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 | |
| WHP Gains | 20-30whp | 10-15whp |
| WTQ Gains | 25-35wtq | 15-20wtq |
| Restriction | Least | More |
| Pass Emissions? | No | Yes |
| Price | $300 | $479 |
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.
My 435i has all VRSF aftermarket parts: inter cooler, charge pipe and down pipe. All are working fine and the price was great!
Thanks for the informative article. Lots of good points here. One thing I’d have to disagree with is the point on the exhuast note. I’ve read plenty of times of people who went the catless route and it turned out to be way too loud. Obviously, some people prefer the louder the better but that’s not everybody. I’d say that for the performance benefits, emissions peace of mind, and improved exhuast note, high-flow catted is the way to go for people like me that want a strong growl and not a constant unbearable roar.
Hi David,
We appreciate the feedback. Interesting – I guess each to their own. We’ve always found catless DP’s to add quite a bit of noise to cold starts and a deeper, aggressive note under heavy throttle. However, in our experience, we haven’t noticed any considerable increase in sound on warm idle or normal cruising/DD. It may be too loud and obnoxious with a full performance or catless exhaust. Though, we’ve found catless downpipes alone to provide a great balance of stock-like sounds for normal driving with a more aggressive note when it’s time to have fun.
Best Regards,
BMW Tuning
hey there i’ve been looking around because im unsure wether you need a tune with a catless downpipe on a b58. i assumed you would but haven’t found confirmation, if you could help thatd be great
thanks
Some fantastic info here! …Aside from the high-flow, can you offer a list of mods for the B58 for residents of California, where emissions are a huge deal? Thanks!
Great summary! Thank you for publishing that.
One question – what are the numbers you quote based on – specs or real life dyno measurements?
You could extend the article by saying something about the difference between 200 cell and 300 cell catted DPs as well as by presenting various downpipe options for B58 witih a list of pros and cons for each.
How would the backpressure effect overall performance? I have a jb4 on map 2 and a agency power intake with the stock cat.
Quick question. With a catless down pipe. Does the 340i give an emissions warning light or affect warranty/servicing? As I’m thinking of just the air pod and down pipe for now and love noise and bang for bucks.
Hi Jimmy,
Yes, a catless downpipe will throw a SES light with the associated engine fault codes indicating an emissions problem. This light can be removed with tunes that essentially just hide the SES and engine code. It can affect servicing under warranty. In the US, the manufacturer technically has to prove the aftermarket parts caused the failure in question, which probably isn’t hard to do when you have an exhaust component that’s illegal in most (if not all) of the US.
Best Regards,
BMW Tuning
Thanks team, do you know if the high flow cat might keep the warning light away?
Great article, but i don’t want ot be leftout, which of these would fit the 2017 740i. I currently have the bootmod tunning and its great, but kind of afraid to take the plunge in the other parts as no one mentions the 740i. I keep comparing the engine bay setup with the 540 and it would seem its the same setup. Apologies for my past post without the spell check.