Since the very first horseless carriages, the pinnacle of automotive technology always comes in the form of racing cars. But while most drivers will never get the chance to drive on a race track or in a competition-spec race car, even the average car buyer has the opportunity to purchase a wide range of street-legal sports cars. Sports cars today are extremely complex machines that combine all the advanced tech of a computer with the increasingly complicated underlying mechanical components of any other automobile. But the basic elements that make a sports car desirable have remained unchanged for more than a century: a sports car must be powerful, it must handle well, and, as always, it must look good.

Updated March 2022: If you own a performance car, or you're planning to buy one, you'll be happy to know that we've updated this article with more details regarding illegal (yet awesome) modifications.

The world's most advanced sports cars can even exceed the technological advances of actual competition racers, but they can also cost well into the millions of dollars, meaning only the richest people in the world will ever get the chance to drive the likes of a Koenigsegg Agera or a Bugatti Veyron. Everyday sports cars, however, are more attainable - especially on the secondhand market. Given constant advances in engineering, used sports cars are typically not quite as fast as newer ones, and an entire industry of aftermarket modifications has emerged that allows for sports car owners to beef up the power, handling, and style of their cars. But not everything sold online is legal to add to a vehicle that travels on city streets. Keep scrolling for 20 of the best illegal sports car mods, some that are easy to get away with and some that are totally obvious to the cops.

20 Racing Seats & Harnesses


For drivers who want to take their sports cars to the track and enjoy a day of pushing themselves and their vehicles up to, and past, the limit, racing seats become a must-buy modification. Normal car seats just don't provide the kind support necessary to hold drivers (and brave passengers) back in their seats when cornering at high speeds, and simultaneously don't provide the same kind of safety support that could result from pushing a sports car too far past its edge.

But racing seats and harnesses can also be against the law for use on city streets for the simple fact that they weren't part of the car's design when it passed safety tests during development.

Related: 15 Mods To Make A Car Go Faster (And 5 Things That Don't Work)

19 ECU Tunes


Modern automotive powerplants are massively complex machines that combine all the elements of an advanced computer alongside the normal mechanical components of an internal combustion engine. And just like bolt-on physical mods can improve power and efficiency, the engine's ECU can be reprogrammed to up output significantly - especially for cars utilizing forced induction like turbo and superchargers.

ECU tunes alter air-fuel ratios and timing to improve combustion in the cylinder, but sometimes they can result in unlawful levels of emissions that won't meet government standards, although they can be hard to spot unless paired with other bolt-on modifications that will make the ECU tune apparent from the outside.

18 Exhaust Cutouts


Much like Volkswagen is currently under investigation for faking emissions test results, many sports car drivers have taken to adding electric exhaust cutouts to their exhaust systems. Essentially, for most driving the exhaust is routed through emissions equipment and a muffler, but during performance drives, all those restrictions can be bypassed and the engine can exhale much more smoothly.

But trying to sneak around by leaving everything in place while allowing for the occasional use of a cutout is going to rouse the ire of any smog station inspector who recognizes what is going on under the car that they're inspecting.

Related: 12 First Step Truck Mods (And 13 First Step Car Mods)

17 Long Tube Headers


For sports car drivers willing to invest heavily in their cars, and for the backyard modders who perform their own work in their home garage or driveway, one serious commitment to improving an engine's airflow is to install a set of long-tube headers. The headers are the first part of an exhaust system and are a crucial aspect that can choke exhaust as it leaves the engine.

Even the physical route the headers take makes a difference in the flow of micropulses of gas as they leave through the exhaust valves, and longer tubes can help smooth out the process while leaving in place later, legally-required restrictions like mufflers and catalytic converters, which are calibrating to take into account distance traveled from the cylinder.

16 Nitrous Oxide (NOS)


The Fast and Furious franchise introduced many audience members to the insane performance, and risk, that Nitrous Oxide offers for use in sports cars. Sure, a nitrous system can provide huge spikes in power output, but that's because nitrous oxide is a fuel that is more flammable than gasoline. Of course, that makes it dangerous for use in cars that might get into an accident at any time.

Now nitrous oxide in itself isn't against the law, but regulation for its use in sports cars varies from place to place - typically, carrying nitrous in a car is okay, as long as hoses are out of the driver's reach and not connected to the engine.

Related: 15 Easy Mods That Immediately Improve A Car's Performance (5 That Improve Nothing)

15 License Plate Covers


For serious road racers, escaping police officers in high pursuit isn't just a typical Friday night, it's what makes a Friday night great. And running from the cops is pointless if they can just find out a home address the next day by typing a license plate number into a computer, so some aggressive drivers have added electrically operable license plate covers that they can activate from inside the car.

Obviously, even having one on a car without using it is illegal, and cops will undoubtedly be very displeased to find that a driver is intentionally trying to evade officers of the law.

14 License Plate Paint


Another way that aggressive drivers have found to keep their license plate numbers from being easily viewable is to apply various kinds of paint to the whole plate. Some scatter light like the angular edges of a stealth bomber, while some just make the plate's reflection entirely too bright.

Whether intended to fool actual police officers in hot pursuit after breaking up a late-night road race, or just to prevent red light cameras from being able to provide a solid identification of a vehicle, adding any kind of covering paint job to a license plate is clearly not ok, and may well lead to further harassment if cops actually notice.

13 Aftermarket Turbos


Forced induction delivers the ability to transform an otherwise solid sports car into a powerful monster capable of burning even the widest tires. If there's any doubt just how insane the gains can be, look at the insane supercharged and turbocharged upgrades coming out of Hennessey Performance in Texas, with figures easily reaching into the 1,000 horsepower range.

But for the average sports car enthusiast, bolting on a couple of turbochargers requires a wide range of modifications, and in regions like California, the changes to intake and exhaust will likely make the project illegal almost from the get-go.

12 Enormous Intercoolers


One disadvantage of forced induction via superchargers and turbochargers is that the process heats up air as it increases pressure. To combat the inefficiencies of sending hotter air into the cylinder, many builds utilize massive intercoolers which are effectively like coolant radiators but for charged air.

For optimal performance, enormous intercoolers need to be mounted at the front of the vehicle to help with airflow, and builds that include this level of modification often require the deletion of standard safety equipment like adequate impact bumpers alongside the changes to engine tuning and emissions that go along with the rest of a forced induction project.

Related: 10 Sick First Step Mods For Used Cars (10 To Avoid)

11 EGR (Engine Gas Recirculation) Delete


On modern cars including sports cars, one component that many drivers overlook is the Engine Gas Recirculation system that helps to draw vaporized fuel that didn't combust in the cylinder out of the exhaust, from where it is then recompressed and circulated back into the fuel delivery system to be reused.

Of course, this improves efficiency, since less fuel is going out the tailpipes, but the process does reduce overall power output. Simply deleting the EGR system entirely can help unlock a few extra horses, but the result is a car that might be slightly more powerful but will definitely struggle to pass a smog test.

10 Racing Slicks


Even the most amateur sports car owner should know that good tires are the single best investment to improve performance, from acceleration to braking and handling. Other than aerodynamic drag, every aspect of a sports car's design and engineering comes down to how much surface grip the tires have with the ground.

In the interest of daily drivability and safety, tire companies make tires that perform well in various different conditions, from dry roads to rain, snow, and mud. While racing slicks may be great for track days, they are illegal for street use because even the slightest patch of moisture or oil will result in a loss of traction.

Related: 10 First Steps For A DIY Sleeper (And 10 Mods That Are Always Mistakes)

9 Studded Tires


Rally racing and gymkhana, as well as The Fast and the Furious: Tokyo Drift, have brought drift culture to the forefront of the modding crowd's style. For off-road rallies, especially on muddy and snowy days, rally racers may use studded tires to aid in traction as they blast full speed around a tight curve, but studded tires are against the law in most regions that don't see excessive icy weather for months at a time.

Studded tires may sound great for hardcore rally fans to add to their cars, but they also wear out roads faster, and can present a dangerous scenario should a blowout occur around other drivers.

8 Rear Taillight Safety Switch


For sports car owners who regularly engage in unofficial road races, evading other drivers, as well as the police, can become a serious consideration. One way that some sneaky modders have found to help them escape during a nighttime chase is to install a rear taillight safety switch so that they can seemingly vanish into the darkness from the perspective of anyone chasing them.

Steve McQueen notoriously had safety switches installed on his cars that he frequently raced on Mulholland Drive, but the practice is clearly dangerous and definitely not ok because it can easily lead to dangerous accidents due to lack of nighttime visibility.

7 Suspension Drops


Most true sports cars leave the factory with taught suspension setups that blend the requirements of track racing with the comfort needs for daily driving. Plenty of owners are willing to sacrifice smooth driving on bumpy city streets for even the slightest improvement in handling, which is part of the reason suspension modifications of all kinds are so widely available.

The style factor is another element of suspension mods, though, as some drivers like to ride as low as they can go - but before considering a massive drop with a cup kit, coilovers, or even airbags, keep in mind that some states make it unlawful to lower a car more than two inches below its factory ride height.

6 Laser Jammers


In addition to radar detectors, laser jammers are yet another way to prevent police officers from being able to catch speeding drivers. The cops won't appreciate laser jammers, which are clearly intended to allow for users to break the laws in place to maintain the safety of every driver on the road.

Plenty of laser jammers can be easily installed in hidden locations however, making the likelihood of getting actually caught for having one relatively small unless the road is entirely empty except for one car that the cop's radar gun struggles to get a read on, a clear sign of something fishy.

5 Rally Lights


Classic car owners, especially sports car owners, are increasingly turning to inspiration from the rally world to help customize their collections and increase the perceived uniqueness of their prized possessions. Off-roaders well know, however, that rally lights and rally bars can be cause for a police officer to pull a car over, as their brighter-than-bright lights - originally intended for long-distance endurance races that lasted all night or passed through inclement weather - can be blinding to other drivers on the road.

Classy Cibies like on the Porsche above will probably go unnoticed even if they are against the law, but excessively bolted-on rally lights will likely draw unwanted attention.

4 Cold Air Intakes


A relatively inexpensive and simple step that many sports car owners take to help their engines breathe better is the installation of a Cold Air Intake. The idea is to allow for better airflow through a larger filter and intake hose, while simultaneously keeping the air away from warmer engine components.

Many drivers incorrectly locate CAIs, though, but regardless of location, some CAIs will be illegal if not properly manufactured or installed - especially for drivers who spend time on California roads, where CAIs need to be approved by the California Air Resources Board (CARB) before being legally sold as aftermarket bolt-on kits.

3 Radar Detectors


Outside of race tracks, pushing just about any modern car - much less a real sports car - to the limit can easily result in a speeding ticket or even imprisonment on reckless driving charges. Instead of obeying the posted speed limits, however, plenty of drivers decide to invest in a radar detector so that they'll know when the cops are out trying to catch speed demons.

The cost for one can be less than a single ticket, but it's important to remember that many regions have outlawed radar detectors, ostensibly in the name of public safety though undoubtedly also to pad the coffers, as well.

2 Cat Delete


A successful sports car combines many crucial elements, including power, light weight, and excellent suspension geometry, to produce a wonderful driving experience. And there can always be more power, less weight, and better handling - but typically the first thing many modders will try to gain is more horses.

A quick and relatively cheap way to improve power is to help the engine breathe better, both on the intake and exhaust side, and deleting emissions equipment like catalytic converters can quickly produce sizable bumps in output. But those catalytic converters are mandated by just about every governing body imaginable, and removing them is highly illegal.

1 Straight Pipes & Muffler Deletes


Any way to improve airflow either to or from an engine is a good way to increase power, and stock exhaust systems are designed by manufacturers to combine efficiency, cleanliness, and performance to meet legal guidelines.

Straight exhaust pipes that remove the muffler may also have the benefit of opening up that wonderful engine roar, but both the sound they create and the removal of government-mandated emissions equipment makes it highly likely a cop is going to notice, and then write an expensive ticket for, any driver who decides this mod is their next best step for beefy power upgrades.

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