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BMW N54 Downpipes: Catless vs. Catted High-Flow Downpipes

About Zach Mayock - DieselIQ

Meet Zach

Zach is a BMW enthusiast with a passion for performance. With over 10 years of experience modifying and performing DIY work on BMWs, he’s developed a deep understanding of virtually every BMW engine. He’s also the proud owner of a 600whp N54 with upgraded twin turbos and an E30 325i drift car and has a particular affinity for the S58 engine. Zach is highly knowledgeable about all things BMW, but his expertise in tuning and performance mods sets him apart. His experience as an enthusiast, combined with his technical knowledge, makes him an essential resource for anyone looking to improve the performance of their BMW.

Downpipes are one of the most significant modifications that you can do to any turbocharged BMW engine, and that is certainly true for the N54. The N54’s factory downpipes are not only hampered by massive factory catalytic converters, but also by their overall shape and design which prevents the turbochargers from expelling exhaust gasses as efficiently as possible.

By upgrading to either catless or high-flow N54 downpipes, you’ll not only see a significant increase in power, but also a number of other performance benefits due to the turbos working more efficiently. In this article, we’ll cover upgraded BMW N54 downpipes and the benefits that both catless and high-flow downpipes have on N54 engine performance.

Table of Contents

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Why Upgrade My N54 Downpipes?

One of the biggest restrictions on BMW N54 powered 135i, 335i and 535i are the factory catted N54 downpipes. Downpipes bolt directly to the turbochargers and the factory catalytic converters create significant back-pressure which results in slower turbo spool, increased strain on the turbos, and an array of other turbo performance issues.

In addition to the restrictive factory catalytic converters, stock 2.5″ N54 downpipes are also restrictive due to their shape and multiple kinks and bends which create undesirable turbulence within the exhaust system. That’s why most aftermarket upgraded N54 downpipes contain very few bends and are as smooth as possible internally.

Modifying your N54 with catless or high-flow catted downpipes results in faster spool, very notable power gains, and an aggressive exhaust note. Upgraded downpipes are a great modification, especially for N54s that have dual cone intakes and other performance modifications like a tune and upgraded turbo inlets and outlets. Dual cone intakes bring more air into the engine, and the more that flows in, the more that has to flow out. When paired with a tune, you can expect even more significant power gains from upgraded downpipes, as the DME can accommodate for the additional exhaust flow.

BMW N54 Catless vs High Flow Downpipes

When upgrading your N54 downpipes, you ultimately have two types to consider. Those include catless and high-flow catted N54 downpipes. The difference between them is in their name. Catless N54 downpipes remove the catalytic converters entirely, leaving an unobstructed path for exhaust gas to escape. High flow downpipes retain catalytic converters, but replace the restrictive stock cats with ones that flow much better.

Catless Downpipes

As with any turbocharged car, exhaust backpressure post-turbine, leads to a significant loss in turbo spool and power. While there are quite a few boring details surrounding the science of backpressure pre/post turbo, we won’t go into too much detail here. However, it is important to know that the relationship between backpressure before and after the turbos affects turbo spool and how the turbochargers behave.

The drop of pressure from the exhaust gas pre-turbo to post-turbo plays a key role in why a turbo is able to spool. A larger pressure drop causes the exhaust gas to accelerate quicker leading to an increase in turbo spool. In a perfect world, the best post-turbo exhaust would be no exhaust at all. Of course, for those of us daily driving our N54s on public roads, having no exhaust is not a viable option.

Catless Downpipes Performance Benefits

Catless downpipes help mitigate the downside of having an exhaust system post-turbo; they lead to a considerable decrease in backpressure and allow the turbocharger to spool faster. Additionally, as air is accelerated faster towards the turbine, boost pressure increases thereby causing an increase in power. Here are some of the reasons why I believe catless downpipes are one of the best mods for the N54-powered 135i 335i and 535i:

  • +15-25 Horsepower and Torque
  • Incredible turbo spool
  • Intoxicating Sound

I run VRSF catless downpipes on my 335i, which I am very impressed with; the quality and welds look excellent. Combine that with a lifetime warranty for fitment and defects and a price of only $305 it’s hard to pass up on them. I bought the downpipes with the goals of spool, power, and torque in mind and boy did these deliver.

The turbo spool is immediate and the instant low-end torque is unbelievable. A loud car is never something I have been interested in – I love that the 335i appears to be a sleeper on the surface. Once again, the VRSF catless downpipes deliver. While idling or cruising at any speed it is hard to notice a significant difference between stock downpipes and catless. However, at WOT or on a cold start the catless downpipes are intoxicating; the exhaust note is very aggressive without being overly intrusive.

High Flow BMW N54 Downpipes

If catless downpipes don’t sound like your cup of tea, there is another option if you are still looking to upgrade your downpipes without dealing with some of the annoyances of being catless. Most of the major manufacturers that offer catless downpipes for the N54 also offer high-flow catted downpipes as well. There are benefits and shortcomings to high flow N54 downpipes, just like there are with catless downpipes.

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BMW N54 VRSF High-Flow Downpipes

Starting with the benefits, high flow N54 downpipes are a better option if you are looking for a more tame and subdued build while still picking up a fair bit of power. High flow DPs are far less noisy and stinky than catless DPs due to the fact that they still have cats to muffle sound and collect exhaust impurities. Perhaps the biggest benefit of running high flow downpipes is that (depending on where you are located) they can allow you to pass an emissions test without having to switch back to the factory downpipes.

Moving onto the shortcomings of high flow downpipes, they do not yield the same power gains as catless DPs and are notably more expensive than catless DP options as well. While high flow downpipes make use of higher cell count catalytic converters allowing for better flow, they are still a restriction in the exhaust system. As a result, they typically yield around 5-10 horsepower less than their catless counterparts. Catalytic converters aren’t cheap either, so high flow DPs generally cost a few hundred dollars more than catless ones.

High Flow Downpipes Performance Benefits

Ultimately, high flow N54 downpipes offer very similar gains to catless downpipes and boost horsepower, torque, and improve turbo spool time in a nearly identical way. As we mentioned in the section prior, while high flow cats significantly improve flow over the stock downpipes, the cats are still a restriction, leaving some power on the table. Overall, the benefits of high flow downpipes really boils down to the fact that you’ll likely be able to pass an emissions test unlike with catless downpipes.

  • +10-20 Horsepower and Torque
  • Incredible turbo spool
  • Intoxicating Sound

Listen to Zach’s 2007 BMW 335i with BMS Dual Cones, JB4, and VRSF Downpipes

BMW N54 Upgraded Downpipes Summary

Overall, if you are looking for the best power and spool catless downpipes are the way to go. High flow catted downpipes are typically more expensive than their catless counterparts and do not offer as much of a benefit. However, if you are worried about having an exhaust system that won’t pass inspection then high flow catted downpipes are the right choice for you.

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3 Comments

  1. Jeff – Yes, so long as the JB4 is “turned on” you will not get a CEL. Do note that even though there is no CEL, you will still fail inspection unless you get a special add-on wire for the JB4 which I think is like $100 new.

    1. Thank you for your great information around N54! I can’t have such information in Japan.

      Are there any MHD Custom Flash Maps which can pass inspection with catless DP? It is OK that the flash map produce lower power. I use the map while inspection only.

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