Exhaust theory on N54 & turbos in general

gmagnus7

Corporal
Dec 3, 2018
133
I wanted to talk exhaust theory and get some opinions from people:
I've been reading a lot on these forums and it seems that most people are content with the stock exhaust (modified or not) or go with single 3" systems. This kind of bothered/confused me because I always read that the lower the back pressure post turbo the better. It just made no sense considering all the effort and money people are willing to put into their cars but left one critical piece out. This is a piece taken from an article i read:

"Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure."

The stock N54 downpipes are super restrictive and I'm not even going to talk about them. However the aftermarket catless downpipes are mostly all 3" necking to 2.5". Companies claim that having the full 3" available all the way to the 2.5" flange is beneficial to the engine for increased power. If we do some math and calculate the cross sectional area of each exhaust size it really doesn't make sense to keep the stock exhaust at all. The 335i has a dual 2.36" exhaust (I think), and the 135i has the single 2.75" for reference.

Dual 3" piping has a csa (cross sectional area) of 6.5" squared and with two pipes it would be 13" squared
Dual 2.5" piping has a csa of 4.4" squared and with two pipes it would be 8.9" squared (rounded)
A single 2.75" would have a csa of 5.4" squared
A single 3" would have a csa of 6.5" squared
A single 3.5" would have a csa of 9.6" squared
A single 4" would have a csa of 12.6" squared

Keep in mind that a larger single exhaust would have less boundary flow than two smaller pipes and would flow more for an equivalent csa dual setup, theoretically. So two things that pop out to me are 1) that 3" downpipes really accomplish nothing aside from just deleting the cats for a stock exhaust systems and 2) to match the flow of dual 3" downpipes you would need a full dual 3" system (obviously) or a single 4" system. This would be the case for a 135i which is limited to a single exhaust exit.

So I guess my question is has anybody tried a full dual 3" or single 4" system? If so have they noticed reduced WGDC in any logs, or better performance with the butt dyno? Does it really even matter or would it just make a couple extra hp and be mostly insignificant? What are your thoughts?
 
Last edited:

matreyia

Major
Apr 19, 2017
1,642
I wanted to talk exhaust theory and get some opinions from people:
I've been reading a lot on these forums and it seems that most people are content with the stock exhaust (modified or not) or go with single 3" systems. This kind of bothered/confused me because I always read that the lower the back pressure post turbo the better. It just made no sense considering all the effort and money people are willing to put into their cars but left one critical piece out. This is a piece taken from an article i read:

"Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure."

The stock N54 downpipes are super restrictive and I'm not even going to talk about them. However the aftermarket catless downpipes are mostly all 3" necking to 2.5". Companies claim that having the full 3" available all the way to the 2.5" flange is beneficial to the engine for increased power. If we do some math and calculate the cross sectional area of each exhaust size it really doesn't make sense to keep the stock exhaust at all. The 335i has a dual 2.36" exhaust (I think), and the 135i has the single 2.75" for reference.

Dual 3" piping has a csa (cross sectional area) of 6.5" squared and with two pipes it would be 13" squared
Dual 2.5" piping has a csa of 4.4" squared and with two pipes it would be 8.9" squared (rounded)
A single 2.75" would have a csa of 5.4" squared
A single 3" would have a csa of 6.5" squared
A single 4" would have a csa of 12.6" squared

Keep in mind that a larger single exhaust would have less boundary flow than two smaller pipes and would flow more for an equivalent csa dual setup, theoretically. So two things that pop out to me are 1) that 3" downpipes really accomplish nothing aside from just deleting the cats for a stock exhaust systems and 2) to match the flow of dual 3" downpipes you would need a full dual 3" system (obviously) or a single 4" system. This would be the case for a 135i which is limited to a single exhaust exit.

So I guess my question is has anybody tried a full dual 3" or single 4" system? If so have they noticed reduced WGDC in any logs, or better performance with the butt dyno? Does it really even matter or would it just make a couple extra hp and be mostly insignificant? What are your thoughts?
Adding more chees is not necessarily the best solution. The bigger the diameter the less velocity of exhaust. The more restrictive the diameter the faster the velocity of exhaust. The aim is to find the proper diameter that allows for good exhaust velocity that produces less back pressure but still creates a vacuum behind it which will help you engine intake valves suck in more air to burn with the fuel.

If biggest is best, then people would delete exhaust systems all together and just let the exhaust spit directly out of the engine. Finally, the proportions involve engine rpm calculations which will reveal proper diameter and length of pipes.
 

gmagnus7

Corporal
Dec 3, 2018
133
Adding more chees is not necessarily the best solution. The bigger the diameter the less velocity of exhaust. The more restrictive the diameter the faster the velocity of exhaust. The aim is to find the proper diameter that allows for good exhaust velocity that produces less back pressure but still creates a vacuum behind it which will help you engine intake valves suck in more air to burn with the fuel.

If biggest is best, then people would delete exhaust systems all together and just let the exhaust spit directly out of the engine. Finally, the proportions involve engine rpm calculations which will reveal proper diameter and length of pipes.
I thought that only applied for natural aspirated engines, along with scavenging theory? That's why big turbo competition cars have exhaust pipes just sticking out the sides or the hood.
 
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matreyia

Major
Apr 19, 2017
1,642
I thought that only applied for natural aspirated engines, along with scavenging theory? That's why big turbo competition cars have exhaust pipes just sticking out the sides or the hood.
I went to read about it and found a few articles and I was under the impression that even turbo cars did that to a certain degree.
 

The Convert

Captain
Jun 4, 2017
1,334
I went to read about it and found a few articles and I was under the impression that even turbo cars did that to a certain degree.
Wrong. Like op said, straight race cars go straight out of the hood. You can improve low end torque on turbo motors with exhaust backpressure though.
 

Mikejones1208

Corporal
Dec 6, 2016
109
Savannah, GA
Everything you said is true, but with stock twins its pointless to go that big due to how small they are, very little power to gain out of them past downpipes. People have gone 4" all the way out on single setups many tmes and it does allow for a nicely flowing system, and theyre really isnt enough space under these car to allow for dual 3" without clanking against the car or ground
 

rac

Corporal
Nov 14, 2016
164
Australia
It is the velocity that is helping system inertia during transient changes in airflow. If you operate in narrow band of rpm all of the time or doing steady state dyno work you shouldn't notice the affect and maximising the differential across the turbine is always good. However, even although airflow inertia is less applicable post turbo than an in an N/A scenario, it can still have an affect on transient response of the whole system which is why there is the thought process that back pressure helps, which is just a function of of the pipe diameter that is increasing velocity. If that back pressure is there because of sharp bends then it's going to be bad thing all round.
 

KevinC39

Corporal
Jun 27, 2017
201
I have a Black Market Racing 3.5" exhaust but my god is it loud. I can't tell you about gains or anything though on my tiny stock turbo N55.
 

matreyia

Major
Apr 19, 2017
1,642
I have a Black Market Racing 3.5" exhaust but my god is it loud. I can't tell you about gains or anything though on my tiny stock turbo N55.
I am not a fan of loud exhausts without reason. To me if a car be loud, it should be because of an actual technical reason related to performance. If not, then it's just for show and for me, that is a bit embarrassing to drive a loud car that has nothing to back up the sound. Just my 2¢.

You should see those Mustang 5.0s and Shelbys... super loud and super confused when I pass them with my girly convertible.
 

KevinC39

Corporal
Jun 27, 2017
201
I agree. I've watched every possible exhaust video on YouTube before buying and this one didn't seem like it was going to be all that loud but you never know until you hear it person. Downsides to buying stuff online I guess. I put about 30 miles on it and took it right back off. I couldn't deal with it.

I know all about the straight piped mustang guys. There's one in my neighborhood and it's just obnoxious.
 

gmagnus7

Corporal
Dec 3, 2018
133
Yea a single 3.5" or 4" would definitely be loud, but these cars are pretty muted from the factory with their tiny turbos. Also @Erichale77 that exhaust system looks super sexy and I'm surprised you actually fit a full 4" single. Hows the ground clearance, it looks like it would be totally fine?
 
I helped a mate custom build a single 4 inch system on a mates single turbo kit from 4 inch dump all the way through with a 4 inch custom cat and muffler to a modified eisenmann exhaust and its a lot quieter than my akra evolution system by like a lot even when the bypass flap is open

I was quite shocked how quiet the system is, mind you it was a lot more to build than an akra eveolution system complete with down pipes and sound kit.

Ground clearance is not really an issue due to being custom and hand built to maximise it.
 

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iminhell1

Sergeant
Jun 17, 2018
252
Put a shop vac on the exhaust, rev, remove.

Now let me know if you want more vacuum in the exhaust, after all the oil smoke clears. LOL
 

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