Controlling Cabin noise/ exhaust drone in a C4
 

My Car (1991 model) is deafening - on the inside!.... and I mean I think it is literally doing damage to my hearing! I often drive the car to meetings, 1-2hrs away.... it sometimes take a few hours for my hearing to come good again!
The engine is modified with Cam & computer (not sure on specs as it was done when I purchased at auction)... It has headers, and a full 2.5" dual exhaust, with 1 into 2 rear mufflers, and the exhaust drone is the main culprit on noise.

My exhaust guy said it's the 1 into 2 mufflers that are the cause - essentially the noise hits the wall where the muffler splits in 2, and bounces the noise back into the cabin, and he has suggested a straight through system will go a long way to improve the problem.
As pretty much all C4's I have seen have dual tip mufflers either side, I'm thinking this is either a common problem - or not the problem at all... Anyone have any experience to offer?

I was also looking today at the AcoustiSHIELD insulation kits being sold by ZIP - are these worthwhile.

I love driving the car, but it would be much more enjoyable if I wasn't deaf when I got out, or if I could actually have a conversation with a passenger in the car!

it's the mufflers, most free flowing mufflers drone.

Gibson and Walker are quitter.

The closer to stock the less drone.

I have an 85, insulating the rear compartment did not help much

I was told plugging one of the outlets on each helps a lot. Maybe try that and if it gets better perhaps invest in different mufflers? IIRC once you go to headers there will be some drone no matter what you do.

Don’t know what mufflers are on currently but the Walker Quiet-Flow are basically a little better flowing stock muffler with two functioning tips

Corsa

Headers will not give you drone. The mufflers control the drone.

If you really want a quiet ride - go to Turbo mufflers which just flow a little better than stock and will quiet the ride.

I run Quiet Flows on my '86 with a Dynomax cat-less front Y-pipe and main cat delete and its quite tolerable. There's still some bass to it and the exhaust note has a bit of texture like a stock late sixties C3 but I wouldn't call it loud. I imagine cats, they'd be even quieter. I know where you're coming from. Drone can even be painful depending on how bad it is.

The resonance you describe is basically coming from your mufflers, how loud it gets depends on everything else in the system. Headers and true duals of whatever diameter will only make it louder. The C4 coupe has that huge resonance chamber right behind the driver that makes it worse too.

Corsa catbacks are probably going to be the best way to cut it down, but typically what I'd recommend is putting in crossover (H/X) pipes to eliminate it. If you did both, you'd see a big reduction. Corsa tends to be very quiet in cruising or idling and only loud when you stand on it.

Blocking up one outlet of the 2 on each muffler with something like a Brilo pad does help a little but is not a long term fix. Adding sound insulation, especially behind you, is not a bad idea but is not a true fix either.

I have an 85 and the drone was so bad , I didn’t enjoy driving the car. I was going to install Corsa, but decided to try Corvette Centrals ‘no drone’ mufflers. I was very skeptical, but have to say, there is no drone, and my car is enjoyable to drive. I hope this helps

Another good reasonable price solution is to find a pair of stock LT1 mufflers and swap them on. They flow well and don't drone. I have a set
on my car with a cam, headers, and no cats. No drone but they are plenty loud when you lean on it. It may not be as simple as i make it seem
but if you run across a set, its a good option.

I had several mufflers on my prior '88 and couldn't get rid of drone till the Corsa cat-back.

If you need a quick and cheap solution, try plumber's test plugs (steel, not plastic) in the inner outlet of each muffler -- leave the outer outlet open. I did this on the three-inch B&B system that was installed on my car by the previous owner, because the resonance was just unbearably horrible. It made a huge difference. I replaced the wingnuts that came with the plugs, using standard hex nuts instead, and squirted the visible parts flat black which made them nearly invisible. The long term solution, which I eventually went with, was a Corsa cat-back. It doesn't drone.

Live well,

SJW

Thanks guys, I can see I'm not alone!
Looks like the Corsa setup is the way to go, but only problem I have is that I am down under in Oz, and the cost for freight on heavy/bulky items like mufflers is prohibitive.
But sounds like my exhaust guy is on the right track with his suggestion for the mufflers... and seeing he is only talking about $300-400 to swap them out, it would be a much more cost effective solution. I'll also talk to him about a crossover...
Out of curiosity has anybody installed straight through single pipe mufflers, as my guy has suggested?
Then I'd just need to work something out for the exhaust tips to make it look OK on the back of the 'vette....

Drone is a frequency that's pretty hard to completely get rid of. There are a few products that help with it but require you to remove your interior and permanently apply it to the entire floor of the vehicle. They do help. Depending on how bad and how quiet you want it, we have the Blockit which is an engineered 4 layer sound deadening mat system that takes about 60 seconds to install.
It consists of one large pre cut mat and 2 small mats that go behind your seats. All mats lay on top of the carpet.

Now if your looking for dead quiet, this it not for you. However if you want to quickly take the edge off tire, road and exhaust noise, this is a great 60 sec install.

Click here to check out the Blockit

To take it up a notch we also sell DeadMat. Dampening mats which also help with vibration & drone
Click here to check out the DeadMats

The Corsa only comes as a complete cat back so you won’t be able to use the rear y pipe with your dual set up it’s also around $1300 so that’s up to you if you think that’s worth it just for a pair of mufflers

You didn’t say what mufflers are on the car now causing the drone but I take it they’re not the stock 91’s?

:iagree:

Packing the spare tire carrier also helps. It acts like a drum.

They sell wraps for the mufflers - they help somewhat, the exhaust tips must be at least 2" behind the rear panel to help .

Benny is correct. Anything else will not get rid of your drone except a full on Corsa system.

I have long tube headers and Borla cat back complete with resonator(no cats) and it drones pretty good, and not even very loud.

Not sure how that relates to my Corsa suggestion :confused: