Catless Downpipe 101: Emission, Power, Legal & Warranty

Catless downpipe can help you gain 50 additional horsepower with tuning – this is slightly more than a regular catted downpipe. 

However, in exchange for that power, catless downpipe is not legal and will not pass the emissions test. The exhaust gases from your car will be toxic both to the environment and yourself. 

I personally have not installed catless downpipe on my car. However, I do know quite a lot about it – especially after my countless research about catted vs catless downpipe. 

As you can tell, I went ahead with a catted downpipe but does that mean catless downpipe is not good? Well, of course not. I have a couple of friends with catless on their track cars and daily cars.

Just keep reading because I have a lot of things to share with you about the catless. I spent days and weeks looking into catless downpipes. Now I share them with you in this nice article.

I will cover things like downpipe emission, power, tuning, check engine light and even the laws around emission tests and warranty.

Catless downpipe is an aftermarket downpipe that does not equip any catalytic converter (CAT).

This is done to remove exhaust restrictions – allowing exhaust gases to flow faster and spool the turbo even harder.

A typical downpipe comes with 2 CATs. These CATs exist to convert toxic exhaust gases into non-toxic ones.

Specifically, 1 CAT is used to reduce toxic Hydrocarbons and another CAT is used to reduce toxic Nitrogen Oxide gases. 

Catless downpipe does not come with CAT.

These CATs on your downpipes are crucial for the environment and also to pass the regular emissions test. 

However, they are made of very fine honeycomb-like shapes – which increases backpressure significantly. Which means, CATs are good for the environment but not good for horsepower.

Thus, enter the catless downpipe – a downpipe without any catalytic converter. One that is good for power but horrible for emission. 

CAT has very fine honeycomb structures – which makes it restrictive.

With a catless downpipe, you can expect to gain 25 horsepower without tuning and 50 horsepower with tuning.

This is just an estimate. The gain differs depending on engine size, turbo size and even intakes. 

Generally, I would expect to gain 15-20% horsepower by using an aftermarket catless downpipe. So the bigger engine and turbo you have, the more horsepower gain you can expect. 

Personally, I gained about 40 horsepower on the dyno with an aftermarket catted downpipe and tuning on my 2016 Civic. 

It should be a little more with a catless downpipe, about 20-30% more is a valid guesstimate from my opinion. 

Catted and catless aside, a downpipe is the most ‘worth it’ aftermarket mod in my opinion. Then comes the headers, exhausts and intake. 

Catless downpipe is not legal in every state of the US (almost in the majority of countries, I would say).  Catless downpipe is illegal to drive on the street because of high emission concerns. 

If you are planning to install a downpipe for your daily driver, then you should look for a catted downpipe instead. It gives you less power but it could still be legal and pass the emissions test. 

Read up my other article about aftermarket catted downpipes here – Downpipe 101: Horsepower, Engine Light, Sizing & Is It Legal? 

With catless downpipe, you are going to fail any sorts of emissions tests – be it OBD2, visual or sniffer tests. 

There’s no way to trick the emissions test as well. The emission from a catless downpipe is severe and your exhaust gases can even smell bad as a result.

Consider catless downpipe only if you are off-roading or tracking. 

Catless downpipe does not void the entire warranty of your car. Instead, it voids the warranty of your stock downpipe and possibly the exhaust, turbo and CATs – depending on the actual damage. 

With the enactment of Magnusson Moss Act, a dealer must prove how your aftermarket installation is a direct cause to the problem. Then only can they void your warranty. 

Which means, if you install an aftermarket catless downpipe and have problems with your AC, muffler or headers shortly after – then you should still be protected under warranty. 

Unless of course, the dealer can prove to you how your catless downpipe is a direct cause to your problem.

Most of the time though – it highly depends on the dealer and your relationship. Therefore, it’s always a good idea to be respectful and communicate well with your dealer.

Just because there’s the Magnusson Moss Act, it doesn’t mean that you can cruise to your dealer and demand a warranty on your broken parts.

Be respectful and work with the dealer and you claim warranty with lesser problems. 

I cover more about downpipe and warranty in this article – Will Downpipe Void Warranty? 

Catless downpipe is definitely going to cause a Check Engine Light. This is because with the lack of CAT, exhaust gases passing through your O2 sensor will be highly toxic and trigger Check Engine Light.

With a catless downpipe, there’s no way around a CEL except for tuning. You could bring your car for a tune and turn off the CEL. 

For catted downpipes, you could get away with a CEL using a simple O2 spacer. This is where you increase the spacing between the O2 sensor and the exhaust gas. 

The extra space causes an inaccurate reading for your O2 sensor. This tricks the O2 sensor into thinking your exhaust gases are not as toxic and turn off the CEL. 

However, from the experience of my friends with catless downpipes, an O2 spacer didn’t work for them – their CEL was still on.

Although, I suggest you could still try. An O2 spacer is cheap and can be installed within minutes.

o2 spacer

I highly recommend bringing your car for a tune after installing a catless downpipe. 

That’s the only way to gain maximum horsepower and you could also turn off CEL and tweak other parameters  to make your car even more aggressive.

Running a catless downpipe can make your car go a little lean – instead of gaining performance, you can even lose some and make your car overheat. 

Without tuning, a castles downpipe can give you about 25 horsepower. Whereas with tuning, getting 50 horsepower is expected – you can even gain more with additional aggressive tuning. 

My suggestion is to definitely tune your car after installing a catless downpipe. Get the air/fuel ratio right, turn off the CEL, tweak the torque mapping and remove any REV limiter. 

With all the above, you can expect a monster power gain out of your car. Even more if you couple a catless downpipe with more parts that are geared to reduce exhaust restriction: headers, turbo inlet and catback exhausts. 

Catted downpipe provides slightly less horsepower but it’s not as toxic as a catless and is more suitable for a daily driving car.

Catless downpipe on the other hand, provide more horsepower but have extremely toxic exhaust gases that can even smell bad at times. 

Catless is more suited for racers who care about every little horsepower.

If you don’t already know, a catted downpipe is an aftermarket downpipe that comes with CATs installed – typically 2 of them. 

A catted downpipe is still good and can give you about 40 horsepower with tuning. I recommend going for a catted downpipe for the majority of car guys.

A catted downpipe looks very similar to a stock downpipe – which means a regular traffic cop is not going to notice anything.

Catted downpipe typically have 2 CATs.

Personally, I have not smelled any bad exhaust smell with my catted downpipe. But I can definitely smell bad exhaust gases from my friend’s car with a catless downpipe. 

Catted downpipe vs catless downpipe is a much bigger discussion than just this. Thus, I recommend you checking out my other article about that.

What’s Better: Catted Or Catless Downpipe?

You should only install catless downpipe if you are into racing on the tracks. Catless downpipe causes heavy emission and is illegal in all states in the US. 

For 90% of enthusiasts, I would recommend looking into catted downpipes instead. They provide slightly less horsepower but do not have as much hassle as the catless downpipes. 

That aside, downpipe is an absolutely great mod. It provides the most power for the money. More than an exhaust, header or cold air intake. You should definitely look into it.