Finally enjoying my catback exhaust...no more drone!

I purchased an MRBP cat back about a month ago for my '14 TRD sport (DCLB). I know we can't make a V6 sound like a V8, but like others on the site I've been looking for a more aggressive sound at idle and acceleration. The MBRP exhaust is great, nice deep tone but as others have reported, and similar to other catback systems, it has the dreaded 1,500rpm drone. It seems to be most pronounced in 5th gear, but not limited to that scenario.

What I've being doing this last month as a workaround is to drop down to 4th to bump up the rpms and get out of the "drone zone". I'm thinking at this point maybe just giving up and putting the stock system back on to eliminate the drone. However, before admitting defeat, this evening I was doing some research about how to eliminate or reduce the drone without having to abandon the catback. It sounds like some have used a glasspack or resonator in front of the muffler with success, but I don't really want to quiet the MBRP or I might as well go back to stock.

In my reading of several threads it kept coming back to the resonance between the exhaust system and the vehicle resulting in that magically annoying tone. Some posters on various owners forums have found stupidly simple solutions, each specific to their system- for example, someone mentioned wrapping the exhaust to change the resonance, others talked about drilling a hole and using a bolt to change the frequency etc. What was interesting is that several people talked about changing/modifying the piping after the muffler and not before the muffler to reduce the drone. I then read a thread here noting that the poster tried a resonator or two without success, so maybe that wasn't the best approach

I went outside and crawled under the truck with my trusty headlamp and looked at the assembly from the rear of the muffler. There is a section of pipe that exits the muffler and goes up and over the rear axle, where it then joins the final section that comes down and makes a 60 degree turn to the right where it exits behind the passenger side rear wheel with a nice stainless tip. I removed the tip first to see if that would make any difference, and surprisingly it did change the tone. As the tip expands from the 2.5" piping up to 3" it seems to produce a bit of a megaphone effect. I went for a quick drive to see if that changed the equation under load, but the 1,500rpm drone was still present.

I then tried removing the last section of the piping from the system (i.e. coming down after the rear axle to the exit) and I didn't really hear much of a difference at idle, but I decided to test drive for the sake of science...and voila, the drone is gone!! As a final test I re-installed the tip at the axle dump section and it lowered the tone at idle again (due to the megaphone effect) but the drone was gone under load.

So, it seems like the final bend in the system was the likely cause of my drone and by removing that piece (1.5' of pipe with a bend) it's now a nice mellow exhaust note that doesn't overpower the radio or conversation at 1,500rpm. Thought I would post in case anyone else was in the same boat; if you have a catback which assembles in sections then it's pretty easy to test.

 

good to know info !

thanks for sharing