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?

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?

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