06 T-RED S/C GT
forum member
Even with the stock exhaust in place, Flowmasters have a horrible drone to them, to begin with aka Dronemasters lol. Once you modify from the stock exhaust by eliminating the OEM cats and especially going to LT headers, the frequencies that produce interior resonance drone are amplified twice as much. With the stock exhaust in place, there are 5 aftermarket axle back exhaust mufflers that are designed to either reduce or eliminate interior resonance drone altogether. Corsa performance exhaust, followed by Ford Racing M-5230-S aka GT500KR axle backs, MBRP, Roush extreme cat-back exhaust and Borla touring axle backs which are also an excellent choice for off/road, LT header applications for keeping resonance drone to a minimum. All are straight thru design, rather than chambered as the Flowmaster/Dronemasters are lol. Other than for the Borla touring axle backs, I can't really say for certain how effective the other exhaust systems mentioned would work out on off/road, LT headers applications, but have read on this site along with other Mustang forums that the Borla touring axle back mufflers do in fact work very well, so you may want to consider trying out the Borla Touring mufflers for your particular application. As for Helmholtz/J-pipe applications? there are 2 manufacturers I'm familiar with. Synapse engineering, no drone resonators http://synapseengineering.com/synapseusa/products/nodrone.html and Solo performance exhaust. Just keep in mind that Solo performance only offers J-pipe/resonator exhaust systems for the S197 2011-14 and current S550 2015-20 Mustang to the general public. However, they may be willing to custom fabricate a one-off system for your particular application? Anyway, here's the link if your interested in contacting them directly https://shop-solo-performance.com/en/31-ford-mustang Meanwhile, hope you find the info provided as useful.Just got my '07, and it has serious drone problems, first at 12-1500 rpm and again just past 2000. Above 2800 the sound sweetens, but it's garbage down low. JLT3 up front.
Original owner installed BBK LT's - 1 5/8" to 2 1/2", followed by BBK catless x-pipe and Flowmaster Am Thunder - all 2 1/2". Somewhere along the way a PO added M-80's before the axle - no doubt to try to deal with the drone. They didn't work, and with 1" lowering springs they occasionally bottom on hard bumps at higher speeds.
This car is a cruiser, with no plan for track use, so I'm OK with running a lot quieter so long as the tune doesn't get messed up. It appears to be a Bama 93 that came with an old Diablo. Not crazy about the tune, but it gives good economy at cruising speeds. As an old hot rodder buddy put it, a lot of these canned tunes seem to feature hair-trigger throttle response off idle to make you think your car is a fire-breathing dragon.
At some point I'll want to add some grunt to the mid-range, but for now the priority is getting rid of the drone. It seems there are two choices for doing this: replace the Flowmaster AT's with stock-like mufflers, or add a Helmholtz setup. So far, this site's Search function hasn't returned any first-person accounts of a successful Helmholtz installation, but I'm willing to spend a little MIG wire on 2" and 4" pipes to fabricate a system.
The math is pretty simple, so long as you know the offending frequency - which by all accounts is around 80 Hz in the 12-1500 range. If anyone has a more specific number please let me know; I was using a freebie sound spectrum phone app that jumped back and forth between 60 and 120 as I droned along. A Youtube Helmholtz site featured a little chart that suggested a V8's problem frequency is right around 80 at the lower rpm range, so that looks like a good bet to plug into the equation.
The value I'm less sure about is the temperature of the exhaust gases within that 12-1500 range. I have no idea how to measure that within the area of the resonators - short of installing a set of old snowmobile EGT gauges that I used for tuning a road race car. After the 4.6 is warmed up, idling exhaust at the tailpipe is 175 degrees F, but I'm thinking that the critical vibration is coming from up around the M80's. I'm guessing it's 300 degrees F at 1350 rpm there, and have included the recognized speed of sound for 300 in my equation. If I install the EGT's, it will be through holes that can then be enlarged to weld in the J-pipes.
So, with 300 degrees and 80 Hz, the Helmholtz equation indicates that a 10.5" J of 2" pipe into a 4" can that's 8" long will effectively cancel the drone.
All suggestions, comments, criticism and so on are welcomed - including advice on moving this request to a different forum. Sure, you guys have beat the exhaust question to death for years, and every car is a little different with its tune and what-not, but S197forum does not seem to have much practical advice ion Helmholtz that I could find. Plenty of good ideas about exhaust and noise, though.
Enfield - 2007 GT convertible 4.6 5-spd with JLT3, BBK LT and catless X, M80's, and Flowmaster American Thunder axlebacks. Also lowered, and with 3.73's.
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