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Old 06-29-2018, 07:48 AM
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Default sound reflection of exhaust in rear

At highway speeds I have the issue that the exhaust is causing too much interior noise to comfortably listen to the radio.
Now the engine has a dual exhaust system with a straight through type bullet style muffler in the middle and 2 straight through magnaflows in the rear. Exhaust sound in itself is good.
The exhaust tips are straight out of the back, so not angled down as was the case with the original style exhaust tips.
Exhaust pipes are 3".
I have heat shields over the pipes in the middle of the car and under the battery trays.

For some unexplicable reason I have in cabin noise coming from the rear compartment which seems to amplify the exhaust noise. I have a hard time explaining why this happens as I would assume that with the rearward pointing exhaust tips this shouldn't be reflecting into the car.
To me it seems that the mufflers which hang under the rear frame rails emit noise that reflects onto the walls of the cavity behind the rear wheel where they get reinforced.
Did someone here ever experience this and solve it ?

I was thinking of glueing some isolation material onto those rear cavity panels, but am a bit afraid that they might get wet and create other issues.
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Old 06-29-2018, 01:00 PM
Red77 Red77 is offline
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How rigid are your exhaust and muffler hangers? Sometimes a resonant frequency can transmit back through the mounts if it is too rigid and the rear compartment could act as an amplifier and speaker for this vibration. Is it worse at some RPMs and vehicle speeds more than others? Back when I was a Ford/Lincoln/Mercury, I dealt with NVH issues quite often. With straight through mufflers it may not be the typical droning issue that chambered mufflers often cause, so pay close attention to the mounts and vehicle and/or engine speed.
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Old 06-29-2018, 06:00 PM
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Quote:
Originally Posted by Red77 View Post
How rigid are your exhaust and muffler hangers? Sometimes a resonant frequency can transmit back through the mounts if it is too rigid and the rear compartment could act as an amplifier and speaker for this vibration. Is it worse at some RPMs and vehicle speeds more than others? Back when I was a Ford/Lincoln/Mercury, I dealt with NVH issues quite often. With straight through mufflers it may not be the typical droning issue that chambered mufflers often cause, so pay close attention to the mounts and vehicle and/or engine speed.
I think they are the plastic/rubber newer type of hangers (in all sorts of colors) from which the exhaust is suspended. In the rear the mufflers hang on the same rubber mounts as the originals and in the front I use the dual mount exhaust support that bolts to the transmission mount. Kind of difficult to check while driving
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Old 06-29-2018, 08:24 PM
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Been a while since I drove mine but I think your issue might be the floor of the rear storage area and the big open chambers that define the wheel wells. It's like a giant upside down bucket. And the floor in the rear storage area, behind the storage/battery wells is a large piece of unreinforced, unsupported fiberglass.
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Old 06-29-2018, 08:40 PM
Red77 Red77 is offline
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Try to make sure that none of it is able to touch the body anywhere. Mine had the newer style hangers on it and with simple no-name brand turbo style mufflers, it was too loud to hear the stereo while driving and would set off car alarms in parking lots while idling. BBShark may be onto something as well because the rear compartment area is basically like a big drum, so any sound pressure in the area is likely to get picked up. With a 79 model, you have that much more area/volume, so basically a bigger drum. My 77 has a later model rear clip on it, so I'm in the same boat. I'm planning on insulating the entire interior with Dynamat or something similar to combat this as I reassemble my car.

We used to have a Honda CR-V as a family cruiser and it didn't have any sound deadener in the rear other than the carpet. On smooth pavement it was fine, but on rough roads the rear compartment was NOISY. Noise comes from vibration whether it is road noise or exhaust, so if there is nothing to dampen it, you are going to hear it.
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Old 06-30-2018, 02:47 AM
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3" pipes are pretty big not sure how to quiet them down...maybe an x pipe or H pipe will eliminate some droning
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Old 06-30-2018, 07:52 AM
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Belgian1979vette Belgian1979vette is offline
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I already have an X in the exhaust. Right under the diff.

I'm going to glue some tarpapers on the inner panels. Hopefully that'll reduce vibration from exhaust pulses. I'm still baffled about how they could even reach the inside of the rear as the tips stick out ca. 5 inch behind the bumper

Last edited by Belgian1979vette; 06-30-2018 at 11:04 AM..
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Old 06-30-2018, 05:49 PM
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Quote:
Originally Posted by Belgian1979vette View Post
I already have an X in the exhaust. Right under the diff.

I'm going to glue some tarpapers on the inner panels. Hopefully that'll reduce vibration from exhaust pulses. I'm still baffled about how they could even reach the inside of the rear as the tips stick out ca. 5 inch behind the bumper
Try an H pipe up front, at the header collectors (or equivalent location). The forward location is more efficient for reducing drone, and it made a significant improvement in cabin noise in my '69.
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Old 06-30-2018, 05:50 PM
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Quote:
Originally Posted by BBShark View Post
Been a while since I drove mine but I think your issue might be the floor of the rear storage area and the big open chambers that define the wheel wells. It's like a giant upside down bucket. And the floor in the rear storage area, behind the storage/battery wells is a large piece of unreinforced, unsupported fiberglass.
I agree.
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