Mr. Whippy, could you post the weight of the back box you have so we have a comparison standard. Maybe a picture or two? Thanks
Yay, won a 3.0i exhaust for £150!
- Thread starter Mr Whippy
- Start date
I haven't got the exhaust yet, it should arrive next week.
I will be doing lots of number crunching/analysis for everyone to be able to reference!
I'm not sure how easy it is to break down, but I'll see when I get it and then get as much data as possible on the standard system.
Ie, weight of each part, what it has as part of it, how the silencers work, and then remove them one by one to see the effect on sound/performance. I'd like to record the back pressure variance too, so perhaps tapping the exhaust near the head, and maybe after the cats, and then record that too...
Lots to do. Some easy, some difficult, but now I have a good seperate system it's a perfect opportunity to do the job right and take my time because I can
Hopefully some good and/or interesting data will come from it!
Dave
I will be doing lots of number crunching/analysis for everyone to be able to reference!
I'm not sure how easy it is to break down, but I'll see when I get it and then get as much data as possible on the standard system.
Ie, weight of each part, what it has as part of it, how the silencers work, and then remove them one by one to see the effect on sound/performance. I'd like to record the back pressure variance too, so perhaps tapping the exhaust near the head, and maybe after the cats, and then record that too...
Lots to do. Some easy, some difficult, but now I have a good seperate system it's a perfect opportunity to do the job right and take my time because I can
Hopefully some good and/or interesting data will come from it!
Dave
Nothing like a good old fashioned science project.Mr Whippy said:I will be doing lots of number crunching/analysis for everyone to be able to reference!
I'm not sure how easy it is to break down, but I'll see when I get it and then get as much data as possible on the standard system.
Ie, weight of each part, what it has as part of it, how the silencers work, and then remove them one by one to see the effect on sound/performance. I'd like to record the back pressure variance too, so perhaps tapping the exhaust near the head, and maybe after the cats, and then record that too...
Lots to do. Some easy, some difficult, but now I have a good seperate system it's a perfect opportunity to do the job right and take my time because I can
Hopefully some good and/or interesting data will come from it!
Dave
Just a very small update for now
I haven't done anything yet bar weigh the exhaust roughly, and it was 27kg or so... possibly 30kg. Not light really. I'll get a nice accurate number as I chop it up.
I just got this as well http://www.pistonheads.com/sales/1610397.htm
So now I have a flappy valve thing (and some spare bits of pipe that should be similar grade and size to use when tinkering), I can run a vacuum pump at the back of the car (like the 330Ci has), and switch it on/off with say the sport button or DISA activation 12v signal, and maybe integrate a flap into the exhaust to have it go loud/er when needed.
Been really busy recently but now the days are longer I want to start tinkering with this stuff. Just need a few exhaust joiners so when I make cuts I can join it up again
Dave
I haven't done anything yet bar weigh the exhaust roughly, and it was 27kg or so... possibly 30kg. Not light really. I'll get a nice accurate number as I chop it up.
I just got this as well http://www.pistonheads.com/sales/1610397.htm
So now I have a flappy valve thing (and some spare bits of pipe that should be similar grade and size to use when tinkering), I can run a vacuum pump at the back of the car (like the 330Ci has), and switch it on/off with say the sport button or DISA activation 12v signal, and maybe integrate a flap into the exhaust to have it go loud/er when needed.
Been really busy recently but now the days are longer I want to start tinkering with this stuff. Just need a few exhaust joiners so when I make cuts I can join it up again
Dave
Loving you work
Very much looking forward to the results!
Very much looking forward to the results!
Thanks for the update. I knew the oe boxes were heavy, but I had no idea they were in that weight range. Changeout to an aftermarket box would be a great performance mod just due to the weight reduction. Question; are the spot welds for the internals on the shell of the 3.0i box visible? These would make great reference guides for opening specific areas of the shell.Mr Whippy said:Just a very small update for now
I haven't done anything yet bar weigh the exhaust roughly, and it was 27kg or so... possibly 30kg. Not light really. I'll get a nice accurate number as I chop it up.
Dave
I hope to do all that kinda spot weld checking when I cut mine up. I'll fully document it for others to use as reference 
I'll weigh each section and work through it too. First single mod for power and weight savings is removing the front cats, replace with as smooth a bends as will fit. Hope it passes on a warm engine, and probably will save a few kg and add a few bhp essentially for nothing... while also sprucing up the sharpness of the sound a bit!
Dave
I'll weigh each section and work through it too. First single mod for power and weight savings is removing the front cats, replace with as smooth a bends as will fit. Hope it passes on a warm engine, and probably will save a few kg and add a few bhp essentially for nothing... while also sprucing up the sharpness of the sound a bit!
Dave
Dave, that Blueflame thread reminded me of this one... you had any more time to play yet?
Not sure where you are no the design, but it may be worth having a look to see if there are any details / schematics published for the Porsche 996 PSE, a switch operated valve thingy changes between quiet and
Not sure where you are no the design, but it may be worth having a look to see if there are any details / schematics published for the Porsche 996 PSE, a switch operated valve thingy changes between quiet and
Not done much yet.
Haven't had a look at my 330Ci backbox yet, but I am going to try pick it up from my dads this weekend and try get time to chop it up purely for analysis of how BMW set it up for quiet and loud (apparently the 330Ci sounds pretty meaty and nice with this valve stuck open all the time) modes...
Mainly just got it for a good idea of how BMW implemented the system, have a valve to play with, and a few feet of pipe of a good BMW quality to use for my new exhaust if needed too
First main job on my Z4 will be to create two appropriate sections to replace the secondary (main?) cats and then go to a nice MOT centre and see if it'll pass emissions test in that config!
Will also note sound change, weight difference, and do some power runs before/after
At some stage I'll have to chop the OEM Z4 back box up too, but don't think I'll have time this weekend... might do though, just need to get the grider well warmed up and a camera to do documenting
Dave
Haven't had a look at my 330Ci backbox yet, but I am going to try pick it up from my dads this weekend and try get time to chop it up purely for analysis of how BMW set it up for quiet and loud (apparently the 330Ci sounds pretty meaty and nice with this valve stuck open all the time) modes...
Mainly just got it for a good idea of how BMW implemented the system, have a valve to play with, and a few feet of pipe of a good BMW quality to use for my new exhaust if needed too
First main job on my Z4 will be to create two appropriate sections to replace the secondary (main?) cats and then go to a nice MOT centre and see if it'll pass emissions test in that config!
Will also note sound change, weight difference, and do some power runs before/after
At some stage I'll have to chop the OEM Z4 back box up too, but don't think I'll have time this weekend... might do though, just need to get the grider well warmed up and a camera to do documenting
Dave
Anxiously awaiting!Mr Whippy said:Not done much yet.
Haven't had a look at my 330Ci backbox yet, but I am going to try pick it up from my dads this weekend and try get time to chop it up purely for analysis of how BMW set it up for quiet and loud (apparently the 330Ci sounds pretty meaty and nice with this valve stuck open all the time) modes...
Mainly just got it for a good idea of how BMW implemented the system, have a valve to play with, and a few feet of pipe of a good BMW quality to use for my new exhaust if needed too
First main job on my Z4 will be to create two appropriate sections to replace the secondary (main?) cats and then go to a nice MOT centre and see if it'll pass emissions test in that config!
Will also note sound change, weight difference, and do some power runs before/after
At some stage I'll have to chop the OEM Z4 back box up too, but don't think I'll have time this weekend... might do though, just need to get the grider well warmed up and a camera to do documenting
Dave
Well today I got a chance to chop up the 330Ci back box a bit... just to see what the logic was behind the flap system etc.
Exhausts:
Double skins tough to cut through, not sure you could weld it perfectly, but it'd be ok:
Blue ones are the inlets. They enter chamber 1 on the left and carry on through the perforated (yellow dashes) of the chamber 1/2 (red) panel where they open out.
Chamber 1 is hollow and empty. The blue pipes here are perforated, so probably a resonance chamber.
Once the gases are in chamber 2, they have two outlets. One is the long green hockey stick pipe, which reverses the flow through a solid pipe through chamber 1, then back into chamber 2, then through into chamber 3 and right out the back of the car in one solid pipe!
Or, the other outlet is via the top-green pipe, a short straight pipe which runs into chamber 3, where the pipe is then perforated and silencing material added. This outlet is blocked until the flap opens.
Empty chamber 3:
Chamber 2:
Best shot of chamber 1 (right hand corner, did chop this out more just to see if the inlets were perforated):
I'm not quite sure what the logic of the system is now. In theory the flap being open is a reduced flow strategy since two inputs have two equal bore and equally un-impeded outlets.
When the flap is closed back pressure will be increased heavily, however, the level of silencing appears to be zero from the chamber 2 hockey stick pipe onwards. Essentially the back box in the 'flap stuck shut' configuration is just a resonator, and then a straight through pipe (bar the weird 'buldge' on the pipe in chamber 3, but I guess this may just be another resonator)
It may well be that the restriction and existing resonance chambers are enough to make this quite a silent exhaust.
The opening of the flap would likely reduce back pressure by over double through this part because the pipe is around half as long, and has no reversal of the gas flow. However, it is surprising to see the presence of silencing around this outlet.
Perhaps the exhaust now becomes so loud that the silencing is needed.
Bar a bit more chopping up (the small chamber on the J section outlet in chamber 3), I'm not really sure what I think to it.
No wadding/perforated silencing on the 'quiet' outlet is surprising, and wadding on the loud outlet is yet more surprising. I might have expected those to be the other way around!
The really interesting thing will be when I dissect the Z4 back box to see how that works, and any similarities or differences to the 330Ci one!
I am certain though, that a 330Ci backbox has oodles of potential to make it sound much different by simply messing about inside the back box!
Ie, you could cut the J section pipe so both exits from chamber 2 were short and straight, and then perforate the lower pipe to quieten it off a little, and then clad the upper pipe (or cut/swap the two), to make the flap-open mode much more free-flowing!
Interestingly, this exhaust wasn't so old, and was removed so a free-flow stainless back box could go on the car. The flap was siezed badly, closed. The new exhaust probably felt loads nicer, but possibly because this old one was not working properly!
Dave
Exhausts:
Double skins tough to cut through, not sure you could weld it perfectly, but it'd be ok:
Blue ones are the inlets. They enter chamber 1 on the left and carry on through the perforated (yellow dashes) of the chamber 1/2 (red) panel where they open out.
Chamber 1 is hollow and empty. The blue pipes here are perforated, so probably a resonance chamber.
Once the gases are in chamber 2, they have two outlets. One is the long green hockey stick pipe, which reverses the flow through a solid pipe through chamber 1, then back into chamber 2, then through into chamber 3 and right out the back of the car in one solid pipe!
Or, the other outlet is via the top-green pipe, a short straight pipe which runs into chamber 3, where the pipe is then perforated and silencing material added. This outlet is blocked until the flap opens.
Empty chamber 3:
Chamber 2:
Best shot of chamber 1 (right hand corner, did chop this out more just to see if the inlets were perforated):
I'm not quite sure what the logic of the system is now. In theory the flap being open is a reduced flow strategy since two inputs have two equal bore and equally un-impeded outlets.
When the flap is closed back pressure will be increased heavily, however, the level of silencing appears to be zero from the chamber 2 hockey stick pipe onwards. Essentially the back box in the 'flap stuck shut' configuration is just a resonator, and then a straight through pipe (bar the weird 'buldge' on the pipe in chamber 3, but I guess this may just be another resonator)
It may well be that the restriction and existing resonance chambers are enough to make this quite a silent exhaust.
The opening of the flap would likely reduce back pressure by over double through this part because the pipe is around half as long, and has no reversal of the gas flow. However, it is surprising to see the presence of silencing around this outlet.
Perhaps the exhaust now becomes so loud that the silencing is needed.
Bar a bit more chopping up (the small chamber on the J section outlet in chamber 3), I'm not really sure what I think to it.
No wadding/perforated silencing on the 'quiet' outlet is surprising, and wadding on the loud outlet is yet more surprising. I might have expected those to be the other way around!
The really interesting thing will be when I dissect the Z4 back box to see how that works, and any similarities or differences to the 330Ci one!
I am certain though, that a 330Ci backbox has oodles of potential to make it sound much different by simply messing about inside the back box!
Ie, you could cut the J section pipe so both exits from chamber 2 were short and straight, and then perforate the lower pipe to quieten it off a little, and then clad the upper pipe (or cut/swap the two), to make the flap-open mode much more free-flowing!
Interestingly, this exhaust wasn't so old, and was removed so a free-flow stainless back box could go on the car. The flap was siezed badly, closed. The new exhaust probably felt loads nicer, but possibly because this old one was not working properly!
Dave
Great info, very good pics and comments.Mr Whippy said:
Please keep the information coming. Once you can get into the Z4 box, it might be possible to devise a way to do a small "surgical" cutout to provide for slightly better flow/sound.
That's it... hopefully there might be some little bits I can tweak around to do something smart... one pipe to retain the nice silenced level of standard for most times, then on sport button, open up the other pipe and give it less silencing...!
I'd like to try that first anyway.
Another job is making up some de-cat sections so I can try without those!
Dave
I'd like to try that first anyway.
Another job is making up some de-cat sections so I can try without those!
Dave
Excellent work Mr. Whippy, im especially looking forward to the Z4 gutting as i have a spare - when are you thinking of doing this?
Very very surprised also at the lack of sound deadening on the hockey stick pipe! However what is wrapped around the pipe in your "Empty Chamber 3" photo? That looks very unusual, almost like a thin peice of steel pinched on at either end...
Keep it up
Very very surprised also at the lack of sound deadening on the hockey stick pipe! However what is wrapped around the pipe in your "Empty Chamber 3" photo? That looks very unusual, almost like a thin peice of steel pinched on at either end...
Keep it up
I will be busy this weekend with other things, but I think I'll get chance to take a look inside the Z4 back box next weekend.
I also want to chop into that weird crimped pipe on the J pipe in chamber 3, to see what is inside there... weird!
Another job I need to do is replace the cats with some straight through pipes, so at some stage I'm going to have to go to an exhaust fabrication place to get those made, since I don't think I can nicely gut the existing ones of their internals without cutting very close to their ends, which would mean I'd have to re-weld to get them together again (a painful task)
Proper de-cat sections would be a better move
That is one of the first jobs I want to do as I think it'll have one of the biggest benefits in one swoop if it'll still pass an MOT in that config
Dave
I also want to chop into that weird crimped pipe on the J pipe in chamber 3, to see what is inside there... weird!
Another job I need to do is replace the cats with some straight through pipes, so at some stage I'm going to have to go to an exhaust fabrication place to get those made, since I don't think I can nicely gut the existing ones of their internals without cutting very close to their ends, which would mean I'd have to re-weld to get them together again (a painful task)
Proper de-cat sections would be a better move
That is one of the first jobs I want to do as I think it'll have one of the biggest benefits in one swoop if it'll still pass an MOT in that config
Dave
Well, a little update just to keep momentum up...
DaveG decided to chop a back box up the other day, which yielded a description of the back box interior...
So here is a picture of what that looks like... ish.
I'll try do a logic based analysis of the layout vs the 330Ci back box, and try make some assumptions about why BMW did what they did.
Then hopefully, following their logic, we can decide what might be the best step to change the noise for a bit more of a raspy, barky, crackling on hte over-run, but not much louder exhaust note
Dave
DaveG decided to chop a back box up the other day, which yielded a description of the back box interior...
So here is a picture of what that looks like... ish.
I'll try do a logic based analysis of the layout vs the 330Ci back box, and try make some assumptions about why BMW did what they did.
Then hopefully, following their logic, we can decide what might be the best step to change the noise for a bit more of a raspy, barky, crackling on hte over-run, but not much louder exhaust note
Dave
What is that a diagram of?
Yes, the standard back boxes look surprisingly light of absorbtion material.
I'm just really curious where you could get some extra flow from.
It looks like the 330Ci box doesn't have padding in chamber 1, while the Z4 one does. That might be a good place to lose some material, and then perforate that wall from chamber 1 to chamber 2...
To get loudness I think you want to reduce the volume though, so maybe sleeving off the chamber 1 pipes (so they can't leak), and leaving chamber 1 isolated totally, might just do something interesting.
I still think the secondary cats cause a great deal of the muffling and woofling of the exhaust though. Replacing those is probably the quickest way to a bit more snap crackle and pop and power, without adding too much volume.
Really tough knowing what to do.
Hmmm
Dave
Yes, the standard back boxes look surprisingly light of absorbtion material.
I'm just really curious where you could get some extra flow from.
It looks like the 330Ci box doesn't have padding in chamber 1, while the Z4 one does. That might be a good place to lose some material, and then perforate that wall from chamber 1 to chamber 2...
To get loudness I think you want to reduce the volume though, so maybe sleeving off the chamber 1 pipes (so they can't leak), and leaving chamber 1 isolated totally, might just do something interesting.
I still think the secondary cats cause a great deal of the muffling and woofling of the exhaust though. Replacing those is probably the quickest way to a bit more snap crackle and pop and power, without adding too much volume.
Really tough knowing what to do.
Hmmm
Dave
I just drew it from DaveG's description, I haven't chopped mine up yet
Did you post up your dissection findings? I must have missed it... damn
Hmmm... so what was the end result of your tinkerings? Did you achieve anything worthwhile?
I'm wondering if just blocking out a chunk of that big centre volume might be a good idea, gut the 3rd chamber of fluff, so same overall volume in the back box, give or take, but less sound absorbancy... this is where my lack of optimising OEM exhausts experience is showing. I just don't get how BMW optimise stuff like this in the first place. I guess it's just iterative improvement and changes to get the end result they have...
But how do we get what we want from it. More character but not loads more noise level. Hmmmm...
Dave
Did you post up your dissection findings? I must have missed it... damn
Hmmm... so what was the end result of your tinkerings? Did you achieve anything worthwhile?
I'm wondering if just blocking out a chunk of that big centre volume might be a good idea, gut the 3rd chamber of fluff, so same overall volume in the back box, give or take, but less sound absorbancy... this is where my lack of optimising OEM exhausts experience is showing. I just don't get how BMW optimise stuff like this in the first place. I guess it's just iterative improvement and changes to get the end result they have...
But how do we get what we want from it. More character but not loads more noise level. Hmmmm...
Dave