Stock catted downpipes lying next to new catless downpipesHorsepower is important. That's why you're doing those mods under the hood. You're trying to get as many of those ponies as you can out of your turbocharged engine by increasing airflow as much as possible. So why not get a catless downpipe and remove all obstacles to airflow, like that pesky catalytic converter? Wouldn't that get you the most power?Possibly, but in the U.S. there's the matter of complying with federal and, depending where you live, state law. Installing a catless downpipe (a downpipe without a catalytic converter) would render your car illegal to drive on the street anywhere in America. Also, catted downpipes (downpipes with catalytic converters) do more than just help you pass emissions tests. They also muffle engine noise and remove the smell that would come from untreated exhaust.And does installing catless downpipes really result in more power than installing an aftermarket catted ones? While you'll see more horsepower compared to stock catted downpipes, data suggests that any gains in horsepower over aftermarket catted downpipes are negligible, at best. In fact, you may not experience more horsepower at all. Cost savings are not all that cut-and-dried, either. Sure, catted downpipes retail for a lot more than catless. But catless downpipes may not save you much in the long run, especially when you take into account the tuning and maintenance costs. It's similar to the question of whether straight piping your car is a good idea. Understanding downpipes and what they doGloved hands touching an aftermarket catted downpipe for a BMW N55A turbocharger is a brilliant device. It takes exhaust that would otherwise just be expelled from the engine and uses it to spin a turbine, which in turn spins a compressor wheel, which pulls more air into the engine's intake, thereby producing more power. But the engine exhaust still needs to go somewhere. That's where the downpipe comes in.The downpipe is connected to the turbocharger and runs to the exhaust's midpipe. Factory downpipes come with a built-in catalytic converter that cleans the exhaust of pollutants before it's expelled out of the tailpipe. In fact, some downpipes actually take the place of the exhaust pipe.Why would someone replace a factory downpipe with an aftermarket one? Mainly to get more power from the engine. Aftermarket downpipes are generally wider than the stock ones. This reduces back pressure and lets air flow more freely through the turbocharger, producing more power and less turbo lag (here's an explanation of what causes turbo lag). Most aftermarket catted downpipes come with high-airflow catalytic converters for this purpose. Catless downpipes go even farther and omit the catalytic converter, removing any obstacle to airflow. But regardless of which kind of aftermarket downpipe you choose, you're sure to squeeze more horsepower out of your turbocharged engine, compared to your stock downpipe. Understanding catalytic converters and what they doThe honeycomb interior of a catalytic converterYour car's engine produces a lot of nasty gases, specifically hydrocarbons, nitrogen oxides. and carbon monoxide. Those first two are terrible for the environment and the air quality where you live, and the last one can kill you if breathe enough of it. Hydrocarbons come about from gas that hasn't been burned, carbon monoxide from gas that has been burned, and nitrogen oxide from expelled nitrogen that combines with oxygen when it meets the air. Obviously, we can't have a bunch of cars going around putting these gases into the air, especially since some of us are fond of breathing. That's what a catalytic converter is for.In a catalytic converter, engine exhaust is forced through honeycomb structures that contain platinum, rhodium, and palladium (now you know why thieves love catalytic converters and why catalytic converter theft is still a big problem). When the exhaust contains excess oxygen, the carbon monoxide and hydrocarbons are oxidated by interacting with the platinum and palladium. But when the exhaust contains excess fuel instead, the nitrogen oxide is broken down as it interacts with the platinum and rhodium. This means those toxic gases get converted into water as well as oxygen, carbon dioxide, and nitrogen, which are gases our atmosphere naturally contain. Too much carbon dioxide in the atmosphere is a bad thing, but catalytic converters reduce emissions of the much more toxic carbon monoxide by 96% and nitrogen oxide by 76%. Why some use catless downpipesAftermarket catless downpipe for a Honda AccordLet's face it, you're not doing a high-performance mod to your engine just to play it safe. You're trying to squeeze every bit of power out of that turbocharged beast. Using a downpipe unencumbered by some tree-hugger-mandated catalytic converter might seem to you like the way to go.On paper, it makes sense if your only priority is boosting power. A turbocharger needs airflow to produce power. To some extent, freer airflow means more power, and a catalytic converter is an obstacle to optimum airflow. A catless downpipe removes that obstacle so the turbocharger doesn't have to work as hard to push air through. Therefore, some assume installing a catless downpipe will result in more horsepower. Actually, the amount of horsepower you'd get compared to aftermarket catless downpipes would be minimal, at best. But some choose catless for racing anyway, since every little advantage is important on the track.Another reason why a catless turbocharger may seem appealing has to do with cost. The platinum and palladium found in your catalytic converter don't come cheap. You're looking at a $100 to $400 difference between a catted and catless downpipe. We should note, though, that there are extra costs that come with installing a catless downpipe. It will often require extensive (and maybe illegal) tuning just to keep the check-engine light from coming on. Your car's computer and oxygen sensors are expecting a catalytic converter, after all. After all is said and done, you may not save that much going catless. Why you really should use a catted downpipeAn aftermarket catted downpipe for an Mk7 GTIIf you plan to use your vehicle just on the track or off road, then you're probably fine fitting it with a catless downpipe. But if you also plan to use your car for your turn in the school carpool, you should go with a catted downpipe. That is especially the case if your state has emissions testing. And make no mistake, you will fail an emissions test with a catless downpipe.A catted downpipe includes a catalytic converter which, yes, will slow down the airflow a bit. But the catalytic converters on aftermarket downpipes are built to balance emissions requirements with power. The inside of an aftermarket downpipe catalytic converter is less restrictive to airflow than the catalytic converter that was in your car when it came out of the factory. That's because automakers are generally careful to lean more on the emissions requirement side than the power side. So your aftermarket catted downpipe will produce more power than the one your car came with, but not noticeably more than an aftermarket catted pipe. Sure, some aftermarket catted downpipes may also fail emissions testing, but if you get one up to California emissions standards, you should remain street legal.Besides reducing emissions, a catted downpipe gets rid of the godawful stink that comes with catless downpipes and muffles the obnoxious noise, too. Some might like the pops and bangs that accompany catless downpipes, but we prefer the low beastly growl of a turbocharged engine with a high-flow catted downpipe. The performance difference between catted and catless downpipesA catted downpipe lying next to a catless downpipeLike we said, a catless downpipe should, in theory, help provide noticeably more power than a catted one. Airflow is completely unrestricted in a catless system. That means the turbo doesn't have to deal with backpressure to push the airflow, resulting in more power. But how much horsepower do you really gain with a catless downpipe over a catted downpipe? Exact figures are hard to pin down, because they would differ based on so many factors. One estimate we found states aftermarket catted downpipes put out from 5 to 10 less horsepower than catless ones. That hardly seems worth rendering your car illegal to be driven on the street.However, a couple of tests have been conducted that indicate there's little or no difference in power between catless and high-flow aftermarket downpipes. Hot Rod compared the rear-wheel horsepower (rwhp) when running a twin-turbo LS3 with and without catalytic converters, finding exactly 660 rwhp measured for both: "That's right, ZERO power loss compared to straight pipes." And another, seemingly more scientific test by My Golf MK7 concluded, "The results of the t-Test support the hypothesis that the WHP [Wheel Horsepower] differences are not statistically significant." Similar results were found with torque produced.Aside from possible cost savings, we really don't see an advantage to going with catless downpipes. But maybe you like a noisy, smelly car that's illegal to drive on the street. You do you. Want more like this? Join the Jalopnik newsletter to get the latest auto news sent straight to your inbox, and add us as a preferred search source on Google.
