Noise problem in 1983 280ZX

[Pilgrim]

pilgrim

Super Moderator

This week I pulled out my faithful old Radio Shack sound level meter and went for a drive. A couple of years ago I switched from rubber suspension bushings to poly, and I was convinced that my 83 ZXT is perceptibly louder inside as a consequence.

The meter measures the noise level either of two ways, but (EDIT) I will just quote the relevant one.

• A-weighted, 500-10,000 Hz, which is the primary human hearing range and is the most used rating for workplaces and general sound levels. The human ear doesn't hear low frequency sounds as well as higher frequencies.
• C-weighted, 32-10,000 Hz on this meter, a wider sample but not as representative of human hearing.

The results?

At in-town speeds of 30-40 MPH, a pretty steady A-weighted reading is about 72-78 dB and every crack or expansion joint boots it to over 80dB. Highway sound levels are similar, but slightly higher and any time I go over pavement with expansion joints or cracks it jumps up another 10 dB at 76-86 dB. But anything over 80 is concerning.

No wonder I think the **** thing is loud inside. Even at the A-weighted readings, the sound level is close to 80dB, the accepted level where hearing damage can begin with 2 hours of exposure.

The difference in scale measurement is all low end frequency, and I think that's the result of the poly bushings - every bit of rumble and low end road noise is transmitted into the passenger compartment. I have a bluetooth radio in the car that links to my cell phone, but I can't answer a phone call traveling above 50 MPH because I can't hear the caller and they can't hear me on mic over the noise level. The carpet and sound deadening in the car are original, as the car is very well preserved from new.

I expected a harsher ride, but it never occurred to me that the noise level would increase so much from the bushings. I'm thinking of going back to rubber bushings, as I don't really autocross this car and I honestly don't need them.

Has anyone else installed poly bushings and noticed a noise problem?

Edited August 26, 2020 by Pilgrim

[Zed Head]

The ZX's have a different type of suspension on the back.  Where exactly did you install the bushings?  And what type of tire and rim are you using?  Tire sidewalls are a significant part of the suspension.  Low profile tires probably make more noise.

I used to notice a big change in the Portland area roads when I went from the old high aggregate concrete to the newer asphalt.  The rocky concrete was super noisy.

[Pilgrim]

Thanks for the comment.

The rear sub-frame bushings are still rubber - only changed out the front. However, I have long known that on the car's interior there's really no sound treatment under the back hatch, just that foam filler under the carpet that gives you a level floor. I think adding some sound treatment on the rear floor would definitely help lower the noise level, because occasionally I can hear a bit of differential sound from there.

I had a shop do the bushing changes for me.  They put poly in the front control arms, front sway bar main mounts, the bushings at the end of the front sway bar, and the bushings on the rear sway bar that mount it to the sub-frame. They didn't do the bushings on the ends of the rear sway bar. So most of the changes were made up front, and none of the bushings are very hard to get to.  I'll check the manual to see how hard the bushings on the forward end of the control arms are.

I agree, the road surface makes a huge difference! I can tell that.  Also, every crack or bump in the pavement creates a big low-frequency thump inside the car, which is the reason the sound meter jumps 10 dB when I contact cracks or expansion joints.

Tires are OEM size, 60-series profile Sumitomos from Tire Rack. The rims are the stock "swastika" turbo rims that came with the car.  I'm running the tires about 38 PSI.

[Zed Head]

That is quite a few pathways to the body from the road.  They probably did the TC rods too, didn't they?  Maybe the steering shaft coupler.

Not sure why poly on the sway bar, or anti-roll bar, is supposed to be helpful.  Probably saves a few mm of roll at most, compared to rubber, but transmits a lot more noise.  The control arm bushings would affect steering response so that seems reasonable to do, for performance.

You could loosen up the ARB links and see if noise goes down.  All that will happen is a little more tilt in the turns.  Easy test.

[Pilgrim]

Most of the poly kits out there include sway bar bushings. Steering shaft wasn't part of it.  Not sure what "TC" rods are....? Abbreviation unfamiliar.

True enough, I can loosen the sway bar mounts to the body, and they're quite accessible. I think they're going back to rubber in any case.

I also think that it wouldn't be a bad idea to plan on pulling the seats and doing a layer of sound absorbent under the passenger area carpet as well as the rear deck.

 

Edited August 25, 2020 by Pilgrim

[Zed Head]

Tension/Compression.  The FSM calls it tension for the ZX's and compression for the Z's, although they work in reverse also, so T/C is more correct.  They see both.  The rod that connects the wheel hub to the body and hold it in place during braking.

[Pilgrim]

Thanks. Yes, those are the biggest bushings that went in. They may not be hard to replace.

It really looks like an hour or two would suffice to replace the bushings, but I'm not sure if they're really the source of that much noise. I had the car on blocks or actively working on it for four-plus years, so my memory of the noise level before that isn't trustworthy any more. 

The rubber ones don't cost much, so I suppose if I change them and it makes no difference, I could swap them back. 

Edited August 26, 2020 by Pilgrim

[Pilgrim]

Just for grins, I measured sound levels in my 2008 BMW 3-series wagon today. A-weighted sound level is around 66 dB on the highway, peaking at 71 dB.

I'm going to go back through the service manual and check to see where those front control arm bushings are, as I thought the tension/compression rod bushings were them. I must have missed them while looking underneath.

And I have ordered some Noico sound deadening material to install under the carpets and on the rear deck. If I have enough I'll also pop off the door panels and apply some in the doors. 

Edited August 26, 2020 by Pilgrim

[Pilgrim]

I have the Noico sound mat material installed in the rear end under the hatch, and in the spare tire well which I think was the source of a fair amount of noise.

The next major step will be to pull the seats and  carpet and treat the cabin area.  Doors may be part of that, or  they may be stage 3 treatment.

The hatch area: