Downpipe 101: Horsepower, Engine Light, Sizing & Legality

Aftermarket downpipe can provide a 40 horsepower gain (with tuning).

It’s an absolutely worthy mod for performance. However, it’s illegal for street use and has high chances of getting Check Engine Light.

Personally, I have installed a catted downpipe on my 2015 Civic and I am a big fan. 

Considering the price and performance gain, I think they are the most worthy performance mod out there right now. 

In this article, I will go into everything you want to know about donwpipe. This includes:

1. Performance (Horsepower Gain)
2. Dealing with Check Engine Light
3. Tuning downpipe
4. Legality 
5. My recommend on which downpipe to install 

I guarantee, you won’t have any more questions about downpipe after reading this article (unless you are the class nerd in high school).

Aftermarket downpipes are significantly less restrictive than stock ones – they are wider, have limited bends and equip a less restrictive CAT.

This allows exhaust gases to flow faster and spool the turbo even harder.

If you don’t already know, turbo is an air compressor that sucks more air and forces it into the combustion chamber for power boost.

Turbo is powered by a turbine that’s spun by the flow of exhaust gases – which means, the turbo can work even harder if the exhaust gases spinning it are flowing more. 

This is where an aftermarket downpipe comes into place. By being less restrictive, an aftermarket downpipe allows exhaust gases to flow out much quickly – causing them to spin the turbo much harder. 

Aftermarket downpipe (white) is wider with lesser bending and less restrictive CAT.

Downpipe is extremely effective and can help you gain up to 40 horsepower with tuning. This is significant and believe me, it was noticeable for my Civic while driving. 

When it comes to gaining performance, tweaking parts of the turbo is the best thing you could do – the power gain is usually significant.

This means, you should look into things like downpipe, turbo inlet or even a bigger turbo. Rather than cold air intake, exhaust or muffler for power. 

You are definitely going to cause a Check Engine Light (CEL) when installing catless downpipe. Whereas with a catted downpipe, you have a chance of causing CEL – it depends on the CAT.

With a downpipe installation, CEL is caused by the aftermarket catalytic converter that may or may not come with the downpipe.

Catless downpipe doesn’t come with a CAT. This is definitely going to cause a CEL. The oxygen sensor in your car detects extremely toxic exhaust gases and assumes something is wrong. Thus, the CEL.

Catted downpipe comes with a less restrictive CAT. The idea is to improve your car performance but still equip a CAT to pass the smog test. 

With a catted downpipe, CEL may or may not be turned on. This depends on the CAT that comes with the downpipe. You should probably ask the seller or do your research on that downpipe before you buy. 

You can turn off CEL from installing downpipe by bringing your car in for a tune or by installing an O2 sensor spacer. 

With tuning, you can programmatically turn off the CEL by changing some parameters in your car’s computer. 

You don’t usually tune your car just to turn off CEL – which means tuning is an expensive option to turn off CEL.

The other alternative to turn off CEL from a downpipe installation is by using an O2 sensor spacer. It’s an inexpensive device that extends the gap between your actual o2 sensor and the exhaust gas. 

More gap between them means that the O2 sensor will have a lesser (and inaccurate) reading of the toxic gases. Thus, tricking it to turn off the CEL.

The O2 sensor spacer costs about $10 – $20 and is quite easy to install. It’s basically unscrewing your O2 sensor, and screwing it back on with the spacer in place.

Here’s an O2 spacer from Amazon that I have used in the past and Here’s a YouTube video showing you how an O2 sensor is installed. 

Downpipe does not cause any damage to the engine. All it does is be less restrictive and allow exhaust gases to flow much more easily. 

You can even say downpipe is good for the engine since it reduces backpressure and ensures the majority of injected fuel is utilized during combustion. 

Some enthusiasts are concerned about downpipe engine damage because it causes Check Engine Light (CEL). But this is incorrect. 

Aftermarket downpipes (especially catless ones) cause CEL because the O2 sensor detects that the exhaust gases are toxic. 

This is not bad for the engine but is bad for the environment and even the driver – if the toxic gases seep into the cabin.

So don’t worry about engine damage caused by downpipes. Instead, worry about the emission and law! 

Downpipe is not legal for street use and is only allowed for track or off roading use.

This is because aftermarket downpipes equip less restrictive CAT that can increase emission significantly (depending on the model).

A downpipe typically equips 2 CATs – one to reduce Hydrocarbon and another to reduce Nitrogen Oxide. 

If your aftermarket downpipe has 2 CATs it has a higher chance of passing the emission test and be legal. You need to consult the shop or company that’s selling it. 

Make sure to ask specifically for your state. Each state in the US has slightly different laws – some are stricter than others. Who knows, maybe you are lucky and drive in a state that allows your aftermarket downpipe. 

On the other hand, if you are getting a catless downpipe, then it’s definitely illegal. There’s no way you pass the emissions test with a catless downpipe. 

I will discuss more about catted downpipe vs catless downpipe later. 

An aftermarket downpipe will make your car sound louder – especially when you rev and spool the turbo.

This is because an aftermarket downpipe is much wider – allowing sound waves to travel without collision.  

To explain why aftermarket downpipes improve car sound, let’s talk a little about how car mufflers work. 

If you don’t already know, the inside of car mufflers are made of metal plates. They are designed to bounce sound waves where they collide into each other and cancel out. 

Aftermarket downpipes are much wider with lesser bends – which means the majority of sound waves coming from the engine can flow out easily without any collision.

Also, sounds get louder when travelling in hollow space. Do you realize you sound louder when speaking in an empty house without furniture? Same with aftermarket downpipes. 

The inside of muffler consists of metal plates that deflects sound waves and make them collide with each other to cancel out.

You can run downpipe without tuning but the horsepower gain is limited. 

I recommend you tune your car after installing a downpipe. This ensures maximum power gain, no CEL and a more aggressive car overall. 

Installing an aftermarket downpipe without tuning can gain you a ‘good enough’ horsepower – about 20 horsepower depending on the size of engine and turbo. 

With tuning, you can look to increase the horsepower of your car by about 40. That’s double horsepower gain when compared with tuning. 

Of course, this varies for different people but that’s what I got from the dyno with my aftermarket downpipe. 

After installing an aftermarket downpipe, your engine could be running a little lean. With tuning, you can fix this air/fuel ratio, turn off the CEL and even tweak other parameters like removing rev limiter. 

You do tuning to improve the overall performance of your car. Don’t just tune your car to turn off CEL or fix the air/fuel ratio. You should look into other parameters to make your car more aggressive. 

Here are related articles around tuning that you should read:

1. Running Downpipe Without Tuning (Should You?)
2. How Much HP Can You Gain From Dyno Tune?

Catted downpipe comes with a catalytic converter (CAT) – they provide less power gain but also keep emission in check.

Catless downpipes don’t come with CAT- they provide greater power but are extremely toxic. 

Personally, I would go with a catted downpipe any day. It provides a good enough horsepower gain without any downsides like bad smell, horrible emission and trouble with the law. 

You should only consider a catless downpipe if you are strictly using it for track days and you desperately need that additional horsepower gain. 

If catted downpipes provide you with 40 horsepower after tuning, then expect a catless downpipe to provide you up to 50 or 55 horsepower. It doesn’t give you that much more but definitely more. 

I cover more about this topic in this article – where I talk about detailed pros and cons. Check it out. What’s Better: Catted Or Catless Downpipe.

Catless downpipe doesn’t have CAT. It’s a straight through metal piping.

With aftermarket downpipes, you should aim to get one that’s slightly wider piping than your stock but not too wide – about 20 – 30% wider should do. 

Downpipe that’s too big can significantly reduce velocity. Which means the exhaust gases travel slowly and it won’t be strong enough to spool the turbo – resulting in a power loss instead.

You need to get a downpipe that’s a little larger than stock but not too large. This ensures exhaust restriction is reduced, but still keeping the exhaust velocity. 

Different engines and turbos require different downpipe sizing. I suggest you discuss this with the shop that sells the downpipe.

I definitely recommend installing an aftermarket downpipe. As I said, it’s the most ‘worth it’ performance mod considering the power gain and cost. 

If you are not a racer or regularly go on track days, I highly suggest going for a catted downpipe. They give you much fewer problems – suitable for the majority of enthusiasts. 

If you care a lot about performance, then you could also save more money and equip your car with an aftermarket exhaust, cold air intake and downpipe.

Once you have these 3 mods, bring your car to a tune and expect more monster performance. Considering that your car already has turbo, adding 3 of these into the mix will be an absolute beast.