1991 mustang, 331ci with twin s366 turbos.

Same engine, turbos, engine mounts, and transmount that got moved from my last car. Last car was dead silent with the windows rolled up, no car drones unbareably. Last car the turbos were supported by mounts that bolted to the heads, this car the turbos are mounted to the frame. Last car was full interior with oem sound deadening, this car is full interior with aftermarket sound deadening.

No flex or bellow between the engine and turbo, could this be the issue even though the engine has solid motor mounts? The car originally had 3.5" exhaust coming out behind the front tire, i moved it to in front of the front tire, that made no difference.

#ad

 

It had solid mounts before?

Can you talk about the sound a little more? Drone is where a pressure wave and exhaust wave line up and it stacks in the vehicle, it is periodic, meaning it will come in and out, and usually occurs at a very specific range. It is murder on the ears and different than a rattle or vibration, but usually causes rattles and vibrations, you can often feel it in your chest.

If you are using the same exhaust and different turbos and have a drone now, something changed in the exhaust frequency. Helmholtz resonator (usually a J-pipe) is a good solution, or changing the muffler/exhaust length. What RPM is the problem occurring?

 

Yes, solid motor mounts just like before. same headers with vband flange but new 2.5" piping from collector to turbo mounting flange. 3.5" exhaust after the turbo but quite a bit longer than on the last car.

Definitely drone, not a vibration or rattle. The drone is rpm and load specific. I covered most of the firewall and the floor with sound deadening and in some areas it has killmat and sound deadening over lapping. I did just sound deadening on the roof as well as in the hatch and the rattle from the hatch area went away. Wearing ear buds helps a lot with the drone.

 

Drone is tough. Vibrant makes a decent resonator, but they don't always hit the frequency range. Look at making a J-tube for it. Build it with a little adjustability, usually they will completely eliminate drone in a ~300rpm range.

 

I assume the j pipe would be used after the turbo, correct?

 

You can put it anywhere in the exhaust stream. It is a tuned length, usually a quarter wave, sound waves of that length hit the pipe, bounce back and cancel themselves. So it will kill that frequency, so it doesn't care where it is in relationship to anything else in the exhaust, just put it wherever it fits for you. Doesn't have to be the same diameter as the exhaust either. You can also build a Helmholtz box but that tends to be a little harder to fit under a car.

 

Frame-mounted setups can transmit more vibration and noise into the cabin compared to engine-mounted, especially if the engine uses solid mounts which don't absorb vibrations well. The lack of flex or bellows between the engine and turbo might also mean that vibrations aren’t being dampened as they were before. You might consider adding some flexible couplings or additional sound-deadening materials around the cabin or near the mount points to help reduce this noise.