Today's Project: Exhaust Quarter Wave Tube

[nrubenstein]

(Often referred to as a helmholtz resonator, although they are technically different.) I re-did my exhaust yet again because of the 6 speed's clearance issues (the bracket and the exhaust tube wanted to occupy the same space), and because I was tired of having a plain 3" tube sticking out the rear valence. It looks fine on a 318, with the single tip valence, but the gap on a 325 just needs two tips (or to be filled in, but I didn't want to go down that road...).

Unfortunately, the new Borla had an annoying drone right at cruising speed. I decided that rather than try different mufflers yet again that I would attack the problem directly.

I have to say that I am thrilled with the results. The exhaust is now virtually silent cruising at 70, where before it had a really annoying spike at about 140Hz. Even better, the tube dramatically cut down on the 1,800-2k drone that I wasn't bothered enough by to try to attack (the tube would have been 40" long instead of 27" in that case).

Even better, I assembled it entirely out of leftover scrap, so my total cost is zero.

So... Here they are:

[619E30]

Interesting...

[PeteD]

Just to be clear, the third dead end pipe stopped the resonance and it is shaped like that because of fitment issues.

[nrubenstein]

Just to be clear, the third dead end pipe stopped the resonance and it is shaped like that because of fitment issues.

Correct. Shape doesn't matter, just length.

Edit: Well, really, the shape is complex because I kind of screwed up when I was cutting the bend that Ts into the main tube. Since that angle was off, it was a case of either redoing it or using more bends. I had lots of scrap bend because I *way* overbought for the tip install, so I got creative.

[ak-]

Ontorosting.

[PeteD]

Correct. Shape doesn't matter, just length.

That's what she said, eh.

Thanks for sharing. At first, I thought you were building a dump pipe to avoid the cat etc; but I like this. I'm curious to see how my B&B Triflow that was on my eta sounds with my s50. It was loud on the eta so I'm assuming it won't be any quieter. I'll try to channel you if I have to get creative to tone it down.

[nrubenstein]

That's what she said, eh.

Thanks for sharing. At first, I thought you were building a dump pipe to avoid the cat etc; but I like this. I'm curious to see how my B&B Triflow that was on my eta sounds with my s50. It was loud on the eta so I'm assuming it won't be any quieter. I'll try to channel you if I have to get creative to tone it down.

For what it's worth, I made a little excel sheet to calculate the length. The big variable is guesstimating the appropriate speed of sound. It goes up as temperatures rise. 1,210 ft/s was my guesstimate, but there is a reason why I made it an input rather than fixing it in a formula. (Blue cells are inputs.)

Shoot me a PM with you email address if you want a copy. Apparently I can't attach anything other than an image and I can't be bothered to host it.

[PeteD]

Ok.

I was curious about how much thought went into this, as I was considering the effect higher RPM's would have on the integrity of the pipe; assuming gasses actually travel the length of the dead pipe, or is it just for sound waves?

[nrubenstein]

Ok.

I was curious about how much thought went into this, as I was considering the effect higher RPM's would have on the integrity of the pipe; assuming gasses actually travel the length of the dead pipe, or is it just for sound waves?

There's some flow, but it's minimal. If you have enough flow to compromise the integrity of the pipe, something is very, very wrong. (Even if both tail pipes are plugged, the engine will shut down before you could build a tiny fraction of the necessary pressure.)

To be clear, there's nothing new about this. Many OEMs (BMW included) have done this on both exhausts and intakes. Edit: The difference is that they often hide them inside a box, presumably to avoid the "hey, what's that sticking out?" question.

[PeteD]

Yeah? I'm not familiar. Seems like an after thought for an OEM application, but that could be my ignorance on the topic talking.

[nrubenstein]

Yeah? I'm not familiar. Seems like an after thought for an OEM application, but that could be my ignorance on the topic talking.

OEM exhausts drone too. The E39 540i exhaust is a great example of one that they resonated the crap out of. There's what looks like a helmholtz resonator next to the muffler (the extra muffler that has an inlet, but no outlet), and the Y shaped resonator in the center may also contain something similar.

Edit: You actually have it backwards. These sorts of resonators are pretty common in OEM applications and comparatively rare in the aftermarket.

[acolella76]

This is definitely interesting. My exhaust used to be extremely loud and I couldn't be bothered to build a tube like this. Now I want the old exhaust back :p Thanks for posting this!!

[nrubenstein]

Well, it won't make a loud exhaust not loud. There's some impact on general noise, but mostly you just get rid of the targeted resonance. However much or little muffling you have doesn't really change.

[Wanganstyle]

To be clear, there's nothing new about this. Many OEMs (BMW included) have done this on both exhausts and intakes. Edit: The difference is that they often hide them inside a box, presumably to avoid the "hey, what's that sticking out?" question.

e39 540i; a couple bonus exhaust protrusions - makes it super VIP quiet.

[DisgustipatedAP1]

S2000's have the Helmholtz on stock exhaust. It is a brilliant, effective way to quiet down any exhaust and get rid of horrific drone at certain frequencies.

Your design on your exhaust looks fantastic. Very well done. :)