Reducing Drone in aftermarket exhaust - side branch resonators

[boom]

A couple of months ago I got a custom made aftermarket exhaust for my 335xi. It's a BEAUTIFUL 3" tig welded stainless steel quad tip exhaust made by a company in Elkridge, MD. called Benchmade Proper. Pix here: Exhaust shop review and advice requested - side branch resonators The exhaust is awesome and I love it at WOT but the drone was considerable at 1.5-3K rpm. I'm on stock downpipes (for now) and know that when I do put aftermarket jobs on it would be unbearable. I contacted BMP and set up a time for Joe and Brian to weld in side branch resonators. There are bunch of articles out there about this technique. Essentially you are creating destructive interference in the exhaust to combat the resonance. All pipes will resonate at different frequencies. The trick is to create the resonance at an RPM that you're not in often. To create the destructive interference you need to add a quarter wavelength of pipe to the exhaust. Big question. What's the wavelength. That is calculated using the number of pistons, rpm, speed of sound in air and temperature of the exhaust flow. Whatever. Fit some pipe up and if it doesn't work adjust. Longer gets lower rpm, shorter gets the higher rpm stuff. Benchmade welded up a couple of capped resonators to my existing setup and basically eliminated the drone. It's freaking badass to have a stock at low rpm sounding 3" exhaust. Enjoy the pix and videos. Still need to get a damn gopro. videos

[airbrush1]

Interesting... The corvette has a horrible drone between 1200 and 1600 when cruising

[boom]

Yeah, I got the idea from a Porsche forum but the most information is from v8 trucks and mustangs. Seems to work very well for them. It cancels almost all drone in my car. Any more and it would be more quiet than stock.
That shop is awesome. They update their Facebook page pretty regularly so you can see examples. Their last car was an e63 amg that's fantastic.

[bull30]

There is a quicker, easier and more accurate way to figure out the length needed for a side-branch tube to counteract exhaust resonance.

You need to take a Real Time Analyzer with you in the car. You can get apps for the iPhone & iPad. Using the RTA drive your car until you hit the resonant frequency then look at the RTA to determine the peak frequency. Now all you need to do is use a closed tube calculator to determine the length of pipe needed to create that resonant frequency. Using the same diameter tube as the exhaust cut a hole in the existing system before the resonator/muffler then weld your closed end side-branch tube to the system. Bingo - resonance gone

There are many closed tube calculators available on the internet.

[boom]

I tried two rta apps on my iPhone and both lacked the resolution to identify the frequency. They were both free and you get what you pay for but the next cheapest one was $10.99. If I didn't already have the resonators I would've done it. In any case, Just reporting back. This mod makes the exhaust completely civilized on the daily. I love it. My friends have told me that it actually makes the tone at wot deeper and less raspy. Highly recommend.

Originally Posted by bull30

There is a quicker, easier and more accurate way to figure out the length needed for a side-branch tube to counteract exhaust resonance.

You need to take a Real Time Analyzer with you in the car. You can get apps for the iPhone & iPad. Using the RTA drive your car until you hit the resonant frequency then look at the RTA to determine the peak frequency. Now all you need to do is use a closed tube calculator to determine the length of pipe needed to create that resonant frequency. Using the same diameter tube as the exhaust cut a hole in the existing system before the resonator/muffler then weld your closed end side-branch tube to the system. Bingo - resonance gone

There are many closed tube calculators available on the internet.

[bull30]

Originally Posted by boom

I tried two rta apps on my iPhone and both lacked the resolution to identify the frequency. They were both free and you get what you pay for but the next cheapest one was $10.99. If I didn't already have the resonators I would've done it. In any case, Just reporting back. This mod makes the exhaust completely civilized on the daily. I love it. My friends have told me that it actually makes the tone at wot deeper and less raspy. Highly recommend.

It's best to use a 1/3 octave RTA if you're looking for accuracy... I used one I purchased for the iPad. I think it cost around $30.00. I used it for a friends Corvette that had a horrible drone at 2500 RPM... The side-branch needed according to the closed tube calculator was 28"... He had the branch installed and the drone completely disappeared.