threcixty

Hey all, after 4 years of keeping my promise to myself, I bought a 89 GXL with 51000 miles... Couldn't pass it up. The only mods I have done are a cold air intake, and a custom exhaust including a homemade presilencer with carbon fiber roving for packing (works better then steel wool.), a magnaflow behind that, and an old dynomax catback I had laying around from a previous car. Anywho, as with my previous 6 rx's, this one was drony as hell right at 3200-3400 rpm. Figures, as I wanted a highway car, and that means right at 75. Just for ***** and giggles, I put some heavy weave fiberglass inside of the well where the spare tire sits, and sprayed a crapton of 3m "rubberized undercoating" all over the inside of the well. I also sprayed the outside on the bottom of the well, and up in front of the rear bumper. All in all, used the better half of 2 cans of the stuff, or about 1.5 pounds. Took her for a drive tonight, and 3200 rpm is the same as 2800 or 4000! No more resonation! FWIW, I know I was hating it and maybe it will help someone.



Good to be back!

Jim

j9fd3s

yeah 3200rpm is loud....



so you insulated the spare tire well and that fixed it? does that mean its like the resonant frequency of the car or something?

threcixty

Thats what I was thinking anyway. If you look at the metal around the spare tire, it is quite thin, and if you tap on it, it sounds like a drum. I was insulating it so that I could try an active noise reduction idea I had, but decided to load it up with undercoating and see if it made a difference. Makes sense as that piece of metal is the same size on all of our cars, and from all of the searches I did, they all resonate around 3k, so yea.. It just makes the car sound as loud as it does at 2800 and 3500. It made the loud spot dissappear, and I can actually hear my stereo now.

Jim

defprun

Another way if you wanted to re-inforce the area you can use 3-4 layers of fiberglass matting in that area to stiffen it up..since seam welding in there is impossible - apparently.

j9fd3s

yeah the drum thing totally makes sense. this little trick will be in the next car, thanks!

Nateb123

Just about to go do this in my new rx-7. I was wondering why it was so screaming loud at 3k (thought maybe there was a rusted through section of the exhaust but it all looked fine). Thanks for the tip!

threcixty

I was wondering about using a little "memory foam" or something like that to put under the carpet on the back panel, as to make it a little quieter also. Does memory foam like you can find for mattreses work for sound deadening??

Jim