A catless downpipe will add more horsepower to your vehicle, but at a cost.
If your goal is strictly more horsepower, however. Then I would install a catless downpipe anyway.
A catless downpipe will increase a turbo’s ability to inject more air into the combustion chamber, adding more horsepower to your vehicle.
Catless downpipes are less restrictive than catted downpipes which helps a turbocharger increase engine power. Less restriction means you can expect horsepower gains of up to 15 to 20%, or an increase of 25 to 50 horsepower. You’ve read this far.
So, I’m assuming you’d like to learn more about how all this works to achieve the horsepower and performance you’re after. Let’s get started.
What is a Downpipe?
A downpipe is tubing connecting the exhaust manifold to the catalytic converter. It aids in allowing spent gases (exhaust) to exit the engine into the rest of the exhaust system.
A downpipe can either be catless or catted. A CAT is a reference to the catalytic converter. A downpipe can come either with or without a catalytic converter – most downpipes come with a CAT.
We’ll look into the reason why most factory and aftermarket downpipes come equipped with catalytic converters as you continue reading.
Catless Downpipes Don’t Come with a Catalytic Converter
Factory downpipes and even most aftermarket performance downpipes come with a catalytic converter – sometimes two making them more environmentally friendly.
A catless downpipe doesn’t come with any catalytic converters. This is the reason why they’re primarily used for race applications.
Making more horsepower is all about how effectively oxygen can be delivered into the combustion chamber. Although turbocharged vehicles come with downpipes, the catted design makes them more restrictive to the outflow of exhaust gases limiting power and performance compared to a catless option. (We’re also going to touch on the legality of installing catless downpipes for street use).
A Catless Downpipe Absolutely Increases Horsepower and Performance
I’ve already given you some of the horsepower numbers you can expect from installing a catless downpipe. Since there’s no catalytic converter restricting the flow of exhaust gas, a greater volume of exhaust can be expelled from the engine allowing more oxygen into the cylinders to create more power.
The process of exhaust gases flowing out of the combustion chamber to allow more fresh oxygen into the cylinders is referred to as exhaust scavenging. A catless downpipe allows for a greater volume of exhaust scavenging.
The Purpose of a Catalytic Converter on a Downpipe
Why does a downpipe even need a catalytic converter? Well, what does a catalytic converter do?
In an exhaust system, a catalytic converter is responsible for transforming harmful exhaust gases and fumes into less harmful emissions before they are released into the atmosphere.
Connected to a downpipe, a catalytic converter works to filter exhaust gases coming from the exhaust side of a turbo breaking down carbon monoxide, carbon dioxide, hydrocarbons and nitrogen oxide turning them into less-harmful exhaust as they pass through to the rest of the exhaust system.
Downpipes with only one CAT are less effective at breaking down nitrogen oxide.
How Downpipes Work with a Turbo
Turbos are an innovative bit of mechanical inventiveness. They were originally created for the airline industry in the late 1800s before making their way into roadgoing vehicles in the early 1960s.
Turbos were designed to increase performance and efficiency.
They’re made up of two halves joined together by a shaft. Hot exhaust gases from an engine spin the turbine on the hot side. This is connected by a shaft to a turbine on the cold side which sucks in fresh air and compresses the oxygen into the engine. The compressed air has a bigger concentration of oxygen which is what gives an engine that extra kick.
Types of Downpipes
Turbocharged cars already come with downpipes from factory to help reduce harmful emissions. Some high-end performance vehicles even come with a performance-focused downpipe. But the pipes are narrower than what you’ll find aftermarket.
You’ll sometimes see aftermarket downpipes referred to as high flow downpipes due to their use of a high flow catalytic converter. The tubes also have a wider diameter than what you get from factory to improve exhaust flow. This means they’re designed to allow a greater volume of exhaust gases to pass through.
Still, because of the catted nature of any of these downpipes. They’re highly restrictive choking out a lot of performance and power for the sake of emissions control.
The Sound of Fury: Cattles vs Catted Downpipes
Although no aftermarket downpipe will be as good at controlling emissions as stock downpipes. Going aftermarket means you can expect a change in exhaust sound. As you can probably already imagine, a catless downpipe will be way more effective at producing a bigger sound since there’s less restriction compared to any catted version.
Now the Negatives: A Catless Downpipe Needs Tuning
You can probably live with a check engine light that’s always on. But that engine light is the computer telling you that something’s wrong. How will you know if the engine light is the result of your catless downpipe installation or some other problem?
Anyone who’s ever owned a car for a significant amount of time can tell you that it’s only a matter of time before small problems become big problems.
While you probably can get away with it, simply adding an aftermarket downpipe – whether it’s catless or catted – won’t do much for performance without a proper tune with the right software.
To maximize the horsepower potential of your ride you should think of an aftermarket downpipe install as part of a total build that includes other performance exhaust modifications like a cold air intake and catback performance exhaust.
The Aroma of a Catless Downpipe
On the other hand, even without a tune an aftermarket catless downpipe will give you significant horsepower gains. Not to mention performance benefits like a more responsive throttle due to less exhaust flow restriction to the turbo. You’ll also notice more burbles that – let’s face it – for enthusiasts like us, makes driving fun.
But in the midst of all this unfiltered exuberance, there are some rather dangerous outcomes of going catless that are worth exploring. Especially because it relates to your health:
Given the higher flow of unfiltered exhaust, an aftermarket downpipe is going to have a noticeable smell. That smell comes from toxic gases that can eventually cause you and your passengers respiratory problems.
And since catless downpipes have no filters, you’re breathing in 100% of those harmful chemicals while simultaneously unleashing them into the environment.
Is Installing an Aftermarket Performance Downpipe Even Legal?
I hate to be such a bummer. As if the health issues weren’t a bad enough reason not to install aftermarket downpipes. It turns out that catted and catless aftermarket downpipes are restricted to off road use only.
This means you can’t install them on cars you’ll be using on public roads.
You’d think that since you can purchase an aftermarket downpipe, you’d be able to install it on your car without legal repercussions. But as it turns out, there are a number of modifications while perfectly legal to purchase and install renders your ride illegal to use on public roads. That’s why most advisors will tell you up front that this modification is only meant for off-road use.
The reason why they’re illegal to use on public roads is because of the way aftermarket downpipes help a turbo produce more power: the wider diameter tubing along with a less restrictive CAT (or no CAT) means you won’t be able to pass emissions.