I think I have a deal on an AOD. Couple of questions. The C4 small bell housing and block plate on the car now had to be trimmed some on the driver's side to fit the 2.5" exhaust pipe. Will the AOD bell housing work with a similar trim or is the big bell going to be a bigger problem? If I need to have the exhaust pipes redone for the bigger bell, will eliminating the X-pipe possibly reduce the exhaust resonance?
I have found that moving or adding/removing X-pipe just changes the RPM at which the resonance occurs.
Resonance occurred at a higher or lower RPM without the X-pipe? Big change in RPM where resonance occurred? If the change was 2X that would be good. (Half the pulse frequency in each pipe might change it by 2X?) I get it now around 2500 RPM. Cruise RPM is currently around 3000 which would drop to about 2000 with the AOD. Moving the resonance down to 2000 RPM would be a bad thing.
When I was running without X pipe, Resonance was around 1500 rpm, Now it is 2500 with the crossover. Can't help you with the math...but it clearly changes the point where the noise lights up.
I don't have any clues for you but I always find the topic of exhaust intriguing. If you are going to make a change, and it wouldn't be too much trouble, please make a before and after video. If I had to guess, I would be scrutinizing the mufflers. If you don't mind it being on the quiet side, a Borla turbo or Borla ProXS turbo mufflers are a good choice. A straight through performance muffler that is irrationally quiet. I have long tube headers, 2-1/2" exhaust, H-pipe, DynoMax Ultra-Flo mufflers, 2-1/2" tail pipes and straight tips out the back. I do have some droning but I don't do enough highway driving for it to be an issue. The only change I made was from 2-1/4' to 2-1/2' tails. Definitely made the tone deeper, possibly just a hair louder. (probably wouldn't have heard the difference except I recorded the idle before and after.)
If I do change the pipes I'll do before & after videos. Won't be for a few months if I do it. I'm going to slowly gather the parts for the AOD swap to spread out the costs out over a few months. The exhaust system is Doug Thorley shorties, 2.5" exhaust pipes with X to Thrush Turbo mufflers, and 2.25" tail pipes. I was wondering if the smaller tail pipes might contribute to the drone so its interesting that yours got louder with the 2.5" tail pipes. Looks like that wouldn't be a good change for me. The Thrush Turbos replaced Dynomax 3-chambers because the Thrushes were going to be not so loud. That did quiet the exhaust but a subsequent 5.0 to 347 stroker swap brought the noise level back up again. I like the sound most of the time but it can get monotonous. The 347 also moved the drone RPM up a few hundred. I would like some quieter mufflers but not going to change them AGAIN.
Resonance Still thinking of ways to quiet the drone. Does anyone have before and after experience with insulating the trunk floor? I have some Peal and Seal like stuff that was sold at a swap meet as car insulation. I'd be willing to put it down if its likely to make a significant difference. Otherwise, I'd rather not. Another possibility would be to put some very short glass packs at the rear of the tail pipes. Anyone think that would significantly reduce the drone?
I have a 347, Doug Thorley shorties, 2 1/4" pipe, Aluminized "H Pipe", Magnaflow Polished Stainless Steel Mufflers, Polished Stainless Steel tailpipes. I have NO drone whatsoever.
Similar exhaust systems but mine has an annoying resonance. It was worse before I replaced the 3-chamber Magnaflows with Thrush Turbos. When my bro and I drove down to the 2011 NPD show he was wearing ear plugs. The switch to Thrush Turbos made it tolerable with the 5.0 but the 347 brought the noise level back to annoying. Main differences between our exhaust systems, when I had Magnaflows, are X-pipe rather than H-pipe and my tail pipes are 2.25". My tail pipes are tucked up in front of the rear valance with turn-downs. To see if the tail pipe ends might be the problem, I stuck short extensions on them that took the exhaust out past the valence. Made no difference. Wouldn't cost me anything to insulate the trunk floor but I don't want to install the Peal and Stick-like insulation if its not going to help.
Have you got the bottom dirty yet? Just couldn't resist asking. Jim, I have 6901's, 2.25 pipes, no crossover or X pipe, Flowtech Redhots glasspacks, pipes exit under the rear valance straight back. NO drone since I went over the axle and out with the pipes. Not very loud at cruise, either. It will talk when you punch it, but quiets down as soon as you level out your speed. Like you said, you might add some of those glasspack resonators at the rear to quiet it down some more. A lot of the big luxury cars did that when they had big blocks.....
That picture was taken at the muffler shop when the car had ZERO miles on it. It has been about 4 1/2 years since those pictures were taken and about 1000 miles. So to answer your question....the bottom is dusty.
I'd like that sound. The resonance gets monotonous when just driving around. Maybe I can think of some temporary insulation to put down in the trunk to see if it helps. Since even luxury cars don't insulate in that area I'm skeptical that its going to help much.
the build of the motor has allot to do with the exhaust drone. the one thing i have noticed is the bigger the exhaust pipe size more chance of a drone. there is a thread on a mustang board (cant find it) that talks about t ing in a section of pipe in the tail pipe that is about half as long as the tail pipe and capping it off. the sound will enter the pipe and bounce back out 180 degrees out of phase. this is supposed to cancel the resonance. there several responses in the thread verifying its success.
