So You Want to Mod Your Car? The Basics in Legality and Liability


First off, I understand this question is likely rhetorical if you are on our website. Modifying cars is what we do as a business and have been doing since 1996. With that, we have also heard just about every concern someone might have regarding "if I do X modification to my car, do I have to  worry about *insert liability, legality etc*". 

Now I am going to use the Nissan R35 GT-R as reference in this blog for a couple reasons. First, it's something we have specialized in since 2008 and a vehicle I have an adequate level of knowledge with. Second, most cars today are using turbocharged platforms, and while the GT-R has some unique complexities, it's still just a twin turbo V6 engine under the hood. I will do my best to explain things in a general sense and also limit this post to what most would describe as bolt on modifications. I'll leave speaking about complete engine / transmission modifications to another post. 

Just another day in our shop :)

Just another day in our shop :)




For many, this is the first modification one may look to putting on their new vehicle. You might see a small increase in power and of course you'll almost certainly have a louder exhaust note. 

First I want to specify that when speaking of exhaust, I am speaking of the cat back portion of the exhaust system. This is everything that follows after the down pipe(s) or header(s). 


  • Almost no liabilities need to be associated with installing just an exhaust on your vehicle. For some high end vehicles equipped with a valvetronic system from the factory however, a solution may be required to keep your vehicle from throwing a check engine light. See our article about how we deal with this on the 2017+ Nissan GT-Rs here

Couldn't have said it better Kristian! People like to go fast!(Disclaimer - This is above 95 decibels)

Thanks to Copaze for the video!




Another common modification made on new vehicles is the changing of the intake system. This one is often not as straight forward as an exhaust change and often required a tuning solution for the car to run correctly. If a tuning solution such as COBB or EcuTek is not used in conjunction with an intake, the car may have idling issues, bogging, and an overall negative impact from the modification. 


  • Installation of intakes is viewed as legal in 49 states with the outlying 50th state being California. We have the wonderful California Air Resource Board (CARB) to thank for this law. 
  • You will NOT pass SMOG in California with an aftermarket intake installed. The visual inspection will be a fail. 
  • The above two bullets are true EXCEPT when the intake purchased comes with a CARB legal sticker. This information on a specific intake will always be made extremely clear from the manufacturer. As it is easy to realize, if the manufacturer went through the trouble and expense of certifying their intake with CARB, you can bet your ass they're going to make it known. 


  • As I briefly explained earlier, an intake can harm performance if the proper steps are not taken. The factory ECU in most cars are quite flexible but have their limits. The installation of an aftermarket intake which most certainly has larger piping than stock can often confuse the readings being sent to the ECU from the Mass Air Flow (MAF) sensor. This confusion results in poor performance, idle issues, bogging, and harm to the engine. The solution for this is to install a proper tuning solution that can allow a custom calibration to be installed onto the ECU that allows for utilization of the new intake. 
  • Hydrolock is also a concern that anyone running a cold air intake (CAI) should have. If the intake piping routes towards the bottom of the vehicle, submersion into water will result in the intake sucking water into the engine. There are "hydro shields" sold by companies such as AEM that can help protect you against this. But anyone running one of these intakes in a state that sees heavy rainfall or flooding should be aware. 
AEM's "Dryflow" shield used to help prevent hydrolocking caused by an aftermarket intake

AEM's "Dryflow" shield used to help prevent hydrolocking caused by an aftermarket intake



Down Pipes & Headers

For most every car I have come across, there is not a better bang for the buck modification than headers or down pipes when combined with a tuning device. However, in many cases this also results in the removal of the factory catalytic converter and oxygen sensors. 


  • Removing of the factory catalytic system is illegal in all 50 states. There is zero exception to this on cars built after 1975. 
  • Temperament of the oxygen sensors often found around the factory catalytic system is illegal in all 50 states.
  • You will not pass smog. 
  • There are "high flow performance catalytic converters" which may be used in conjunction with either shorty headers or down pipes. The legality of these depends on your state's laws regarding them. Please review your state laws if this is a concern of yours. 


  • Similar to the intake, installation of down pipes or headers will regularly not result in a direct increase in performance. This is due to the oxygen sensors in place in these areas which cannot properly read the air flow created by the new parts. Anytime the ECU experiences confusion from the signals it receives, the results are almost always going to be negative. Use of a tuning solution is highly recommended by us to take advantage of the performance available in upgrading your headers or down pipes. 
  • Without a proper tune performed on the vehicle after installation of these parts, the ECU's confusion will likely result in damage to the engine or turbos over time. 



If I wanted to run over all the parts that are installed on a car, and in great detail, I would end up writing a book within this one blog post. What I would like to get across to people is that modifying your car is awesome when done right. But certain things should be noted when you begin your modification journey. First, find a reputable shop from which you can trust to work on your vehicle along with handle all of your tuning needs. Lay out some of your goals and see if they fall in line with your budget after discussing things with your shop

Lastly, understand that you may be performing modifications on your vehicle that are at odds with state law. It is your responsibility to research the laws within your state. From our experience, many law enforcement officers are awesome guys, love cars (we have quite a few LEOs as customers!), and have a job to do. In many cases, a simple fix it ticket is the most you'll have to deal with which can be signed off with removal of the item deemed illegal. 

In the coming weeks I'm going to try and put together more information regarding other modifications such as fuel systems, coilovers, body kits and so forth. 

Have a question? Leave a comment or shoot us an e-mail and I'll get back to you as soon as I can!

- Rory