Quiet Flow with Resonator
Drone Cure for Kolak (or other) 3" Custom Exhaust

Side Branch Resonator & Muffler

This mod relies on a simple side branch resonator and WalkerQuiet Flow muffler, both shown above.

A Quieter Modification of the
Kolak Performance and Offroad
Exhaust for V8 Jeep Grand Cherokee
WJ series (1999 - 2004)

Kolak's exhausts are generally well regarded within the Jeep performance community. Unfortunately I was disappointed with the setup provided for my '99 JGC Laredo ( 4.7L V8, Quadra-Drive, up country, tow package). I found it was too loud and drone prone for my needs. (What can I say, I guess I'm old!)

Fortunately, after a lot of homework and three followup attempts, 
I'm happy to say I finally have a configuration that fixed the noise issue to my satisfaction.

Here are the components I am running:

•  MagnaFlow 94409 catalytic converter / resonator
•  2.25" x 30" custom side branch resonator
•  Walker Quiet Flow 21054 muffler
•  Kolak 3" mandrel bent tail pipe

This is not intended to be a one size fits all recommendation. Consider it to be more general guidance. Details may vary according to circumstances
. My suggestion would be to first install the Quiet Flow muffler, then check for drone, and finally add a resonator tuned to eliminate any actual drone. An alternative, if you're happy with the overall sound of your exhaust and just want to cure drone, would be to try to figure out a way to fit a resonator while retaining the existing muffler.

My Experience. When I first contacted Kolak about an exhaust I explained that I wasn't young anymore and I did not want a loud system. Kolak recommended that I go with a "stealthier" MagnaFlow muffler rather than one of the Flowmasters he typically supplies. He provided a system consisting of his custom tail pipe, a MagnaFlow 12289 24" case muffler and MagnaFlow 94009 high flow catalytic converter.

I still found this system to be too loud, particularly the drone at expressway cruising speeds (1,700 - 2,200 RPM). There was also cabin interior resonance (a "beating" sensation I could feel on my ear drums) around 1,600-1,700 RPM. Kolak's response was that most customers are happy with the sound level of his systems, but that sometimes variations in the installation might cause drone. He also mentioned that the early 49-state-emission WJs without pre-cats do tend to be louder. In an attempt to address my issue he replaced the catalytic converter with a larger MagnaFlow model 94409 which includes a resonator. The new converter did quiet the system somewhat but there was still annoying drone at highway cruising speeds.

Still dissatisfied, I had the system completely reinstalled by a different exhaust shop, one with a top-notch reputation for custom work. My hope was that a better installation might correct any other issues contributing to the drone. I also had the MagnaFlow muffler swapped for a Walker Quiet Flow model 21054 - the quietest muffler I could find which would fit and still provide good flow with the 3 inch pipes. This system was significantly quieter overall but there was still obnoxious drone, though now it was at 1,500 - 2,000 RPM. These changes had moved the drone downward about 200 RPM, trading some of the cruising drone for still annoying around-town drone.

Finally, after spending a lot more time researching the cause and possible cures of exhaust drone, I had my shop fabricate and install a side branch resonator. I tried to tune the resonator length close to the center of my drone band, but leaning a little toward the upper RPM.

I was really pleased when the new resonator actually eliminated nearly all drone. It seemed almost magical. Only a slight drone remains right at 1,500 RPM. Perhaps that could also have been eliminated if the resonator length had been tuned to work best at slightly lower RPM, but when I was considering the tuning I wanted to be certain to cover all cruising RPM drone and I wasn't sure whether one resonator could cover the entire range from 1,500 to 2,000 RPM. In view of this choice I am really happy it worked as well as it did over almost the entire band.

The exhaust is now far quieter than before. The overall sound is comparable to a fairly subtle original equipment high performance or heavy duty truck exhaust. It's not as deep, throaty and loud as typical aftermarket performance systems, but there is considerably more V8 character than with the stock exhaust. 

Most side branch resonator installations I've seen on the internet have been mounted on the tailpipe. However there is limited space under this vehicle, especially with a 24" muffler attached directly to the tailpipe, so the resonator had to be attached in front of the muffler and routed along the inboard side between the muffler and driveshaft. There is enough room there because the Quiet Flow muffler is relatively narrow and also has an offset inlet and center outlet (opposite of the Kolak and original equipment setup). That configuration shifts the casing away from the driveshaft providing more room on the inboard side. It might be possible to install a side branch with a wider muffler mounted in the stock position, but that would likely require some clever pipe bending to fit in the limited available space.

One additional consideration with placing the resonator in front of the muffler is that the temperature there is probably a little higher than it would be at the tailpipe. This would increase the speed of sound within the resonator. Which means the tuning length needs to be a bit longer to function at the intended frequency. Based on some later calculations, it appears that ~" to 1" longer tuning might possibly have worked better for hitting the center of my drone band. This assumes a resonator temperature of 165 F rather than the 150 estimate I initially worked with.

Below you'll find charts, formulas and links to calculators other people have provided to help determine the correct tuning length. These tools provide only estimates. Perfect tuning requires either luck or trial and error. One trial and error approach would be a temporary clamped-on resonator pipe that could easily be resized, and then replaced with welded pipe once the optimal tuning was determined. Fortunately tuning doesn't have to be perfect to do a decent job. I haven't seen one report of anyone trying this with estimated tuning and not realizing improvement.

It's easy to find information, misinformation and speculation about the cause of exhaust drone on the internet. After a lot of reading I concluded that the drone in my system was caused by resonance of exhaust gas within the tailpipe. This occurs when the tailpipe is effectively tuned to resonate at a certain frequency, similar to a pipe in an organ. The tuning is determined primarily by the length of the pipe. Whenever the engine speed reaches the RPM which produces the "tuned" frequency a resonance occurs within the pipe exaggerating that sound.

The actual resonant frequency can change somewhat depending on the temperature and composition of the exhaust gas. The sound level will also depend on the exhaust flow (a function of throttle opening in addition to RPM).
On some vehicles the resonance may be amplified by sympathetic vibration of the exhaust pipe. Whether this happens can depend on the mass of the pipe, the position and stiffness of its mounts and again on temperature. In some cases the floor and/or interior of a vehicle can also resonate at the same frequency making the problem even worse. I think those other factors are usually secondary. I believe controlling the resonance of the exhaust gas within the pipe is the most important factor in curing drone.

I have a further theory that the drone frequency may actually be determined not simply by the length of the tailpipe alone but also the length of the final muffler chamber (or chambers) in combination with the tailpipe. Connected together I believe they may form one (or more) longer effective chamber(s). This would explain why the frequency of my drone was lowered when switching from the MagnaFlow to the Quiet Flow muffler. I haven't been able to verify this, but I believe the final chamber in the Quiet Flow may be longer than the one in the MagnaFlow. This theory may also explain why my drone band is so wide, i.e., potentially there are effectively two or three of these combined "chambers", each resonating at a slightly different frequency.

Side Branch Resonator. A side branch resonator is a simple approach for eliminating low frequency exhaust drone. (This principle is used on a few original equipment exhaust systems, very large stationary engines, and also on engine intake systems.)
  • Capped pipe “T”ed into exhaust. 
  • Length determines tuning for target sound frequency. 
  • Sound wave for tuned frequency reflects back 180 out of phase and cancels that frequency from exhaust.
resonator sketch
The tuned length is equal to 1/4 wavelength of the target frequency. This means the reflected wave is 1/2 wavelength out of phase with the primary exhaust wave which results in
cancellation. Seems like magic but it’s actually physics.

There is some approximation involved with the tuning, since it depends on the speed of sound within the resonator and that is dependent on the composition of the exhaust gas and operating temperature, which can usually only be estimated.

The resonator tube size does not need to be the full diameter of the main exhaust pipe. There is some reduction in effectiveness using a somewhat smaller diameter tube for the resonator but it will still remove most of the drone.

With Different Converter or Muffler. I listed the exact catalytic converter and muffler I am using because I am very satisfied with my current setup. Many people have
successfully applied side branch resonators to cure drone using a variety of exhaust components. So there is no reason you could not do the same with the smaller (louder) 94009 converter that Kolak usually supplies. You would simply have to tune the resonator for the actual drone occurring with that setup. It could also work with a different muffler, but finding space to install the resonator might be a challenge.

Credit. Primary credit for sharing this concept should go to Graham at the Mustang corral.net forum who pioneered and wrote about it back in 2004. Lots of others also deserve credit for sharing their experience. All I've done is put all the useful information I've found together on one page. Also credit to Keith's Muffler and Brakes in Westland, Michigan for the fabrication and installation. 

My Side Branch Resonator Intallation Detail

resonator connection

resonator attach

resonator cap

Side Branch Resonator Tuning Notes and Sources.

Approximate (maybe a bit too short) 1/4 wave resonator length to cancel 8 cylinder drone

Resonator Length


V8 RPM Audio Frequency
Primary Sound frequency (Hz) generated at engine RPM (V8).
(Freq = RPM 15, where 15 = 60 sec / min 4 ignitions / rev.)

V8 RPM Drone Resonator Length
Resonator length (in.) to cancel drone frequency at engine RPM (V8) @ 150F

"To find the exact length*, take 54,491.4 and divide your RPM into it.
Say your drone is at 1900 RPM.
Take a calculator and type 54,491.4 / 1900 and it will equal 28.679 inches."


*Well not "exactly". I'm afraid the guy who wrote what I'm quoting is guilty of false precision. He's using really precise values (lots of decimal places), even though the margin of error may actually be 5% due to other factors. I think the length calculated with his formula also may be a bit too long, but is likely more accurate than the simple chart at the top of these notes. The perfect tuning length depends upon the actual gas temperature and composition, which we don't know for certain. So the charts and formulas above are only estimates of what might work best. They should be pretty close though. The calculators linked to below may help provide better estimates if you have better data.

Additional note in response to reader questions. For 4 and 6 cylinder engines the math relating drone frequency and resonator length to RPM is different than for eights. Since there are fewer ignition firings per revolution with fours and sixes compared to an 8 cylinder, the frequency of the primary exhaust note at any given RPM will be lower. (i.e., the wave length is longer.) This means that the resonator also needs to longer to cancel drone at that RPM. For a four cylinder it should be twice as long. For a six 1.5 times as long.


Quarter Wave Tube Calculator

Exhaust Gas Temperature Guess

Speed of Sound Calculator @ Air Temperature  
(Note speed of sound in exhaust gas is not the same as in air,
so this is only an approximation for the wave resonator.)

Estimated Speed of Sound in Exhaust I

Estimated Speed of Sound in Exhaust II

PerformanceTrucks.net Drone Cure Discussion

Mustang Corral.net Forum Drone Cure Discussion

Engine Exhaust Noise Control Presentation

Finally. It took some time to put this write-up together, but nowhere near as much as it took trying to get an exhaust I could live with. Without members of a couple of internet forums sharing their successes on other vehicles, it might not have happened at all.

So this is for any other Jeep Grand Cherokee owners and others out there who might be in the same situation.


Updated: 27 June 2012