Over the course of a season, it’s hard to mask performance without just being rubbish and throwing away a title, but early in the season performance figures are all up in the air. At this year’s Roar Before the 24 test day at Daytona, every single GT team was busted for underperforming. This was partly because they didn’t want to show their hands to other teams too early, but also because IMSA uses the test session to figure out Balance of Performance specifications for the first race of the season - if the data shows a car to be slower than it is, the restrictions will be lower and the cars will perform better.

Speaking at the time, one official noted: “We simply took the top-five fastest laps of every car and we were able to identify in every single one of those laps, in every single car - all 54 of them - an area of underperformance. In every single one of them.”

During the actual race, the Lamborghini Huracan absolutely ran away with everything - at least until the leading pair of Bulls wiped each other out. Lamborghini has been given a $25,000 fine for, in Layman’s terms, manipulating the data handed over to get an advantage. Rumour has it, however, that Lamborghini used an outlawed version of the restrictor plate that Audi had used in 2014, which nullified the intended restrictions.

But strangling performance isn’t only used to keep the racing fair, sometimes they’re used for safety. Probably the best known example of this is in Nascar. Back in 1987, cars were reaching speeds well in excess of 200mph. This culminated in a huge crash for Bobby Allison at Talledega Superspeedway, in which his car got airborne and took out a large section of catch fencing. Fortunately, nobody lost their life that day, but a number of fans were injured and it was clear speeds had to be reigned in. The very next season restrictor plate racing began, however the plates are only used at circuits where top speeds reach ‘dangerous’ levels.

The way it works is actually rather simple. The plate has four holes drilled into it and, depending on the circuit, the holes differ in size, altering how much performance is available. The plate is inserted into an engine’s intake, restricting the amount of fuel and air allowed into the engine, which naturally curbs the amount of power produced.

This isn’t without its own controversies, however. Thanks to the restricted performance, the cars all tend to run in a very tight pack. This looks great on TV and results in multiple position changes during a lap, but it’s extremely dangerous and increases the chance of a ‘big one’ happening. The cars are also designed to be incredibly slippery in order to be able to draft easily, which coupled with the tight racing, often results in cars three-wide on track.

However, as Nascar legend Tony Stewart has pointed out, this takes some of the race craft away, since you’re stuck in a tight pack without room to manoeuvre. He’d like to see restrictor plates removed to allow speeds of up to 230mph during a race:

“Instead of teams working so hard to build cars that are slick, they could try to build cars that actually handle again. There’s always talk about why we get into these big crashes and it’s because there’s 40 of us running in one big pack, and we’re so close together you could throw a blanket over us.

“If you had cars running in the 215 to 220 mph bracket, you wouldn’t be able to run that close to each other because you’d have to move around on the race track to find clean air. We wouldn’t be able to stay down on the bottom of the track. We’d have to drive around like we do at Michigan and find places on the track where our car drives better. I think it would make for exciting racing to be able to do that again.”

Balancing performance is a complicated science, and no matter how it’s implemented there will always be complaints. Nobody likes to have their cars strangled, but it’s a necessary evil in GT racing, where variety and close racing improves the spectacle. Teams will always try to game the system, though, which is why IMSA has implemented a new data logger that analyses many aspects of a car’s performance to help it come to a decision about restrictions - it costs $18,000 per car, but it’s vital in closing the gap between competing racers.

In Nascar, whether the plates are still necessary is rather more up for debate. The original introduction in 1988 was much needed - the technology in the cars had overtaken safety technology - however modern race tracks are far safer places to race and spectate. The barriers and cars absorb impacts better, protecting drivers from heavy impacts, while catch fencing, aero tricks to minimise airborne cars, and better safety procedures for fans mean that the risk of harm is massively reduced. Lifting legislation that has not only been in place for nearly three decades, but was implemented on safety grounds, would be a huge step, but it’s definitely something worth discussing for the sake of the racing.