When it comes to exhaust systems on turbocharged vehicles, specifically BMW turbo vehicles, everyone talks about the downpipe instead of the traditional cat-back system that people are most familiar with upgrading. That’s because cat-back exhaust systems and expensive and don’t really provide any power benefit. They sound great, but we’re bargain performance guys here!
The downpipe is the most restrictive part of the OEM exhaust system on the B58. And that’s why people upgrade it. Less restriction = more power!
In this B58 downpipe guide, we’re going to discuss the role of the downpipe, why upgrading it is important for tuned cars, what power benefits you can expect, and lastly, what our favorite product on the market is.
This guide is applicable for all:
2016+ M140i / xDrive
2016+ M240i / xDrive
2016+ 340i / xDrive
2016+ 440i / xDrive
2016+ 540i / xDrive
2016+ 740i / xDrive
Additionally, this guide is relevant for all Z4’s and X-series BMW’s with the B58 engine, however, the products we link to will not fit these vehicles.
Why Upgrade Your B58 Downpipe?
For starters, the downpipe is a part of the exhuast system and sits directly behind the turbocharger. The goal of the downpipe is to pass air from the turbo to the cat-back portion of the exhaust system which then sends the air out into the atmosphere. In traditional naturally aspirated vehicles, you would have headers that bolt directly up the cylinders. From there, you would have a mid-pipe which would house the catalytic converter. On the B58, the catalytic converter is housed inside of the downpipe.
Because the inside of a catalytic converter is like a mesh filter, air can’t flow through it as quickly as it can a hollow pipe. The OEM catalytic converter on the B58 downpipe is very restrictive in the sense that is creates significant air flow limitations. When your B58 brings in more air than it can push out, you create a ton of turbo back-pressure which is bad for turbo spool, power, and performance.
On a stock B58, you’re not sucking enough air in for this to be a problem. While just upgrading your downpipe will still provide a meaningful power increase, the benefit is amplified when used in conjunction with other modifications. More specifically: an upgraded air intake, and a tune.
Upgraded Downpipe + Air Intake + Tune = Maximum Performance Benefit
With zero other mods, a B58 with an upgraded downpipe will add approx. 10-15whp. This is simply from better turbo spool and more efficient turbo performance. However, the gains become amplified on cars that have other mods.
With an air intake, you are now sucking a lot more air into the car. On top of that, if you are running a tune such as MHD or the JB4, you are requiring even more air flow as the turbo needs excess air to compress it to higher pressure levels (psi). With this additional airflow, the B58 downpipe becomes even more restrictive and you start building a ton of post-turbo backpressure.
Back-Pressure and Turbochargers
I’ll keep it light on the technical detail – but – at a high level, you want a lot of air pressure before the turbo, and zero air pressure after the turbo. The higher the air pressure is before it enters the turbo, the less work the turbo has to do (more efficient). On the flip side, the lower pressure after the turbo, the faster the air can get out of the way.
When you have high air pressure after the turbo, called back-pressure, which pushes air back towards the turbo and reduces spool, power, and efficiency. Here is my most simplistic explanation:
When your turbo dumps a bunch of air into the downpipe, but the downpipe is restrictive, the air bounces back from the downpipe to the turbo. This bounce back creates resistance for the turbo compressor, which reduces spool.
B58 Catless vs. High Flow Downpipes
You have two options for upgrades: removing the catalytic converter completely, or simply upgrading the catalytic converter to something that is still legal, but can flow a higher volume of air.
Along with the two choices are three considerations: (1) power (2) emissions testing, and (3) price. Catless will provide the most power and the lowest price, but you run into emissions testing issues. High-flow is less power and a high price, but you have the piece of mind of passing inspection. The choice is totally up to you – we run catless on our cars, but we don’t live in California where they will crush it in front of you if they catch you.
|B58 Catless||B58 High-Flow|
B58 Upgraded Downpipe Performance Benefits
- 20-30whp for catless or 10-15whp for catted high-flow
- 25-35wtq for catless or 15-20wtq for catted high-flow
- Increased exhaust note, without being too loud or obnoxious
- Faster turbo spool
- Better throttle response
Our Product Recommendation: B58 VRSF Catless & High-Flow
VRSF is the best bang-for-the-buck manufacturer in the BMW community. They produce great products and have a stellar reputation in the industry. Ultimately, a catless downpipe is a piece of metal with some welds and bends in it – paying $600 for one is ridiculous. Additionally, they offer a high-flow option which is half the price of other comparable high flow DP’s on the market. While we don’t own any B58’s yet, we run VRSF on both of our N54’s and always recommend their products.