using SCIENCE to ELIMINATE exhaust drone: I FINALLY did it!

I know, I know! You guys are probably getting sick of seeing exhaust builds on the GMT800 Stepside build, but this cammed 4.8 LR4 is kinda annoying do drive when the exhaust drones, and I'm bound and determined to defeat this issue once and for all, while maintaining 4" exhaust. After exhaust version 4, we have the overall volume to an acceptable level, but the resonance and drone were killer, so with a little help from science and a quarter wave resonator, we're gonna tackle this issue head on.

#exhaust #resonator #magnaflow

0:00 What is a Quarter Wave Resonator
4:08 Building the Resonator
7:27 Fabrication Notes
10:00 Oops...
12:05 Test Drive
15:05 0-60 Testing
17:39 In Depth Calculations

[LawrenceTolman]

The LSA install starts next! I gotta say, this exhaust journey was a long one, and usually it doesn't take quite so many changes to make me happy with the sound, but I think we're finally there guys.

[jaredshaffer]

You talked about doing a full length or half length resonator. Just FYI, those wouldn't work because with a half length (full length when you consider the wave going down and back) and full length, the wave would be back in phase. With quarter wave length, (half wave length full travel distance) it is 180 degrees out of phase, which causes the cancellation.

[steveh]

I love your scientific approach to the drone! Essentially, you installed a Helmholtz resonator. I've installed them in piping systems in natural gas processing facilities where pulsations from reciprocating compressors will cause vibrations and noise due to pulsations in the pipe. A large part of you did was change the natural resonant frequency of the piping (exhaust) system farther away from 117hz. You did this by adding pipe length of the resonator (which is why drone now occurs at lower rpms). The further away your resonant frequency between pulsations and exhaust system, the less drone you'll have. Its the same idea where a tire that's out of balance vibrates at a certain speed on the highway, and when you speed up or slow down it goes away. Those tire vibrations were matching the suspension's resonate frequency. I wouldn't be surprised if you went back to the original muffler you wanted to use (the hush power, as I recall), you'd still keep the drone effect down because the resonator is now installed. Maybe an experiment for another day. Great work!

[fullautophx]

I’ve made many many of these over a 20 year custom exhaust career, adjustable and various lengths. For some reason 28” is the magic spot, it never fails to relieve the drone.

[Avitor]

Make the resonator so it can slide like a trombone. (Slightly larger pipe over slightly narrow pipe). Then you can tune it in. The drone is also likely to shift some with other mods. Once happy make the final version.

[danh]

I think you win the award for messing with an exhaust. And i felt it in my chest when the exhaust hit your cross members, but glad you figured it out.

[psdaengr]

Slicing the end of the resonator tube at a 45 degree angle would make it work over a wider range of frequencies just as the 90 degree bend did. Also the placement of the resonator at the position along the length of the exhaust where the resonant vibration is highest would make it most effective.

[stanleysteemr]

Pure Genius.! Compliments from a retired mechanic, age 64, former Navy engineer, drag racer. I need to hear my engine at extreme RPM, not the exhaust.

[Comoestas]

Not sure if it applies here but as a loudspeaker designer, if you increased the diameter of the pipe it would effectively double the length of the pipe at the same wavelength. For example: @2" I.D. x 28", or @3" I.D. x 56", and so on. This is strictly for tuning a specific port frequency and there is no hard and fast resolution because it is alignment dependent. Cool video!! I've never seen science like this applied to an exhaust. I'm glad I was able to learn something different.

[stAxelKain]

When I built the Helmholtz resonator for my 454 burb, I actually got a more effective result by calculating by the affected RPM, after trying it by a frequency reader like you did, which ended up needing to be a little bit shorter, and solved my drone completely at 1800 rpm!

[steve]

What do you think about using a slip-on extension to your resonator so you can adjust the length and tune the sound of your exhaust

[El_Chompo]

You remind me of my old days when I cut off my exhaust system and would drive around at exact RPMs so that it wasn't obnoxious xD I had such a small window I swear there was like a 100 rpm band to keep it in or else it would be sooo obnoxious.

You are absolutely right, the large 4" exhaust is the primary reason behind the drone. Too bad you have fancy muffler tips, if you had a normal exhaust pipe coming out the end you could easily temporarily slip on a reducer or downturn or reducer + downturn combination just to see how it sounded because I'm sure it would make a huge difference.

To understand why the size of the pipe makes such a big difference in drone, if you make an annoying droning sound with your voice and open and close your mouth from wide open to barely open, you will instantly understand what is going on with your vehicle exhaust. When your mouth is wide open, it's like the room you are in is part of the resonant chamber. You are basically INSIDE and not separated from the noise generation. Once your mouth is small most of the noise gets captured inside and resonates inside of your mouth, and the room your in will be much more quiet, because now there is a separation between the source of the sound and the room you are in. It's exactly the same with exhaust. When you have a 4" exhaust, it's essentially like living inside of the engine. There is nothing separating you from its sound, you have become part of the sound chamber. Now when you reduce the muffler exit size, a lot of the sound starts reflecting back up into the system which captures it and creates a separation so that now you are no longer INSIDE the sound chamber, you are outside of it. And once you are outside of any sound chamber compared to inside of it, the difference in drone will be huge. That is essentially what drone is, being inside of a sound chamber that resonates.

And let me tell you, if you put a reducer on the end of your exhaust pipe to go from 4" to 3", I bet it would be hard to even notice a difference on a dyno. But it would make a huge difference to your ears. If you could easily slip them on and off with clamps it would be fun to try different sizes for science. If you reduced the end from 4" to 2.5" or even 2" the drone reduction would be so incredible it might be hard for you to believe. The ideal situation in my opinion is having a cut out so that you can have a nice quiet system for normal driving, but if you want 100% no restrictions maximum performance to put the hammer down, you can open the cutouts. That way it's quiet the majority of the time, and the tiny fraction of the time that you want maximum power you can have it too without annoying everybody's ears all of the time including your own.

[blairanthony]

May I suggest an adjustable resonator. Similar to a trombone slide. Fully tunable an adjustable

[flubyux]

Just to be clear, it’s a hemholtz resonator. The drone frequency happens in the range of rpm that is a multiple of how many cylinders you have. The temp of exhaust gas will alter that rpm point a little due to the speed of sound at those given temps. Cold exhaust pipes will resonate at a slightly diff rpm than hot exhaust pipes. And you either choose the rpm at which your engine drones/resonates or the rpm at which you spend the most time driving in. These rpms are usually different. So it’s ultimately up to each persons needs to determine the final length of pipe.
Edit: ps: the diameter of the resonator should be the same size as the exhaust pipe it’s connecting to. Sometimes that’s not possible so the equation just has to be adjusted.

[prjndigo]

Make an 9x11x17 inch can with the through holes at your 4" diameter but offset to one side on the input and dead center on the output then run a set of 4.5 inch holes through 9 baffles that are spaced progressively tighter towards the rear in the line from the input to the output. You'll enjoy the results, doesn't need to be perfect. It can go after your rear muffler with a dumb tip on it. Mid 90's I was playing around with ways to reduce the shock-pulse of the exhaust of aircraft engines on air-boats and came across this design - it is mathematically a nautilus first derivative.

[leebarker]

15:27: "um...pick my spot wisely." What a jewel of a moment. Thanks for the science and the smile.

[VJohnny]

Hey man nice work on this is something that I am planning on doing on my exhaust on my truck as well. I've played around with exhaust damper weights and they work as well but the quarter wave resonator is something I'll be definitely doing myself.

One idea that I've had when I do mine, is to get a slightly larger diameter piping for the very end so that you could slide it over the main section of the quarter resonator so that you could fine-tune it. And then you could use a lap joint clamp so that you could tighten it down once you find your right length. It's just a thought because it would allow you to gain a few inches or shorten it depending on your targeted frequency. When I had spent a lot of time researching this over the past year I found that 28 to 35 seem to be the ideal length for a lot of the V8 vehicles but it's not always easy to confirm which one is best per application.

Nice work

[russbayne]

Persistence pays off LT, I’m glad you got it the way you wanted it 👍😀 great job and nice work

[bobbystubblefield]

That exhaust sounds great, like the in-depth approach you took to get it right!

[terryvincent]

I built a pair of these for my 04 Titan after header install. Worked amazingly well!