What’s Better Catted or Catless Downpipe?

So you want to install an aftermarket downpipe but don’t know whether to go catted or catless? Then you are at the right place 🙂 I will provide you with a super detailed comparison of the two – basically everything you can know about catted and catless downpipes. 

For a quick answer, catted downpipe is better than catless downpipe if you are driving on the street – especially if the car is a daily driver. Catted downpipes provide slightly less horsepower (about 5-10 lesser) but they have much more pros than the catless version: significantly less emission, no unpleasant smell, and has lesser chances of turning on check engine light (CEL).

Actually, there are many more comparisons to be done. I will cover all of them in this article. Read on 🙂

This recap section is for the beginner enthusiasts. If you already know the answer, skip right ahead to the comparisons.

That being said, I assume you already know what’s a downpipe right? Downpipe is only needed if your car has turbo. It allows exhaust gases to flow through a turbo and exit into the exhaust system. I cover more about downpipe in this article: Up-pipe Vs. Down-pipe (And Why Modify Them)

Turbo is an air compressor that sucks surrounding air, compresses them and delivers them directly into the combustion chamber for more horsepower. The beautiful thing about turbo is that it’s self powered by a turbine.

Exhaust gases flow through the turbo and spool it’s turbine – generating power for the turbo to do its work. The heavier and faster the exhaust flow, the faster the turbine is spooled. Thus, generating more power for the turbo to work and more horsepower for the car. I cover more about turbo in this article: Which Is Better: Supercharger Or Turbocharger

Stock downpipes have narrow piping and equip restrictive catalytic converters (CAT). This is good for emission but restricts the power generated for turbo to work – reducing the horsepower gain. For car enthusiasts like us, horsepower is more important right? 🙂 

We install aftermarket downpipes to improve exhaust flow. Aftermarket downpipe has wider piping and they equip a less restrictive CAT. This means the exhaust gases can flow harder and generate more power for the turbo to do its work – resulting in more horsepower.

Aftermarket downpipe that equips a less restrictive CAT is called catted downpipe. Whereas a downpipe that does not have CAT is called catless downpipe. Each has its pros and cons and that’s what we will discuss in this article. Read on.

1. Catalytic Converter

Catted downpipe comes with an aftermarket catalytic converter that’s less restrictive than stock. This means exhaust flow is improved and emission is still being controlled – although at a reduced capacity. 

Catless downpipe does not equip any catalytic converter. This provides maximum exhaust flow but emission is horrible. The toxic exhaust gases are not treated at all – allowing them to exit into the environment. 

2. Emission

Catted downpipe has worse emission than stock downpipe but still at a manageable level when compared to catless downpipe. If the emission test is visual and uses OBD2 port, then your catted downpipe will pass this one. If the emission test uses a sniffer, then any catted downpipe with highflow CAT can fail.

A typical stock downpipe equips 2 CATs – 1 to breakdown CO, CO2 and HC (HydroCarbon), while the other breaks down NOx (Nitrogen Oxide). If your catted downpipe has 2 CATs then you can pass the emissions test.

If your catted downpipe only has 1 CAT (most common), then you will only pass the emissions test for CO, CO2 and HC but fail for NOx. 

Catless downpipe has horrible emission. You will fail all the emissions tests – whether it’s visual, OBD2 or sniffer. The only way to pass emissions test is to swap it into the stock downpipe before going for emissions test. What a hassle.

3. Performance

Catted downpipe provides horsepower gain but not as much as catless downpipe. You can expect catted downpipe to provide additional 40 horsepower with tuning (20 hp without). This is a decent horsepower gain given its price point.

Catless downpipe provides even more horsepower gain than catted downpipe. You can expect catless downpipe to provide 50 horsepower with tuning (25 hp without). Performance is the biggest advantage for going catless.

4. Sound Improvement 

Both catted and catless downpipe will provide great sound improvement. Your car will sound more aggressive with lower tones. Catless downpipe will provide a slightly louder sound than catted version. I covered why downpipe can provide sound improvement this article: Does Downpipe Make A Car Louder? 

5. Price

Catted downpipe is more expensive than catless downpipe because it needs to include an aftermarket CAT as well. A catted downpipe can cost around $350 and up. 

Catless downpipe is cheaper because it does not have any CAT. A catless downpipe can cost around $150 and up.

6. Smell

Catted downpipe causes a slight fume smell from the exhaust. This is not significant and you will not smell it very much. Maybe a little bit when your car is idling and you exit the car. Not a big problem, in my opinion.

Catless downpipe causes a significant fume smell from the exhaust. You can definitely smell it when your car is idling and you exit the car. You can even smell it from inside the car when you are idling at traffic for some time.

7. Is It Legal?

Both catted and catless downpipes are not street legal. They are only sold for tracking or off-roading purposes. Though, if you are using catted downpipe, you probably won’t have any issue with traffic cops because they are not visually apparent.

8. Will It Void Warranty?

Installing aftermarket downpipes should not void warranty. You are protected by Magnusson-Moss Warranty Act. It’s a federal law that prevents you from being denied your warranty just because you installed an aftermarket part.

With this law, dealers can only void your warranty if they can prove that the damage is directly caused by your aftermarket install. Otherwise, they cannot and should not void your warranty.

Even if they do void your warranty, it should only be the warranty for the downpipes, turbo and the exhaust system. Your warranty for the electrical equipment like lights and air-conditioning should be intact. 

For more information on Magnusson-Moss Warranty Act, check out my other article: Does Muffler Delete Void Warranty? It’s not exactly about downpipe but the warranty act is the same. 

9. Daily Drivable 

Catted downpipe should be daily drivable. It provides additional performance but still keeps the emission, noise and smell acceptable. With catted downpipe you can also pass the visual inspection or pop up inspection that uses OBD2. 

Catless downpipe is not daily drivable. They are more suited to be used on off-roading or tracking where every little performance counts and emission doesn’t really matter.

10. Needs Tuning?

If you want the most performance, you should tune your car after installing any downpipe – regardless whether it’s catted or catless. Tuning your car after installing a performance mod can yield twice the horsepower.

In the case of downpipe, 25 horsepower without tuning and 50 horsepower with tuning. I covered more about tuning and downpipe in this article: Running Downpipe Without Tuning (Should You?)

11. Check Engine Light (CEL)

Both catted and catless downpipe will cause the check engine light to turn on. This is because the CATs are modified. Your car’s computer will assume that your CAT is not working properly and turn on the check engine light.

You have two options to fix this:

1. Bring your car to a tune. Tuning can help you change some settings and keep the check engine light turned off when it comes to CAT.
2. Use O2 Spacer. Spacer allows you to adjust your O2 sensor away from the direct flow of exhaust – where it’s not so concentrated. By doing this, the O2 sensor will have less emission reading and keep the check engine light off.

For tracking purposes – there’s no question, you should go for the catless downpipe. It will provide you with maximum horsepower that you need.

If you are looking to equip a downpipe on your daily driver and drive it on the street, then go for the catted downpipe. It gives you a reasonable horsepower boost while still being manageable.

• Lesser emission
• Much higher chance to pass emissions test 
• No fume smell 

In regards to tuning, I suggest you don’t bring your car to tune right away. Wait until you have more complete performance mods like cold air intake, exhaust header and catback exhaust. Once you have all these, then bring your car for a tune. You will see significant improvement in horsepower.