why x pipe?

[p1nk50ck]

anyone know why we even bother with an x pipe? wouldn't it be more efficient just to have two pipes going straight back?

are there any setups that completely eliminate the x pipe? any idea what that would do to sound? i don't really come from a BMW background, so i don't know if this has been covered before in the e46 world or not.

[ZStig]

it think it combines the sound waves and exhaust pulses from both sides, thereby making the exhaust sound much better. I think it also cancels out some bad frequencies. The stock one also has a resonator built in.

Changing to a free flowing x pipe often increases rasp, if you have other exhaust mods like headers, it can make the rasp terrible and unbearable

[pokeybritches]

Yes, there is a reason (two actually). One is for performance, the other is for sound.

Every time a cylinder fires, it generates an exhaust pulse. Each pulse has a high pressure "head", "body", and low pressure "tail". When an exhaust pulse is generated, the previous one actually helps suck the air through the engine and pull along the next pulse. If you have a crossover, all six cylinders collectively help pull the air from each other. With no crossover, only three are working in harmony. The effects are fully realized when you have equal length piping (thus the need for equal length headers), because the pulses need to be timed to reach the crossover point in the same amount of travel time.

The second reason is for sound. The first Vipers that came out sounded like two 5-cylinder engines, because they had no crossover in the exhaust. Everyone who has heard my Supersprint exhaust on my non-M has commented on how the car sounds "like it sounds like it has 700 hp," when it actually has maybe half that. The reason is that car has a Y-pipe, and the exhaust note has much more harmony than before (where it was two pipes straight back).

By the way, I just moved to Alexandria. I'm sad I'm going to have to get all season tires now

[p1nk50ck]

some interesting theories here. i never thought about sound cancellation, but that makes sense. if the pulses are opposite of each other, then you get a smoother exhaust note. if there is no crossover, it'll just sound like dual high revving 3 cylinder motors, which will probably sound like total crap :-/

nice! welcome to the area! if this winter is anything like the last few winters, we had maybe 3 total days where there was no more than 2 inches of snow on the ground, and by the next morning it was slush. every 5 years or so, it dumps like 6-10 inches and we celebrate cause everything shuts down, and the grocery stores are raided of milk, bread, and bottled water. hopefully i'll get to meet you at one of these meets. we have early saturday morning cars n' coffee meets in great falls VA (about 15 mins from you), and thursday nights in ffx there are meets as well.

[RADRACR]

If it's better, why is it not OEM in most cases? Especially a performance vehicle.

[The HACK]

Originally Posted by aestheticstorm

If it's better, why is it not OEM in most cases? Especially a performance vehicle.

Most of the time it's probably due to cost. When my old company was working with GM to produce a part for the Camaro, they were parsing the cost of the part down to a single plastic washer we were using. They wanted us to use a cheaper material that would save them about 1/2 a penny per unit. The washer ended up shearing on our tester before it hit 100,000 operating cycles, so we told them NO.

Ended up scrapping a $100,000 order because they then said "f**k it, design us something where we don't have to use said washer AND can survive 100,000 cycles."

I suspect all major automotive manufacturers work like this. Unless it's for a small run type vehicle, like say a super car like a Ford GT. We ran into a similar case where Mobis (umbrella company for Hyundai and Kia) also scrapped a huge potential order because they deemed a difference in $1.00 per unit is too much. After we've already spent tens of thousands on R&D and prototyping to get a part ready.

So imagine if adding an X pipe cost say, $0.50 more, but then you and add $0.05 worth per unit of glass fiber in the muffler to take out the drone? Guess which solution will they use?

[RADRACR]

Perhaps. But it seems like good ROI equipment from this thread. It's just pipes. Wonder how much that extra ///M badge on the motor costs. oh well.

[Westersund]

I'm not sure if I believe in X-pipes. The Pagani Huayra doesn't have a X-pipe, if it would produce better sound and performance the Huayra would have it. I saw show on Discovery Channel about the development of the Huayra and Horacio Paganis attention to detail and approach to performance (enhancing) is mind blowing. So if X-pipes was the shit the Huayra would have it.

[Nate4641]

The Huayra is a good reference, although the engine is bigger and things may change due to that. I would be interested to see what the LFA exhaust set up looks like considering it was acustically tuned by Yahama.

[Gallardo Rosso]

The engineers who work on these cars don't have free reign when it comes to design. There are many federal and corporate rules they must abide by.

In the U.S. All cars need to pass what is called a drive by noise test where the car must meet a very specific DB requirement to qualify for sale. Often, there is one tone out of thousands (including tire, transmission, intake and wind noises) that exceeds the limit and is usually the exhaust is the easiest to fix. "Quick, weld a resonator on the crossover pipe before we get fired! We're behind schedule and over budget." This is just one obstacle to market a car.

As for X-pipes making power, it's all relative. Everything pokey britches said is correct about the physics in the system. It goes for the intake as well. But they also vary with RPM and load. Variable cam timing also complicated things by changing variables continuously. One feature like an x-pipe or step in a header may significantly increase power at 8000 RPM but cost you power at 5000 RPM. In order to get a robust power band you need to spread these power increases around. Or you'll end up with a motor that's peaky or gutless up top, it may even stumble at low revs if it's tuned for excessively high rpms. And if it's a production car, it might lose 5mpg off the window sticker because odd tuning. There aren't always right answers in engine design but there are certain to be compromises.

The other aspect is also marketing. Engineers are often shown a picture of the dyno graph before they design the car. The product plan often requires "sandbagging" so they can add some power to a mid-model refresh or not eat into sales of another car.

[Westersund]

The purple parts of the Huayra exhaust system are made to INCREASE the "noise" (read music) because of the enginge being turbo charged. The turbo(s) decrease engine sound, that's why they wanted to increase it. A sound fit for a super car.

[Westersund]

Originally Posted by Nate4641

The Huayra is a good reference, although the engine is bigger and things may change due to that. I would be interested to see what the LFA exhaust set up looks like considering it was acustically tuned by Yahama.

Lexus LFA exhaust system, no X-pipe:


Ferrari F1 exhaust system, no x-pipe (I know, it can hardly be called an exhaust system):

[itdnwiwbp]

Originally Posted by Westersund

This is beautiful. If I didn't miss my car so much when I'm away at work I think this would be my new desktop.

Also, thoroughly enjoying this discussion since it's the last part of my exhaust that hasn't been addressed.

[Westersund]

Originally Posted by itdnwiwbp

This is beautiful. If I didn't miss my car so much when I'm away at work I think this would be my new desktop.

Also, thoroughly enjoying this discussion since it's the last part of my exhaust that hasn't been addressed.

That exhaust is awesome, real master piece. Full titanium exhaust and the purple resonating chambers are formed by pressurizing the titanium to 1500 bar.

[pokeybritches]

The Pagani is a V12 twin turbo, which has totally different characteristics from an NA straight six. I haven't analyzed the exhaust, but I would expect half the engine to feed one turbo, and half to feed the other turbo. If this is the case, you already have six cylinders merging to a single point - a turbo. It doesn't matter if it's in the shape of an X, Y, whatever... so long as they merge.

Secondly, how does a turbo affect exhaust pulses? Do they dissipate at the turbine? I would imagine their effect would significantly decrease after passing through the turbine housing, so scavenging wouldn't play as much of a role.

Third, a V12 at 6000 rpm has far more cylinders firing than an inline six at 8000 rpm. There is a point of diminshing returns. As Gallardo Rosso said, there are many variables, one being variable cam timing. Twelve cylinders firing at varying intervals may cause too many exhaust pulses to collide or negatively interfere with each other, and an additional crossover not worth it. Again, we already established that six cylinders merge at the turbos.

Lastly, the Z4M has long piping due to the front engine design. The Pagani is mid-engined, with less routing. The additional weight may not make it worth it, even if there were performance benefits.

[Westersund]

Originally Posted by pokeybritches

The Pagani is a V12 twin turbo, which has totally different characteristics from an NA straight six. I haven't analyzed the exhaust, but I would expect half the engine to feed one turbo, and half to feed the other turbo. If this is the case, you already have six cylinders merging to a single point - a turbo. It doesn't matter if it's in the shape of an X, Y, whatever... so long as they merge.

Secondly, how does a turbo affect exhaust pulses? Do they dissipate at the turbine? I would imagine their effect would significantly decrease after passing through the turbine housing, so scavenging wouldn't play as much of a role.

Third, a V12 at 6000 rpm has far more cylinders firing than an inline six at 8000 rpm. There is a point of diminshing returns. As Gallardo Rosso said, there are many variables, one being variable cam timing. Twelve cylinders firing at varying intervals may cause too many exhaust pulses to collide or negatively interfere with each other, and an additional crossover not worth it. Again, we already established that six cylinders merge at the turbos.

Lastly, the Z4M has long piping due to the front engine design. The Pagani is mid-engined, with less routing. The additional weight may not make it worth it, even if there were performance benefits.

Firstly, in your previous post you wrote that exhaust pulses help suck each other out of the engine. Why would there be a difference between an inline six and a V12?
And if you're saying that the only thing needed to gain this effect is a merging at some point, doesn't that mean that the Z4M already has this? As the headers are a 2 part configuration, but each part takes care of 3 cylinders each. So 3 cylinders help eachother out to pull the pulses out of the engine.
So if a merge at some point of the headers/exhaust is all it takes, a X-pipe is redundant on most cars.

Secondly, the exhaust pulses hit the turbine and the pulses are diminished. So what you're saying makes sense.

Thirdly, there are other super cars with V12 that doesn't apply the use of a X-pipe. Mclaren F1 is one. Don't get stuck on the Pagani turbo setup.

Lastly, if a X-pipe would help enhance the sound (and performance) of the engine I don't think Horacio would mind adding a couple of pounds worth of titanium to achieve that. Or Mclaren for that matter. Mostly I think X-pipes are a gimmick and the companies that sell them make some extra cash on them. So if you already have equal length headers with a collector there's no point in an X-pipe.

Lambo V12:


Mclaren (BMW) V12:

[pokeybritches]

Tell me, why do you think the Ferrari F1 exhaust you posted is shaped like it is? I'll give you a hint, and it has to do with equal length piping. If it wasn't about exhaust pulses, then what is it about?

I used the Pagani, because that's the example you used. I'm not going to go through each individual exhaust setup and attempt to decipher what the engineers were thinking. I do know the physics behind it, which I explained to you. X-pipes are not gimmicks, and they do have an impact on sound and performance. Going in with the mindset that they are gimmicks, and then trying to find data to prove it while ignoring other relevant data that disproves your theory, is an academic crime in engineering. It works in liberal arts, but not in science.

On the Z4M, three cylinders merging does not equal six cylinders merging. Just like six cylinders merging does not equal twelve cylinders merging. Different applications. Twelve cylinders firing creates a vastly different scenario than six, with many more pulses per second that can interfere with each other rather than help each other. Think of a revolving door- with too few people, each person has to push his way through to the other side, slowing progress. With the right number of people, the door continues to revolve with minimal effort from each person passing through. Too many people getting to the door at the same time will cause a backup.

My 130k mile, 10+ year old, non-M Z4 running 6 psi (non-intercooled) on a centrifugal blower and rich tune puts down more rwhp than a stock Z4M, and it's because of the exhaust work.

[GuidoK]

That lambo eninge has an X pipe, or better said: an Y pipe (1 per cilinder bank), which is basically the same (but used when converting to a 1 muffler setup).

That mclaren does not (or more specific: we do not know what happens in the backbox).

As long as there is a construction that makes the venturi effect possible, (exhaust) scavaging can take place. But it all depends on the harmonics and speed of the flow in the system. So any form of venturi induced scavaging can either result in a lower pressure on the exhaust port or a higher pressure on the exhaust port; depending on rpm, exhaust lenght, exhaust diameter and cilinder size.
An x (or y)pipe is nothing more or less than an extension of how a header works. Why you want the piping in a header to be as equally of lenght is for the same reasons.

Has anybody tried to open the middle resonator in the z4m exhaust? I mean the small box behind the x pipe (z4 3.0i also has a similar small box).
Does that have any (extra) x/h flow in it? Or does it keep the 2 pipes 100% separated? Seems a bit small for that. (the 3.0i box is even smaller I think and that system already has a big mid muffler)


Edit: oh hold on: that mclaren has x pipes, or at least: thats what I can make of it. The 2 seperate tracks (on 1 side of the cilinder bank) are welded together. I can only imagine that the piping wall behind the welds are removed. Otherwise they would have to be dented a bit inwards. I can't imagine they'd do that. You can clearly see it on 0:25 in the video:

[Westersund]

Originally Posted by pokeybritches

Tell me, why do you think the Ferrari F1 exhaust you posted is shaped like it is? I'll give you a hint, and it has to do with equal length piping. If it wasn't about exhaust pulses, then what is it about?

I used the Pagani, because that's the example you used. I'm not going to go through each individual exhaust setup and attempt to decipher what the engineers were thinking. I do know the physics behind it, which I explained to you. X-pipes are not gimmicks, and they do have an impact on sound and performance. Going in with the mindset that they are gimmicks, and then trying to find data to prove it while ignoring other relevant data that disproves your theory, is an academic crime in engineering. It works in liberal arts, but not in science.

On the Z4M, three cylinders merging does not equal six cylinders merging. Just like six cylinders merging does not equal twelve cylinders merging. Different applications. Twelve cylinders firing creates a vastly different scenario than six, with many more pulses per second that can interfere with each other rather than help each other. Think of a revolving door- with too few people, each person has to push his way through to the other side, slowing progress. With the right number of people, the door continues to revolve with minimal effort from each person passing through. Too many people getting to the door at the same time will cause a backup.

My 130k mile, 10+ year old, non-M Z4 running 6 psi (non-intercooled) on a centrifugal blower and rich tune puts down more rwhp than a stock Z4M, and it's because of the exhaust work.

What I want to understand is why it would be beneficial to have a X-pipe if you already have equal length headers. Is more better? If that's the case would it be beneficial to have more than 1 X- or Y-pipe?

Maybe Pagani Huayra was a bad example as it has turbo's, the other engines were merely to show they don't use X-pipes, efter the equal length headers. Is there any scientific proof that there's any benefit of having a X-pipe if you already have equal length headers? At what number of cylinders is it not beneficial anymore?

What happens if the revolving door gets bigger? Is it still beneficial with a X-pipe?

Does your non-M Z4 have equal length headers or just a X-pipe, maybe both? Surely the blower must make much more of a difference on the power gained than the X-pipe.

On a side note, damn you got cranky fast.

[Westersund]

Originally Posted by GuidoK

Edit: oh hold on: that mclaren has x pipes, or at least: thats what I can make of it. The 2 seperate tracks (on 1 side of the cilinder bank) are welded together. I can only imagine that the piping wall behind the welds are removed. Otherwise they would have to be dented a bit inwards. I can't imagine they'd do that. You can clearly see it on 0:25 in the video:

I can see it. Could be that they are just welded together to be more stable. Or it's a X-pipe with a very small opening between the two pipes.

[GuidoK]

I don't think you looked correctly:

Here is a screenshot from the video what I was talking about:


You can clearly see that the two tracks from the headers are welded together. I think that's an x pipe.
There is no x connection between the cilinder banks from what I could see.

And here is a video of a shabby looking guy demonstrating the scavenging effect by the venturi principle:

[GuidoK]

Oh I see you've edited your post.
If they were welded toghether to get more sability, they would have dented the pipes there before welding. Otherwise you can only weld 2 pipes together on one small point. I dont think they'd do that. They would have welded a few brackets on them if they'd want to get it a bit stronger/stable.