Back box modification - With pics.

[Ducklakeview]

Ok, after much searching and looking at the options, I reckon I have 3 options;

1: Chop out the box and go for straight pipes - But worried it may be too noisy, AND possible drone.
2: Bypass pipes - But I'd want to go for the 1" ones, and looking at my box, I don't think they would fit, especially with my Monkey Tips.
3: Cutting open the box and modding it, similar to Yorkie's thread here; viewtopic.php?f=2&t=62691&hilit=back+box+mod

So, as I want to keep the car on the road, on the off chance that we get some good weather I've just bagged this on Ebay for £30

backbox.JPG (66.2 KiB) Viewed 8084 times

I intend to mod it like Yorkie did, cutting it open and joining the in and output tubes, leaving a small amount of perforated tube for sound reduction, and then packing the wadding back in. This way, I can always revert to STD, or make more mods to the box, as I've had some decent sleeves delivered to the shop today, not bad @ £6.49 ea from Euro Car Parts.

20160708_194616.jpg (62.37 KiB) Viewed 8084 times

So, quick question, does anyone know what stainless the box is made from? I'm assuming it is stainless, although I haven't checked.

Will pop up a post/pics as I'm doing it, and a before/after video.


Mike

[jakblade]

This is what I did to mine. Just for info. Last page of post has pics of modded backbox.
viewtopic.php?f=2&t=79494&start=165.

[GuidoK]

So, quick question, does anyone know what stainless the box is made from? I'm assuming it is stainless, although I haven't checked.

My guess is something like 304 like most stainless things bent or pressed. Fairly standard.

[Ducklakeview]

So, quick question, does anyone know what stainless the box is made from? I'm assuming it is stainless, although I haven't checked.

My guess is something like 304 like most stainless things bent or pressed. Fairly standard.

Cheers. Just had a nice 20L bottle of 95% argo dropped off FOC by a customer, so just waiting for the box now.

Mike

[Ducklakeview]

Ok, spotted some info on another post about a guy that offers mobile modding to M3 owners, a bit of a Google turns up this little lot;


Apparently this is some of his work;

https://www.facebook.com/Paulscarwelding/

Now this I LIKE the sound of

https://www.facebook.com/john.hyde.9275 ... 967986090/



And I found this how to, which whilst I appreciate my 3.0 won't sound this good, I think this is the way I'm going to go, joining the pipes as per Yorkie's thread, and covering the perforations with tape and exhaust wrap..



Mike

[Ducklakeview]

So, the used box has arrived

Looks like my thoughts re 1" bypass pipes were right, not enough room to fit them without making the back ends of the tips visible...

20160712_093445.jpg (64.92 KiB) Viewed 7954 times

I thought this may have been an aftermarket box, but it's original BMW, having been "modded" to slide on and off with welded sleeves, apparently the seller had received it with the car and left the "performance" box on it when he got rid. One less job for me to do then!

20160712_093457.jpg (42.57 KiB) Viewed 7954 times

Have now got exhaust wrap tape, aluminium tape, stainless ties, 50mm sleeves and 2 * 300mm lengths of 50mm stainless tube. The weekend should be fun, and hopefully noisy! Cost so far, around £50 all in, the stainless welding gas and wire were freebies, the box was £30 of that total.

Will take some pics as I do it, and a before/after video/sound clip..

Mike

[Machine monkey]

It is most likely 304 any stainless wire will weld on to it it dose not ,matter when welding really. Well not on something like this.
Get yourself a pack of 1mm cutting wheels for you angle grinder. You dont want to be trying to fill a 3mm gap. The stainless is thin and you will start blowing holes. Sorry if i am teaching you to suck eggs. Its just what i have noticed from modding a few M back boxes.

[Machine monkey]

Also plan and look where the spot welds are. And then cut around them if you can.

[Ducklakeview]

It is most likely 304 any stainless wire will weld on to it it dose not ,matter when welding really. Well not on something like this.
Get yourself a pack of 1mm cutting wheels for you angle grinder. You dont want to be trying to fill a 3mm gap. The stainless is thin and you will start blowing holes. Sorry if i am teaching you to suck eggs. Its just what i have noticed from modding a few M back boxes.

Mat, gone for 308LSi wire with 95% Argon 5% Oxygen. Although I'm a dab hand with the Mig, I've never welded stainless before, but I'm sure I'll get by Already stocked up on 1mm discs

Mike

[jimmybell]

FWIW the M3 oem exhaust mod is ... a drastic change. Videos do not convey how loud it becomes (a friend just did his, and has swapped back to OEM verrry quickly). It was ASBO loud - couldnt drive it at night.

[Ducklakeview]

FWIW the M3 oem exhaust mod is ... a drastic change. Videos do not convey how loud it becomes (a friend just did his, and has swapped back to OEM verrry quickly). It was ASBO loud - couldnt drive it at night.

Obviously the M3 OEM exhaust is a fair bit louder than the std one on my 3.0 to start with. I'm (probably) going to join the in & out pipes with 50mm stainless tube, and still either utilise the small slotted sections, or more likely cut similar slots into the joining pipes with a 1mm disc. I'll also be replacing the "wadding" before reassembly.

As you can see in this pic pinched from Yorkie's thread, the std box is made differently than the M3, with no actual pipes running between the inlets and outlets, as they are cut allowing the gasses into the "void" which I assume does a lot of the silencing work. My mod will (hopefully) allow more noise out of the back, but prevent boominess. That's the hope anyway

Std box;

box std cut open.jpg (63.89 KiB) Viewed 7923 times

Whereas the M3 box has actual pipes linking the inlets & outlets across the box, with perforated sections to allow the stuffing to do it's work;

m3 back box std.jpg (68.13 KiB) Viewed 7921 times

If my mod sounds as loud as a stock M3, or maybe just a "bit" fruitier, I'll be happy

Mike

[Marlon]

Watching this thread with interest

[Machine monkey]

It is most likely 304 any stainless wire will weld on to it it dose not ,matter when welding really. Well not on something like this.
Get yourself a pack of 1mm cutting wheels for you angle grinder. You dont want to be trying to fill a 3mm gap. The stainless is thin and you will start blowing holes. Sorry if i am teaching you to suck eggs. Its just what i have noticed from modding a few M back boxes.

Mat, gone for 308LSi wire with 95% Argon 5% Oxygen. Although I'm a dab hand with the Mig, I've never welded stainless before, but I'm sure I'll get by Already stocked up on 1mm discs

Mike

I mostly tig weld i am not so good with mig. But i assume its the same as steel. Stainless and steel mig weld exactly the same.
So you should be a ok buddy all the best

[exdos]

I've gutted lots of OEM silencers for the Z3 MC and the Z4MC (thread posted on here in 2012) and the golden rule that I have discovered is that there must not be a direct line of sight of the inlet pipe into the silencer through the outlet pipe, The exhaust needs take a 90degree turn out of the inlet pipe and then take another 90 degree turn into the outlet pipe: this permits the sound waves to get reflected in the silencer but doesn't impede the gas flow unduly.

[GuidoK]

You can also make a loop system (which obviously also has turns) where the loop is reintroduced in the same chamber. That noise will be in antiphase for some frequencies.
Similar to the effect of resonance chamber now in the backbox (spring-mass system/helmholz resonator) but with a continous pipe so flow is less effected. I think both the z4m backbox has this and exhausts like supersprint twinloop etc.

But if you want more noise and more power, stainless headers and high flow cats is the way to go. The biggest flow restriction on the m54 is by far the exhaust manifold.