[OzBMR]

I just measured my stock downpipe and it's just over 5" diameter (132mm)

From the BMW Group University Technical Training document for the B58 Engine ST1505 p.53 it states that it is a 3-way catalytic converter with 2 monoliths of different sizes and a total volume of 2.8 liters:

• 1st monolith: 600 cells per square inch, 125 x 98 mm
• 2nd monolith: 400 cells per square inch, 125 x 130 mm

I also note on RealOEM that there are several different part numbers for 340i and a different part number for the M140i all with the B58.

I'm interested in understanding the free area calculations (flow rate) of the monoliths in the stock downpipe and comparing to some of the aftermarket options that are smaller diameter but have lower cell density.

Does anyone have any data or measurements of the monolith sizes in popular aftermarket downpipes with cats?

[Wires]

I've never seen that information. All the aftermarket companies post is "CELL count" x diameter of cat with zero reference to size of the cells or the actual flow.

They typically post a "we flow X% more than stock", but with no indication of the difference in CFM. It tends to be just a marketing number.

I'd be interesting in hearing if someone has info. What I do know is the 5" 200 CEL DP that VSRF has sounds drastically different than the 200 CEL DP from ER. Obviously a 5" 200 CEL cat needs to have a larger honeycomb size than a 4" version.

[kern417]

I wouldn't expect anyone to have that data. But CFM isn't king here - velocity is. That's why a pipe that's too big can actually hurt performance. You need something that supports the vacuum effect to pull exhaust gases out of the cylinder head and turbo.

[aadum]

Originally Posted by OzBMR

I just measured my stock downpipe and it's just over 5" diameter (132mm)

From the BMW Group University Technical Training document for the B58 Engine ST1505 p.53 it states that it is a 3-way catalytic converter with 2 monoliths of different sizes and a total volume of 2.8 liters:

• 1st monolith: 600 cells per square inch, 125 x 98 mm
• 2nd monolith: 400 cells per square inch, 125 x 130 mm

I also note on RealOEM that there are several different part numbers for 340i and a different part number for the M140i all with the B58.

I'm interested in understanding the free area calculations (flow rate) of the monoliths in the stock downpipe and comparing to some of the aftermarket options that are smaller diameter but have lower cell density.

Does anyone have any data or measurements of the monolith sizes in popular aftermarket downpipes with cats?

so if you remove 600 one...you have original high flow cat

[JD11937]

Not sure if this helps at all. Can be found in higher resolution on the Wagner website, provides airflow readings.

[Skyhigh]

Originally Posted by kern417

I wouldn't expect anyone to have that data. But CFM isn't king here - velocity is. That's why a pipe that's too big can actually hurt performance. You need something that supports the vacuum effect to pull exhaust gases out of the cylinder head and turbo.

That's only true for naturally aspirated engines. They benefit from the "pull" effect from the velocity of the exhaust gases for optimised performance. Turbo engines don't! The less resistance, the better.

[J3Vr6]

Originally Posted by Skyhigh

That's only true for naturally aspirated engines. They benefit from the "pull" effect from the velocity of the exhaust gases for optimised performance. Turbo engines don't! The less resistance, the better.

Arin @ APR used to say this all the time for VW. I've also heard from other people. I'm sure there is a sweet spot

[weehe126]

To complicate things more, having a larger DP than turbo outlet and rest of the exhaust is actually supposed to help with responsiveness specifically due to the slight turbulence from the diameter change.

[Skyhigh]

Originally Posted by J3Vr6

Arin @ APR used to say this all the time for VW. I've also heard from other people. I'm sure there is a sweet spot

That is what they teach engineers.
No matter who made the engine, it is the physics of it.

[Wires]

The reason you want the exhaust to pull/draft is it improves scavenging from the engine (sucks air out so you get a free "boost" of air sucking in).

I don't know how that volumetric efficiency works on a turbo engine, but I suspect there is probably a lot less gain (none?) in a turbo car since you have the turbo in the exhaust path.

Gut feeling is I'd lean to what Skyhigh is saying since restriction will effect turbo spool time. Now that being said, having a 4" DP that goes to 5" and back to 4" might introduce pressure reflections which might cause more hindrance than help. Pure conjecture here though....

[OzBMR]

Originally Posted by aadum

so if you remove 600 one...you have original high flow cat

Interesting idea, although the second O2 sensor is between the two monoliths?

[kern417]

Originally Posted by Skyhigh

That's only true for naturally aspirated engines. They benefit from the "pull" effect from the velocity of the exhaust gases for optimised performance. Turbo engines don't! The less resistance, the better.

It is true for all engines. That's why a twin scroll turbo is more efficient than single scroll, and tubular manifolds are better than log manifolds.

And vacuum is not resistance. You will lose power by putting an exhaust on the car that's too big. You want a straight through design that minimizes diameter changes and bends. But it's directly proportional to your power output, exhaust housing, etc. Everything has an optimized size for your power range.

[Skyhigh]

On contrary - twin scroll turbo is more efficient as it prevents pressure interference between the cylinders (facilitates easy way out).

Recommend some easy educational material:
https://x-engineer.org/automotive-en...turbochargers/

This one is very simply written, pay special attention to the very last paragraph:
https://vpexhaust.com/exhaust-back-pressure-a-myth/

A turbo-charged engine, unlike a naturally aspirated engine, does not benefit from the scavenging effect created by the gas velocity as it does from reduced back-pressure. This said, it is of course true that it would not benefit from suboptimal gas extraction either. As a general rule - the less resistant the exhaust system (for a turbo-charged engine) - the better.

[kern417]

Originally Posted by Skyhigh

A turbo-charged engine, unlike a naturally aspirated engine, does not benefit from the scavenging effect created by the gas velocity as it does from reduced back-pressure. This said, it is of course true that it would not benefit from suboptimal gas extraction either. As a general rule - the less resistant the exhaust system (for a turbo-charged engine) - the better.

absolutely not true lol. No matter what is attached to it, the engine is what makes power. That power and efficiency is always based on the same principles. The exhaust's only job is to evacuate air from the cylinder. if you don't optimize that, you are sacrificing power. You want an exhaust that's big enough but you begin to approach a region of diminishing returns. I.e. if the whole exhaust was the diameter of the OEM downpipe, don't expect to gain power.

And you keep saying it needs less resistance/less back pressure. That's what the vacuum effect is doing. If you don't focus on the vacuum effect, then you don't fully get rid of the burnt fuel in the combustion chamber. That will reduce efficiency, which then effects power at the highest RPM (where the maximum power can be made). A larger pipe isn't going to compensate for a poorly flowing exhaust system. That's why it's called volumetric efficiency.

Since we're posting links, here's a good one comparing a good vs poor flowing manifold. It doesn't just reduce interference. It actually sucks out more air from the adjacent runners. This also needs to be appropriately sized for your power, along with the turbo exhaust housing, wastegate, etc. Otherwise you can literally put a turbo on your car that is capable of XXXXHP, but your engine won't even be able to spool it up.

You need to read up a lot on how this works. There will always be conflicting opinions, but facts are facts. Every time you talk about intake and exhaust modifications, you miss the mark.

[B50AteYourRide]

There is the header (before the turbine) and the exhaust (after the turbine). I believe the discussion was about the exhaust.

The turbine is your primary source of backpressure. After the turbine, I believe the best exhaust is no exhaust, but that’s just not practical. Not debating the merits of an equal length header, that’s another topic.

[Skyhigh]

kern417, you continue to make one crucial mistake in your analysis. You keep forgetting that there is a spinning turbo charger between the engine and the exhaust system, equipped with a blow-off / bypass valve! Think about that when considering the effect of vacuum on the engine and you will realise it is practically 0.

[Skyhigh]

Originally Posted by B50AteYourRide

There is the header (before the turbine) and the exhaust (after the turbine). I believe the discussion was about the exhaust.

The turbine is your primary source of backpressure. After the turbine, I believe the best exhaust is no exhaust, but that’s just not practical. Not debating the merits of an equal length header, that’s another topic.

All very well said. Exactly.

[johnung]

Originally Posted by Skyhigh

On contrary - twin scroll turbo is more efficient as it prevents pressure interference between the cylinders (facilitates easy way out).

Recommend some easy educational material:
https://x-engineer.org/automotive-en...turbochargers/

This one is very simply written, pay special attention to the very last paragraph:
https://vpexhaust.com/exhaust-back-pressure-a-myth/

A turbo-charged engine, unlike a naturally aspirated engine, does not benefit from the scavenging effect created by the gas velocity as it does from reduced back-pressure. This said, it is of course true that it would not benefit from suboptimal gas extraction either. As a general rule - the less resistant the exhaust system (for a turbo-charged engine) - the better.

I'm probably over simplifying, but less exhaust back pressure means that the turbo can spin faster to create more boost.

I enjoy reading engineering conversations and competing hypotheses about which factor carries the most or least weight. But I also enjoy a well designed practical test in the end that proves which one is correct.

I had a great experience when I spent some time at Fabspeed. Great engineering and fabrication house that work on really exotic cars. Their welders are artisans. I love the fact that they have an awd dyno in the middle of their shop. Every project goes through the engineering and design phases, the mockups and the build. But in the end every one of them is dyno tested to know the absolute result versus the baseline before the project began.

They have floor to ceiling shelves containing the 3D steel templates that they weld up to be able to duplicate any project that they have previously completed. It's fun to go to a Cars & Coffee there on a Saturday morning because you never know what customer car will show up. Lamborghini, Porsche, McLaren, Ferrari, Vettes, Mustangs, BMWs, etc.

- - - - - -

Oh, since there appears to be engineering types on this thread, some may appreciate this science story about doing a practical final test...

It was 1980-something and I was a low level scientist at a big pharmaceutical company's Research & Development facility in Philadelphia, a five story building with many brilliant PhD's. Every lab room had a big hood that you could perform experiments safely that might be toxic, flammable, stinky. It had a think glass window on the front that slid up or down. Your hands go under to work but you could never smell anything. There were big fans on the roof that sucked the air up and out of the room. Constant positive airflow. If you pulled the front window down to a 4" opening to just get your arms and hands underneath they airflow may have been like 80mph. Big time suction!

One day one of my coworkers down the hall was working with something in the mercaptan class of compounds. They really stink. Natural gas has no smell so they add tiny parts per billion amount of a mercaptan to it to give it an odor to be able to smell a gas leak. So this guy managed to spill literally a few drops outside of his hood. The smell was so bad that we had to evacuate the entire floor for three hours until the smell dissipated. People from other floors were reporting the smell too.

Some months later, the R&D administration called a big meeting in the auditorium to dispel rumors. Word had gotten out that on one of the floors in a particular lab, they were planning to start research using the live AIDS virus. Scientists were concerned and getting panicky to be in proximity to something so deadly with no treatment or cure at the time. The company hired this third party company to come in with specialized air flow testing equipment that proclaimed the labs perfectly safe. Companies frequently hire third party consultants to cover their asses and have someone to blame if something goes wrong. And those companies are willing to take on that role when paid a big fee.

So the scientists from this third party consulting company got up in front of this auditorium full of really smart pharmaceutical scientists and went through their testing methodology in great detail. They anticipated every question and had a satisfying answer. There was more airflow theory that day than I ever want to hear again. As the meeting seemed to be coming to a close, I found it strange that all of these scientists seemed like they were going to be satisfied with a lot of theoreticals, a lot of assumptions and no practical testing.

Being in my twenties and still very naive, I put my hand up and asked a question.

"Now that all of the instrumental testing is complete, why not do a practical test? Why not spill some mercaptan in the AIDS testing lab and see if we can smell it any where else in the building?"

I thought it was a practical proposal and said it like I would to a professor in one of my college classes. I wasn't anticipating the reaction that I got.

Well, all of the scientists on stage from the third party company began to gag. Some of them appeared to try to speak, but nothing came out. My coworkers in the audience looked at each other and murmured. I never knew exactly what "murmur" meant, but that was a murmur. These were all older, very well educated accomplished scientists who had published hundreds of scientific papers. This went on for a long time until the head of our department jumped up on stage to grab the microphone. He never answered my question. He just seemed to babble and repeat some of the talking points/justifications that the third party company had presented. Then he adjourned the meeting. It was like watching many of our politicians today try to spin stuff that just can't be spun, unless the audience is really stupid or will accept it for some reason.

Back in our lab I was confused as to why everyone wouldn't just want to run such a simple test. Scientists like to run tests. Running tests is what they do for fun. They live to run tests. Like on The Big Bang Theory! This was like offering ice cream to an audience full of people and everyone immediately turning it down. No one turns down ice cream! This behavior was extremely unusual.

Remember I was still young wide-eyed, wet behind the ears and naive as to the ways of the world. Back in my own lab I attempted to ask my bosses why they wouldn't just run such a simple practical test. They kinda shrugged me off and changed the subject. For months I secretly wished that someone working in that AIDS lab would just spill some mercaptan to give my practical test a try. But as far as I know, no one ever did.

Now I understand that those scientists jobs were on the line if they had publicly expressed any opinion in favor of the mercaptan test. They knew that the company higher ups had made a big money decision to do AIDS research and anyone questioning anything, even the safety of the scientists working in the lab, would be viewed as someone who was disloyal. Someone to be gotten rid of.

It was kind of amazing to me looking back how quickly they all knew in that auditorium to keep quiet. I couldn't understand why anyone would take on the potential risk of death when the answer was such a simple test. Instead they all just kept quiet and hoped that everything would be okay.

Again, it reminds me of politicians today, defying their own common sense to keep quiet, to just go along with what's being dictated to them, and to just hope that everything will be okay. We could have just elected pet rocks. Sorry, I didn't start out with this ending to my story in mind, it just seemed an obvious parallel.

[kern417]

Originally Posted by B50AteYourRide

There is the header (before the turbine) and the exhaust (after the turbine). I believe the discussion was about the exhaust.

The turbine is your primary source of backpressure. After the turbine, I believe the best exhaust is no exhaust, but that’s just not practical. Not debating the merits of an equal length header, that’s another topic.

Every piece of the system has an effect. I just use illustrations with the manifold because he said that scavenging isn't important for turbo applications. Everything has to do with exhaust gas velocity, including the downpipe. It's not just an open dump for exhaust gases. It's a bottle neck that needs to be optimized for your goals.

It's exactly why A/R for the turbo is important. The actual displacement of the turbine defines your spool characteristics. If it's too big, you get peaky response and a crappy powerband. If it's too small, you leave power on the table. Your entire exhaust from cylinder head to catback should be optimized in sizing if you want maximum efficiency. This will maximize the exhaust speed through the turbo to give you the powerband you want. And you'll notice that you can get various A/R housings with the exact same downpipe sizing. Because more power does not mean you need a larger downpipe. If anything, you don't want to increase the size until it's absolutely necessary.

That's why I think it's funny that people are hellbent on running 5" downpipes on their sub-400whp bmw. I know people running north of 600whp on big turbo kits with 3" downpipes. That's how Garrett, Precision, etc. designs their systems. And they are the turbo gurus.

[Skyhigh]

Originally Posted by johnung

But I also enjoy a well designed practical test in the end that proves which one is correct.

I had a great experience when I spent some time at Fabspeed. Great engineering and fabrication house that work on really exotic cars. Their welders are artisans. I love the fact that they have an awd dyno in the middle of their shop. Every project goes through the engineering and design phases, the mockups and the build. But in the end every one of them is dyno tested to know the absolute result versus the baseline before the project began.

100% true. There is only one way to prove theory works - implement it and collect empirical data.
I feel lucky to have observed various bench-tests of airplane engines, which include not only jet engines, but also various petrol engines. And for the fun of it, also a couple of automotive NA and turbo engines (mainly Honda).
Surprisingly (or not) the turbo engine performance drops as soon as one attaches an exhaust system.

Theory is only as good as the empirical data supporting it.

[johnung]

Originally Posted by Skyhigh

100% true. There is only one way to prove theory works - implement it and collect empirical data.
I feel lucky to have observed various bench-tests of airplane engines, which include not only jet engines, but also various petrol engines. And for the fun of it, also a couple of automotive NA and turbo engines (mainly Honda).
Surprisingly (or not) the turbo engine performance drops as soon as one attaches (any) exhaust system.

Theory is only as good as the empirical data supporting it.

Wow, that's cool. One of my Motörhead high school friends was the kind of guy who started by putting big lawnmower engines on go-cart frames, and a few years later tried to jam a big Chevy V8 into a Chevy Vega!

He ended up being an R&D engineer for one of the two big US jet engine manufacturers and racing his Porsche on weekends. Told me that he went for a whole day of interviews with the engineering staff and lunchtime talk turned to cars. When they heard about his racing Porsche they pointed to the engineering parking lot where many would cover their cars before they walked into the building for work. HaHa, they told him he got the job and was going to fit right in!

My buddy took me to his test site once to watch them bench test a new engine design. The engine was in a special stand in a concrete bunker like building with an open wall for the engine flames to fire out. Pretty amazing!

[Skyhigh]

Our engine bench was underground, in a populated area of the city. I was told they were lucky to have received allowance for it years ago. Today it would not have been possible due to noise regulations.

They were however mandated to put several layers of huge noise absorbent panels (see below). That alone was impressive. Height is about 1,8 meters from memory!


And here the "tiny" exhaust You can see the engine at the very end. I guess they did not consider vacuum and scavenging to add value



Btw. your story reminded me of this

Sorry for the OT.