Today's Project: Exhaust Quarter Wave Tube

[SpeedRacer85]

^ I agree on that design also.

[AlphaE]

brilliant!! been lookin at this idea a bit my self. looks good. I'd be intersted in more info on the excel spreadsheet. how u came up w the drone number? that real life?? like it actually drones at 3800rpm??

[nrubenstein]

brilliant!! been lookin at this idea a bit my self. looks good. I'd be intersted in more info on the excel spreadsheet. how u came up w the drone number? that real life?? like it actually drones at 3800rpm??

2,800 RPMs. It was calculated and then I measured on the road with an RTA app on my iPhone for confirmation. The gigantic spike in that range was pretty good confirmation from my perspective.

The magnaflows, on the other hand had zero drone at that RPM, but really get horrible around 4K. (This is 80MPH with a 5spd and a 3.46.)

Shoot me a PM with your email address if you want the sheet.

[bubbajan21]

How would one go about making one of these on a dual exhaust? Does it matter where it is placed in the exhaust or can it just be welded in anywhere it fits?

[cabriodster87]

Couldn't it be designed like a Trombone? Slide the tube different lengths to fine tune.

[nrubenstein]

Couldn't it be designed like a Trombone? Slide the tube different lengths to fine tune.

Absolutely. The downside is that you're more likely to have exhaust leak issues.

Also, frankly, these things operate over a pretty wide frequency range. Precision isn't *that* necessary. (Which is not to say that there isn't room for improvement, just that you can get excellent results by building to a calculated length.)

[nrubenstein]

How would one go about making one of these on a dual exhaust? Does it matter where it is placed in the exhaust or can it just be welded in anywhere it fits?

You'd want to make two of them. Also, if it's a true dual exhaust with no merges, remember that you're calculating based on three cylinders.
You might also try putting a 3 way collector in place of an existing merge if it's practical. (It might not be and collectors are expensive.)

[bubbajan21]

You'd want to make two of them. Also, if it's a true dual exhaust with no merges, remember that you're calculating based on three cylinders.
You might also try putting a 3 way collector in place of an existing merge if it's practical. (It might not be and collectors are expensive.)

And if it's a dual with an x-pipe? Still make 2 pipes but calculate based on 6 cylinders?

Also, does it matter where in the exhaust the tube is placed?

[nrubenstein]

And if it's a dual with an x-pipe? Still make 2 pipes but calculate based on 6 cylinders?

Also, does it matter where in the exhaust the tube is placed?

Dual with an x-pipe would notionally want two, but calculated for 6 as you said. Depending on how much drone you want to get rid of, you might have sufficient results with just one pipe. I speculate that putting the resonator as close as possible to the X would result in cross talk. But I should emphasize speculate there.

There seem to be varying schools of thought on placement. Some suggest that the ideal location is at the tail pipe. After all, this cancels out any resonances induced by the muffler. A lot of folks seem to like before the last muffler (i.e. where I did it), and some suggest all the way up front to cancel noises before the exhaust even starts to resonate.

If that sounded to you like "I don't really have a clue, other than that what I did worked," you'd be about right.

[harv]

Very Very cool!

Maybe I can do something about that rasp blat from 2000 to 2800 on my 318is?

Hmm

John

[nrubenstein]

Got off my ass and uploaded the file. Apparently quite a lot of you want my little sheet:

http://nrubenstein.com/images/330is/quarter%20wave%20tube%20calculator.xlsx

Enjoy! Feel free to share it around, but I would like credit for putting it together if you do. Blue cells are inputs, black cells are formulae.

[CarsSuck]

Did you try your calculations with a shorter length? I know on 6 cylinder e39s BMW incorporated one of these just before the rear cans, but it's much shorter than yours. About 8" iirc off the top of my head. Nice Work!

[nrubenstein]

Did you try your calculations with a shorter length? I know on 6 cylinder e39s BMW incorporated one of these just before the rear cans, but it's much shorter than yours. About 8" iirc off the top of my head. Nice Work!

A helmholtz resonator is more complex than a quarter wave tube. You can make one of those effective by trading trading length for width and scaling the inlet tube appropriately. A quarter wave tube's target frequency is absolutely dictated by length.

[tonytony]

+1 I'll definately be referring back here if my new exhaust has any annoying drone

[quickervicar]

This might just save me. I've got a 3" system with a Magnaflow on my 2.7L and it has a nasty drone from about 2,500-3,000 rpm. I was considering swapping the whole thing out because it's just too annoying. (sounds delicious at WOT though)

A big thanks for posting up the sheet, btw!

As a side note, your pics are just kind of freakish. I've got the same color combo, same Konis, same lift, and same ladder. Weird.