I have a 95 Civic DX that I purchased about 6 months ago and it has the slow D15B7 engine, cold air intake, and a Greddy exhaust complete with an aftermarket header.
First of all just as a preface, I'm not sure if the CAI is the one that makes the cabin noise levels higher, so maybe I need to l look more into that? All the mods that had been done on the car were from a previous owner.

I measured the noise levels inside the car at various speeds, mainly highway since it hangs around 3000 RPM in 5th on the highway. During that time my average dB was around 85 dB, with lows down to 79 dB and max at over 100 dB sometimes if I was accelerating a few MPH. I've had to use earplugs every time I drive because if I don't then I know I will eventually have tinnitus and go deaf from daily driving it.
I looked at this thread about quiet exhausts:
https://honda-tech.com/forums/honda-...haust-2907847/
but I'm not sure what noise levels the cabin is at, especially at highway speeds. I've looked around for the Greddy exhaust and I believe it's this one, or at least one similar:
https://www.vividracing.com/greddy-s...151806913.html
This description says it reaches 91 dBa with their turbo, so maybe it's not the same since I was reaching louder levels inside my cabin. I was going to probably swap to Yonaka's catback:
https://www.yonaka.com/product_p/ymcb001.htm
but I just don't know how much quieter it is. Is there any data on how much quieter it is, in terms of decibels, so I can make a comparison?

 

windshield and door seals.

 

There is gorilla tape for all the doors' window "trim," and when I had to get the windshield replaced the company said that the seals were going bad. They assured that that leaks would not occur, which they have not. Does the trim really make that much a difference on how much gets into the cabin?

 

What trim? please provide details.

 

Your door seals keep most of the wind noise from entering the inside while driving, that would be the 1st thing to look at. There is a trim on the exterior window that snaps into your doors where the glass window enters the door, that also needs to be looked at. The rubber seal that the window rides up to is also another place to investigate. There's also noise coming from the trunk, your rear seats need to be secured when its closed off from the trunk.

 

So I have tape on the outside bottom on all 4 doors, only on the bottom strip. All the rubber from the inside is still there and intact as far as I can tell. I've had my back seats down before and I know they're back up and in place and secure from when I put them up last time. It's not wind noise that I'm hearing when I'm at 3000RPM going 65MPH, it's the exhaust droning that I hear.

 

I found even with stock exhaust the short ram intake would increase exhaust sound under higher rpm situations and WOT situations. Less noticeable in slow acceleration.

Being you are concerned about noise, I would honestly switch back to the stock air box, resonator and intake tube. And then I would also switch back to the stock exhaust with the stock dampened muffler.

Doing these things will quiet the car down considerably from exhaust drone.

 

I’ve ran the world sport system and it was pretty quiet. No drone, and all you could hear is engine. I don’t recommend the yonaka. I have the yanaka 3” and although it’s not rice loud, it’s louder than what your after. If you really have to wear earplugs to drive, convert the car back to its stock form. That will get it quiet. Or at least as quiet as you can get a 90s Honda economy car. These things are like driving pop cans, it’s going to be hard to start modding and keeping the cabin noise down. Even in their stock form there was a level of road noise that came with it as with many economy cars.

 

I was waiting on mods to approve my reply to photos but I guess they still haven't so I'll just describe it. So the outside trim where the bottom of the side windows are are covered with duct tape. The front windshield's rubber is also not properly attached. Here's an imgur album of what I mean:
https://imgur.com/a/3aRCuYC
All the rubber for sealing the doors in is there and looks fine from what I can tell. I just don't know how much an impact that has on hearing the exhaust drone. I don't hear much wind noise, or if I do it is overcasted by the droning I hear at highway speeds.

Will I be able to go back to stock if I have an aftermarket header? I'm not sure what the diameter is, but I'm pretty sure it's wider than stock headers to fit the aftermarket exhaust properly.

 

My apologies, the forum doesn't provide me any indication the attachments (photos) need approval when I enter the thread. Until someone says something I am under the impression everyone can see the photos just as I see them.

They have been approved.

 

Not sure on the header, I’m sure the collector flange is a bit larger than stock. I would imagine a muffler shop could just bend your a stock system and use a larger collector flange to make it work if needed.

 

Get a factory airbox and tube to start. Do you still have the stock cat?

 

I would have to check. The engine, transmission, the header, the air intake, and the exhaust have all been swapped. The guy who I purchased from said "full Greddy exhaust," so I'm not sure

 

A full greddy exhaust is designed to mount to the factory cat. Get under there and snap a pic for us. But the header might be a larger collector, that usually dictates if factory cat can be used or if a testpipe or custom sized cat is required.

 

You can run an aftermarket header to a stock catalytic converter and stock cat back.

I daily drive a bone stock civic si with the really high gear set and never hear engine noise

I also use the cheap ebay sound dampening material on large metal areas, for example in the door panels and interior door panels to lessen road noise. Smooth metal panels will echo the road noise inside the cabin

But right now your main problem is the intake and exhaust.

 

Ok, so its not road noise but exhaust noise. You might want to put it back to stock, airbox included.

 

Believe it or not I actually replaced the ITR muffler with one from a DB integra. It def is quieter but I still have the occasional wind noise.

 

I am not surprised, I noticed doing 70 to 80 mph highway drives with a jdm 4.7 with my GSR swap and JDM 4-1 and catco cat, it isn't exactly quiet inside vs when I had an LS trans/motor and comptech header with oem cat + ITR catback, but it is tolerable enough for me.

 

yeah, your revs are up there but if its anywhere 2k you should be fine. Its just that with the ITR you can still hear the muffler but with the LS its quiet. I had put it up for sale but had to take it back down because it wouldn't be complete w/o the b-pipe.

 

Here's an album of the cat, I'm thinking it might be stock? Firestone fucked up my header pipe because they didn't know how to drive a lowered car.
https://imgur.com/a/anwk7BP

 

that looks like a test pipe, which can also contribute to the the loudness.

 

So should my first step be to replacing that with an actual Cat and seeing if that helps?

 

Yes.

 

Or just take out the interior, then add sound deadener to the floorplan, doors and then add MLV - Mass Loaded Vinyl. Research that, your car will then be quiet enough to drive and you dont have to modify the car any further

 

Any suggestions on a decent stock-like Cat? I'm trying to find a parts catalog but I can't seem to find a decent one. Most common one is Magnaflow, but I don't know if I need a high flow in a weak engine, even if I hit high RPM.