Nice build!! I'm impressed with the fuel mileage you are getting! Mine is not nearly that good. I need to investigate.
How did the acoustic experiment work out? (Eggcrate foam on the roof)
I bought this and just threw it on top of the roll cage. There is enough room above the cage and below the roof that the foam is gently held between. I can still slide it around up there if I wanted:
Amazon product ASIN B00P7FFR6A
It had been a while (~1.5 years) since the top was on last. The truck is actually really quiet now. I can easily hold a conversation while going down the freeway at normal volume. The cabin noise is low enough that I can hear the Orion wining under load.
It's hard to say if it did anything because I changed so much at once. One of the largest improvements in NVH was a result if putting foam in the shift boots. I had insulated the tunnel, but the shift boots don't do much to block sound. You can stuff a few microfiber towels in there to test it out. I have 4 shifters so it definitely made a large difference for me.
Sealing up the bad doors with the new hatch frame and tool tailgate did a ton to make the exhaust quieter as well. The carpet went in at the same time as the top and the foam headliner so its really hard to say.
Most of my noise is the wind in the top left corner of my hard top where the gutter was bent away from the fiberglass. I can see light through there and there is a large gap.
It must be pretty quiet because I can easily pin point and leaks or rattles int he interior now. Most of my rattles from from the lever on the heater box, and the rear hatch lift springs. They only rattle at certain rpms when accelerating and on washboard roads so I don't mind. I'm pretty anal about rattles and it doesn't bother me so that's saying something.
Installing the fuel tank below the drivers seat is likely going to dampen a lot of sound as well.
I have a pretty large stereo and I find my self listening to it at pretty low volume now. The other day on the way to big bear my buddy and I were both on a few phone calls while going down the freeway ~75mph and had no issues at all. I don't think I used to be able to do that.
A little breakdown in my NVH journey in a relative of an order of magnitude:
Getting the engine and transmission mounts dialed in
Vibration isolating the steering wheel
Going from Bias-ply off road tires to radial tires
Getting a proper driveline balance
Proper gasket sealing between all the doors and rear hatch
Isolation of the hood and modified hood hinges
Tightening up door latches and hinges
Taking apart the latches and isolating their vibrations/rattles
Tracking down and fixing air leaks
Making sure all suspension components are tight
Installation of the snorkel- Now this is kind of application specific. My turbocharger is unbearably loud without the snorkel.
Foam under the shift boots
A nice thick rear tailgate full of tools in tool pouches
Isolating rattles from the Hi-Lift jack
Sound deadening on the tunnel
Isolating the D-rings with urethane spacers
Aiming the exhaust towords the ground below the rear bumper- It's a old production car trick
Foam above the roll cage
Carpets
Mounting the ECU behind the engine on a shield to block sound
Up next is probably the body mount replacement.
Other than other obvious things like exhaust rattles ect that's the gist of it.
Actually I can say that the loudest thing below 65-70MPH would be the OEM heater blowing on its hi setting- Maybe that can help quantify the improvements?
Hope that helps,
Mike