Low buck exhaust. Drone at 2k

Discussion on fuel delivery & intake/exhaust tech & mods.
4flats
Posts: 15
Joined: Wed Oct 21, 2020 8:14 pm

Low buck exhaust. Drone at 2k

Post by 4flats »

So I picked up a 2013 caprice and hacked off the back mufflers and I hate the sound. It sounds fine at idle and WOT but at 2k I want gouge my ears with an ice pick.

I am never going to throw big money at this car but I'd like some rumble with out alota' cha-ching.

Cats are a must as I don't mess with carbon monoxide.

A hot rod "know it all" says If I extend the exhaust out past the rear bumper it won't drone at 2k. Says that is the sound bouncing around under the car. Thoughts?

Has anyone ever deleted the forward muffler and kept the rears? Or even just added flow masters in the back?
4flats
Posts: 15
Joined: Wed Oct 21, 2020 8:14 pm

Re: Low buck exhaust. Drone at 2k

Post by 4flats »

I've read a few posts about the J pipes but they are out of budget for this rig.
CapriceX2
Posts: 85
Joined: Sat Jun 22, 2019 10:06 am

Re: Low buck exhaust. Drone at 2k

Post by CapriceX2 »

You just hacked the mufflers off and let the pipes terminate under the car?

Extending the pipes past the bumper should make a significant difference. Based on what I’ve read from others, I’d guess you will still have SOME drone. It is cheap enough to do that you might as well try it and see if it is tolerable for you.
CapriceX2
2011 Caprice 9C3 - TSP 220R W/ DOD delete, 10.8:1 Compression, Custom Stainless Exhaust
1995 Caprice Classic Wagon
1996 K2500 6.5L diesel
DGBryan
Posts: 21
Joined: Wed Jul 24, 2019 3:18 pm
Location: Germantown, TN

Re: Low buck exhaust. Drone at 2k

Post by DGBryan »

In my experience with this, the drone is only a factor when the engine is in 4 cyl. mode. The intermittent nature of the drone is your cue when the 8 cyl./4 cyl. transition occurs. Those big fat OEM mufflers cancelled out the drone but made the entire system very quiet. My cure was to order a Range Technology Active Fuel Disabler. It will keep the engine in 8 cyl. mode all the time, thereby eliminating the drone. At roughly $200 it may be your most cost effective fix.
lastcall190
Posts: 604
Joined: Mon Sep 11, 2017 5:17 am
Location: North Jersey

Re: Low buck exhaust. Drone at 2k

Post by lastcall190 »

I'm gonna echo what CapriceX2 asked... if the tips are behind the rear bumper now, then my goodness extend them out. When I put my G8 system on before having the shop extend the tips, it felt like someone was holding a subwoofer right up to my brain. Some of the magnaflow systems are what you described (no mid muffler, and two rear mufflers) and mine was quite pleasant, even the wife liked driving the car and kids didn't have any complaints on trips.

I understand that drone is wildly subjective, but putting the tips out from under the car will help a great deal I'm willing to bet.

-J
2012 Caprice
2008 CVPI

Both gone but not forgotten...
4flats
Posts: 15
Joined: Wed Oct 21, 2020 8:14 pm

Re: Low buck exhaust. Drone at 2k

Post by 4flats »

I have only deleted / removed the rear mufflers, so the exhaust is duping under the trunk.
Yes the 4 cyl mode is horrible, I am debating the ODBII plug in vs a programer vs a full tune.
4flats
Posts: 15
Joined: Wed Oct 21, 2020 8:14 pm

Re: Low buck exhaust. Drone at 2k

Post by 4flats »

I found a shop that will do custom work. Getting hard to find these days BTW.
We are going to:
1. Extend the pipes out the bumper and run it see how I like it. If it's good to go I'll rock that for now
Option 2. Add Magna flow or Flowmasters at the trunk
Option 3 Straight pipe the mid muffler and run the option 2 at the trunk
Option 4 put it all back to stock.

So Tuesday, I get the out the bumper extensions.

I did learn that the sound I wanted comes from a full cat delete but I am not going to go that route for health reasons.
4flats
Posts: 15
Joined: Wed Oct 21, 2020 8:14 pm

Re: Low buck exhaust. Drone at 2k

Post by 4flats »

So, I had the exhaust extended out the rear bumper. The motor noise is less but the Drone at 1,700 to 2,000 RPM is sitll unbearable!! I've run swamp buggies with straight open headers that were quieter than this harmonic going on in this cab!

I'm ready to try custom J pipes or something, I can't stand it. BTW the actual engine noise isn't bad.
4flats
Posts: 15
Joined: Wed Oct 21, 2020 8:14 pm

Re: Low buck exhaust. Drone at 2k

Post by 4flats »

So I found this post on another forum. By Geroge C

The concept behind drone is pretty simple; nearly everything has a resonant frequency...exhaust systems, electronic RLC circuits, even gravitation has resonance. At peak resonance, the amplitude of the wave at the resonant frequency shoots up to many times higher than normal amplitude. With sound resonance, it pops up when a certain frequency emitted from the engine resonates with the exhaust system and muffler to create that nasty in-cab drone. Unfortunately this resonance often has its peak right in the area where you want to cruise, between 55-75mph.

The solution is a 1/4 wave resonator tube. Simply put, it is a small piece of exhaust pipe with a flat capped end welded nearly perpendicular to the exhaust flow post-muffler that serves to take some of the resonant amplitude and shoot it back into the exhaust pipe out of phase with the resonant frequency to bring down the amplitude. If you've ever taken a physics class, this is in the 'standing wave dynamics' chapter. Mufflers are designed to bring down the amplitude or volume of the exhaust note at all RPMs, but they typically can't do anything about resonance. Corsa mufflers have a built-in resonator that works decent, but only with the entire exhaust kit. Sometimes you'll run across a setup that drones at an RPM the Corsa muffler and exhaust wasn't designed to combat, so theres a chance the internal resonator will do little to no good.

Things you need to know to build the resonator:

-RPM that drone occurs at
-Number of cylinders (engines with simultaneous firing cylinders like the SRT-10 truck will divide this number by two)

You will also need to know the speed of sound. It varies with temperature, but is generally accepted to be 343m/s.

Say your V8 truck resonates at 2000rpms, right at cruising speed. You'll need to find the frequency of the drone. Units here are pulses per second, or Hertz.

f = RPM * pulses/rev * (1/60)

f = 2000 rev/min * 4 pulses/rev * (1/60)

f = 133.34 pulses/second = 133.34Hz

At 2000rpms, your V8 fires 4 times per revolution. RPM is in minutes, so divide by 60 to reduce to seconds. Now you need to find the length of the wave at the frequency you just calculated. Wavelength is denoted by lambda (λ), units are meters.

λ = v/f = speed of sound / frequency

λ = (343m/s)/(133.34Hz)

λ = 2.572 meters

This gives you the length of a full sound wave. The idea here is to reintroduce a sound wave into the exhaust that is 180° out of phase with your drone frequency. To do this, you build your resonator tube at exactly one-fourth the length of the resonant wave. By the time the sound wave enters the resonator tube, bounces off the end and re-enters the exhaust stream, the amplitude is exactly opposite of the drone frequency and will lower or eliminate the volume of the drone.

Dividing the wavelength by 4 gives you 0.643m, or about 2.1 feet. Its long and kind of awkward, but someone who has had to deal with a droning truck on a road trip will likely sacrifice the space to make it fit

The diameter of the tube you'd need is debatable, as I haven't tried different sizes of tubing. When I built mine, I took a shot in the dark and went with 2" pipe on my 3" exhaust. It gives about half the surface area at the end of the tube to bounce off of. I assume a larger resonator tube would allow more sound to bounce back and cancel more of the drone noise. However, space is somewhat limited when you are sending an exhaust pipe off of your existing pipe at some odd angle. You can bend the resonator tube to some extent, but it needs to be mandrel bent, not more than one bend or more than ~30°. If the bend is too sharp, the sound will bounce back too early and the resonator will work poorly or won't work at all.

The reason for this is simply the fact that sound waves do not necessarily 'flow' with whatever medium they are travelling in. Sound is a pressure wave, and any obstacle (bends) will return the wave prematurely. To minimize 'obstables' in the resonator tube, the straighter it is, the better it will work. The intent is not to randomly scatter the sound wave as a radius will tend to do, but to send it back to the exhaust precisely when it is needed.

So, you just need to mock up your resonator tube in a place that it will fit, cut a hole into the exhaust pipe and weld the tube to it and enjoy a drone-free ride.
4flats
Posts: 15
Joined: Wed Oct 21, 2020 8:14 pm

Re: Low buck exhaust. Drone at 2k

Post by 4flats »

Well here is the data:
Drone at 1800RPM in 8 cyl and 4 cyl
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