Drone Warfare

There is a deep primal urge within the lizard-brain of every car enthusiast. It usually surfaces when said enthusiast gets a new vehicle, but it can appear also when routine maintenance is performed on the exhaust system. The desire to undo countless hours of fine engineering work and replace the OEM muffler with something a bit louder is as strong as it is irrational. Rarely is anything besides a few decibels to be gained in doing so, yet we persist and do it over and over again.

One of the greatest risks in changing the muffler is that after spending a little time and often, a lot of money, the exhaust will drone. Drone is an unpleasant experience that happens when the sound waves from the newly loud exhaust resonates the body structure of your vehicle at it's natural frequency. It makes you feel like Manuel Noriega in your self inflicted NVH torture chamber.

I recently did this to myself. Not with Grace, but with my Jeep XJ daily driver, the TURD. The original 1998 exhaust system was on its way out and I got that urge. I picked up a glasspack from Summit racing for about $30, since my friend in high school had owned a glasspack equipped XJ and I remembered it sounding good. I paid an exhaust shop to bend up a new exhaust system and install the glasspack with a new catalytic converter.

Maybe I've just gotten old and maybe teenagers are immune to drone but as soon as I drove the Jeep, I knew I'd made a terrible mistake. The drone at 1500 RPM and 2800 RPM was unbearable, and it sounded so ricey I was a bit embarrassed to drive it. Remember, this is a beat up Jeep with a badge on the rear that says TURD, and the sound was the embarrassing part. I used an app on my phone called Sound Analyser to look closer at the problem areas, and found my biggest issue to be sound levels at 145 Hz.

Sound level readings with glasspack installed. Red areas show unpleasant drone.

After a bit of research, I came across the Flowmaster Super HP2 muffler. It would fit right in place of the glasspack and had a pretty good tone, from what I could find on YouTube. It wasn't cheap, but I figured I'd give it a try.

Top: Flowmaster HP-2   Bottom: Summit Glasspack

Inside Summit Glasspack

Inside HP-2 Muffler

The HP-2 or Hushpower muffler is not the traditional Flowmaster chambered "mousetrap" muffler design, it's closer to being a "straight through" muffler, without actually being straight through. It helped overall sound levels, as well as tone. While it took away the embarrassment of driving my Jeep, it didn't quite fix the droning when at 1500 and 2800 RPM. From my sound level 

Tailpipe as made by exhaust shop

Tailpipe after my modification

Over  the past couple years, I've done a bit of research on different types of noise cancelling resonators for exhaust systems. Most people thing of absorptive resonators when they think of an exhaust resonator, but those don't cancel noise, and they only absorb high frequencies. My issues were centered around 145Hz, which is a relatively low frequency. Of the noise cancelleing resonators, the most common is a Helmholtz resonator. A Helmholtz resonator is like an empty soda bottle that you blow across the top of. A wave resonates inside the bottle, and comes out amplified. If sized correctly and included in an exhaust system, this can actually cancel out sound waves of a given frequency. These are actually included inside many OEM muffler designs. 

A quarter wave  or branch resonator is very similar in function and can be similar in form. It is simply a capped pipe that is teed into the exhaust system at 90 degrees. At certain frequencies, depending on the speed of sound (influenced by temperature inside the pipe) and the length of the pipe, noise is cancelled. After doing a few fairly simple calculations and educated guesses, I chose 30" to cancel my 145 Hz drone. Link to my 1/4 wave resonator calculator. The construction of the resonator was simple and can be seen below.

This is where the resonator tees into the main exhaust system.

I was pleasantly surprised when I went for a drive after finishing. My calculations were correct, or at least correct enough to do what I wanted. The drone at 2800 RPM was completely eliminated, and the drone at 1500 RPM was actually reduced. This can be seen in the plots below. The first plot shows an acceleration without a resonator, the second shows acceleration with a resonator. In the color plots, you can see that the red spot is turned light green, and in the upper chart, you can see that the 145 Hz region was reduced from 74 dB to about 61 dB. A 13 dB reduction means that exhaust sound energy was reduced 20x. The average level (RMS) was reduced by 11 dB. That is an unbelievably large improvement.

I can't say enough about how effective this method is in reducing unwanted exhaust noise.  I was beyond pleased with the result. I've finally found an effective method to please my adolescent lizard brain with a loud exhaust while simultaneously using physics to appease my ever advancing old-man "get off my lawn" requirements.

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PS: This blog post was done, and it sat in my drafts box for a couple weeks. I didn't want to post it without a video of what it sounds like. With the new (old) house I just hadn't had time to do it. Today I finished up the exhaust job and had time to make the video. First I'll show the downpipe fab and then show the video.

I had an exhaust shop do the rear part of the exhaust because it was pretty straightforward. At the front, it was going to be a little more complicated. There were two items to deal with:

1. I didn't want to mess with a two-bolt flange on the header because the bolts always rust and then the nuts are impossible to get off. Instead I wanted to have a V-band connection. To do that, the header needed to come off
2. I also wanted a large diameter, non-crushed downpipe. As you'll see in the pictures below, the downpipe was significantly crushed. It's a common issue on XJs, I guess as the springs sag, the front driveshaft's front u-joint smashes the downpipe and restricts flow. I figure mine was crushed to about 50% of it's original area

Now for the pics:

Crushed downpipe

Left to right: OEM downpipe size, crushed downpipe, new downpipe size

Expanding the collector to mount v-band flange

V-band flange mounted to collector

Manrel bent XJ downpipe

The new downpipe did just what I hoped it would and increased top end power. Previously, the power would die out above about 4000 RPM. Now it pulls very strong from 4000-5000 RPM. Mission accomplished.

For your viewing pleasure: This is what it sounds like. I like it. I think it brings out the inline-six sound nicely. It's not too loud, but it lets you know it's there.

PPS: This is my Michigan Garage V1.0. It's an almost 2 car garage. The structure is musty and starting to lean, so it will probably be torn down and replaced with V2.0 next year. Since most of my attention will be focused on the house for the next year or so, it'll do.