Turbocharging is perhaps the most critical development for tuners. Now, sure, naturally-aspirated engines still have some cult following, especially among purists. However, forced induction systems are still one of the easiest ways to significantly increase the power produced by an engine. Well, ok, maybe not the easiest, but you get the point. Nonetheless, that’s why terms like “blow-off valve” and “downpipe” are becoming increasingly popular in the tuning community.
We have already covered the blow-off valve, and all you need to know about it in a previous article. Now it’s time to tackle the notorious downpipe. What makes it have that kind of reputation, you ask? Well, the downpipe is directly connected to tailpipe emissions. Some aftermarket solutions disregard that quality to add more power. If you are into car culture, you’ve probably seen tuned vehicles fume like crazy.
But what is a downpipe, and how does it relate to emissions? Well, the downpipe is part of the exhaust system of turbocharged cars. This critical part connects the exhaust turbine on one end and the exhaust on the other.
Modern turbocharged cars have downpipes with catalytic converters built-in. Manufacturers do that to reduce harmful emissions, specifically NOx gases and microparticles. Nitrous-oxide gases are poisonous in enough quantity, while the particles often lead to lung cancer and other diseases.
The problem with the catalytic converter is that it restricts the flow of exhaust fumes. And, the less circulation, the slower the turbine spins, which translates into less boost and less horsepower. The last time I checked, less power is a nightmare for tuners and enthusiasts. For that reason, some people opt for downpipes without a catalytic converter.
Luckily, if you are an environmentally-conscious petrolhead, there is also a way to add more power without harming the environment, though. Let’s get into more detail and see how.
Why Would You Want an Aftermarket Downpipe?
The aftermarket tuning scene offers numerous downpipe designs. They vary in diameter but also the type of catalytic converter.
Also, as I already mentioned, some downpipes eliminate the catalytic converter altogether for the highest power gains. However, I wouldn’t recommend “catless” turbo downpipes to anyone unless you only plan to use your vehicle on a track. Even if you just don’t care about the environment, running “catless” is also illegal pretty much everywhere in North America and may get your car towed or impounded.
Now, you probably thought about the catalytic converter in your exhaust system. And, yes, most cars have one there as well. So, why would you need another one in the downpipe? Because catalytic converters work their magic at higher temperatures.
And, the closer the catalyst is to the engine, the warmer the exhaust fumes will be. If the converter is far from the engine, the exhaust gases will cool down, which lowers the catalyst efficiency. Because of that, the catalyst won’t be able to clean the exhaust gases properly before they enter the atmosphere.
See also: What is a Blow-Off Valve? | Tuning 101
If you want to increase power without poisoning the atmosphere, you’ll need a downpipe with a high-flow catalytic converter instead. These catalytic converters offer less restriction while also doing a great job of cleaning them. Moreover, a downpipe with a larger diameter can also improve the flow of gases. Some downpipe designs also have fewer curvatures. This can additionally increase the flow of exhaust fumes.
The gains will not be very significant, though. Depending on the downpipe and the engine, the increase ranges from 10-20 horsepower. Not bad, but you might get the same jump with an ECU update.
Still, in some circumstances, you might need a larger downpipe with a high-flow catalytic converter. For instance, installing a larger turbocharger will also need a downpipe, providing a better flow.
Will Catless Downpipe Pass Emissions?
You certainly won’t pass emission tests with a catless downpipe. Without the catalytic converter, your car’s exhaust gases will be much dirtier. And, the tailpipe analyzers can undoubtedly catch the discrepancy. And, if your vehicle fails an emission test, you won’t be able to register it.
Now, I’ve seen many tuners replace the catless downpipe with an OEM one before the annual inspection. Still, you might get a fine if you do that. Moreover, in some states, the authorities test the vehicles for emissions directly on the road and might penalize you. Remember that before you decide on doing these double-crossing practices.
Will a Downpipe Void my Warranty?
It depends on the manufacturer but also the type of damage. In my experience thus far, the dealer won’t repair a broken engine or similar issues. The downpipe won’t void the warranty of the electronics inside your cabin or the suspension components.
You must understand that car manufacturers will do anything to reject the warranty claim. Repairing damage inside an engine is costly. Considering that, manufacturers will look for any reason not to fix the issue for free. And, honestly, an aftermarket downpipe will be a valid one.
Now, again, you can replace your aftermarket downpipe before bringing your vehicle for service. Nonetheless, mechanics have a way to tell if you replaced some parts, specifically via the ECU.
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Downpipe vs. Test pipe
The test pipe is piping that can be installed instead of the catalytic converter. With a test pipe, you can quickly increase the flow of gas. However, you will also lose the catalytic converter, making your engine worse for the environment.
That said, test pipes are a very cheap and straightforward upgrade. Replacing the downpipe will be much more expensive. Remember, the test pipe is only a part of the downpipe as a whole.
And while I don’t recommend taking out the catalytic converter, you can indeed get good horsepower gains with a test pipe. Besides, having a test pipe instead of a converter will make your engine sound much louder.
Downpipe vs. Catback
Catbacks and downpipes are different animals altogether. The downpipe is the piping that connects to the turbine. The catback, on the other hand, relates to everything after the last catalytic converter and goes all the way back to the muffler.
Still, if you can replace only one, should it be the downpipe or the catback? Well, it depends on what you want to gain. In pure numbers, the downpipe wins easily. The horsepower gains will be much higher – there is no comparison.
Nonetheless, installing a catback has other benefits. Primarily, catbacks can improve the looks of your car. Furthermore, a catback will probably be louder and sound a bit better.
Downpipe vs. Header
A header is a collection of pipes that brings exhaust fumes from the engine to the turbine. In naturally-aspirated engines, the header connects directly to the downpipe.
If you plan on upgrading one of these parts, the downpipe will give you a higher power gain. However, a good header will let your engine breathe better. Both are excellent options if you plan on upgrading the turbocharger.
Frequently Asked Questions (FAQs)
Are downpipes only for turbo cars?
Technically yes, downpipes are only for turbo cars.
Downpipes are a part of the exhaust that fits between the turbocharger and the catalytic converter. Of course, naturally aspirated cars have this part of the exhaust too. However, without the turbocharger in the equation, it’s referred to as the “mid-pipe” instead of a “downpipe”. The mid-pipe fits between the exhaust header and the catalytic converter but, more often than not, will be a part of the header itself — not all N/A cars have a separate mid-pipe.
An important thing to note is that a bigger downpipe is a popular performance mod on turbocharged vehicles. With it, your car can gain a serious power bump when combined with a good remap. As for N/A cars, a performance mid-pipe alone won’t do much for power gains; for these engines, you’d need a better exhaust header to notice a change in acceleration.
Final Words
Replacing the downpipe in your car might be tempting. That’s especially true after you read the marketing speech from tuning parts manufacturers. However, the real-life gains aren’t always as high as it says on paper. Besides, replacing the downpipe requires other modifications, such as upgrading the ECU. In most cases, downpipes are usually collaterals of other mods. For example, tuners usually install an aftermarket downpipe as part of a larger kit when they plan to replace the turbo for a bigger model. In this case, installing a high flow header, downpipe, catback, and intercooler at the same time is usually the way to go. Installing a bigger turbo without installing bigger forced induction system components will significantly reduce the possible gains.
Having said that, if you’re really into it, at least make sure you purchase a high-quality downpipe. It’s is a crucial part of any turbocharged engine, and you shouldn’t mess with it. Moreover, ensure that the aftermarket downpipe meets regulations in your state.
Good luck!
