Controlling exhaust vibration and harmonics

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89vette
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Controlling exhaust vibration and harmonics

Post by 89vette » Fri Dec 07, 2012 4:27 pm

After exploring the issue of engine vibration, I’d like to post/ask about exhaust vibration(s). Though many/most of you will not have dealt with the type of exhaust I’m running, I’m hoping the principles will be the same.

The car in question is a C4 with sidepipes. This particular system was sold as a “cat-back” solution where it connects to the OEM factory converter. That’s a single main converter with a 4-bolt flange. At the bolt-up location, it splits into two outlets, swings over to each side…and exits. Sidepiping is bolted to the vehicle frame using rubber bushings for isolation.

My car has been modified with a 383-stroker, small cam, and headers. Because the cam is “only” 216/216 .544/.544 112LSA, my expectation were for a near factory-like idle/performance. This is important because it’s also a 6-speed that runs as low as 1400 rpms while cruising on the highway! With the improved stroke, higher-than-stock compression, and add’l torque, the 6th gear experience could be even better than stock – if I can eliminate sound/vibration issues. That has presented some challenge.

I’m actually heading in for my 5th iteration of exhaust on the car (while I’ve owned it). The first two changes were easy, straight-forward and resulted in great “upgrades” in sporty-sound. Now that I’ve added headers, the challenge is much greater. That’s because of the severe limiting of space – for sound-reducing components.

When the stroker/headers were installed, I dumped into small, short metallic spun cats. An immediate “jog” was required to hit the center tunnel. There, it connected to a dual-in/single-out muffler that was quite sturdy. (dual 2.5” inputs/single 3” outlet) It’s single 3” outlet connected to the sidepipe system described above. Though the result was relatively successful, highway drone was an issue. That’s because the muffler seemed “tuned” to lower frequencies. In fact, I could tell it put out more low-frequency rumble than the “main” high-flow converter that previously occupied it’s location. (The converter has a raspier, higher-pitched “barK’ than these sport mufflers.)

Because of the muffler ability to block high-frequency sound – while seemingly accentuate low-frequency sound, I decided to remove it and go back to a converter. I needed to reduce low-frequency sound because the system’s resonance seems to be around 1500-1600rpms. That right in the heart of hwy cruising range. Really it’s cruising for all gears. The system also seemed to vibrate a bit more than expected, which is why I posted about engine balancing, dampers, etc. I did this before posting here.

My goal is (was) to reduce low-frequency harmonics in the 1500-1600 rpm range. At the same time, I was looking to increase isolation from the body/frame. So, I chose to add short flex pipes in the pipe routing. After doing this (2 wks ago), my system goes from headers – to spun cats – to flex pipes – to a short Y-Pipe – into a single “main” cat converter – and into the side pipe system. I got the expected low-frequency attenuation, but vibration actually increased! The flex pipes seemed to hurt – not help.

I’m headed back to the exhaust shop tomorrow. I’m asking to have solid pipe restored to the flex pipe location(s). The shop owner also suggested welding (hanging) weights off the piping to help dampen pipe vibration. The idea sounds reasonable/inexpensive…so I’ll probably try that too.

This thread is to solicit for other ideas – and figure out why the flex-pipe didn’t work. My guess is the flex pipe allowed the headers (+ small bullet cats) to float as if mounted on a dragster. IOW, the front part of the system/piping had nothing to support it. Though it’s not clear how normal my low-rpm engine vibration/harmonics are (for a 383 gen1 1-pc RMS chevy stroker), there has to be ways to attenuate harmonics/vibration. (I bet this is a big part of factory designs when developing new automobiles.

FWIW, the weights the shop will be adding tomorrow should slow/lower pipe vibration(s). Hopefully, this will lower/dampen vibration to sub-idle harmonics – and reduce/eliminate them. (Of course, I thought the flex-pipe would do the same thing.) But, I guess it actually “freed” the headers to vibrate even more…but retain enough “solid” connection as to transmit the result to the rest of the car. Seriously, it vibrates like a massage chair around 16k rpms now! (And, yes, nothing is touching the frame.)

For any mom/pop exhaust shop, sound tuning and vibration control can be a trial/error operation. So, I don’t blame the shop at all for not being sure of the next step. Really, the flex-pipe idea was mine. Tomorrow, the return to solid pipe is also mine – but I like the idea of weight to change pipe harmonics. (Though weight in general is never welcome on a sports car! )

If this next option doesn’t work, I may revisit the mounting locations and/or it’s rubbing bushings. Maybe I can improve those? Until then, I thought I’d post to see if any other suggestions pop up.

Thanks!

89vette
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Re: Controlling exhaust vibration and harmonics

Post by 89vette » Sat Dec 08, 2012 9:41 pm

The shop owner didn't come thru with the weights. So I didn't get to try that option. However, since the addition of flex pipe seemed to increase apparent vibration, I opted to remove the flex couplers and go back to solid pipe. Surprisingly, that helped. It's back to the condition before adding those couplers.

I didn't get to feel it myself, but I asked the shop owner/tech to feel the motor and piping. This has in an attempt to determine the source. They said they didn't "feel" the piping (knowing it would still be too hot [from my drive over]. But they did place their hands on the oil pan and said it was "vibrating like an electric sander". So, unless some degree of this is acceptable in a stroker, I have to assume the balancer job (as posted about in adv tech forum) really isn't ideal. Obviously, that's disappointing news -- considering how much I looked for the right person, parts, etc.... I guess it "is what it is".

Hard to say if I could find a local expert "good enough" to put their hand on it, look at pics of the crank balance job, and determine about how long it's likely to last. Or, if it's suprising that it lasted even 5k miles. When I built this, it was a one-time deal -- paying lots of attention to details and shooting for getting it right the first time. How bad does it suck that the only problem may come down to the RA balance job? ](*,)

With regard to the issue of vibration dampening, it would seem that bushing-isolated connection to a frame is best served with sturdy, solid pipe. Flex pipe is often used on transverse engine where the engine flexes further/closer to the pipe. Seems like this is it's best application.

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Re: Controlling exhaust vibration and harmonics

Post by BirdMan » Sun Dec 09, 2012 11:13 am

How about disconnecting the cats as they are nearest to the engine, starting the car and feeling the oil pan again?

a) If vibration is still there I would disassemble the engine and have it rebalanced/checked!
b) If vibration is gone then reconnect the cats and disconnect the next removable item? Do this until you find the culprit?

I have/had a 347 SBF that the first time/run at the dragstrip the stayon starter wire came off and it wouldn't restart, found it had come off and reconnected it. Came back from 2nd run and starter wouldn't engage and found wire was off again! I then replaced the pushon end with a loop end and bolted it on. On the 3rd pass it stayed on but the valve covers would loosen up and I had to retighten them after every pass. The car weight w/me at 3050#'s, C4 auto, 7" 6,800 leave rpm converter, shifting at 8,000 I ran consistent 6.70/6.71's all day.

I was using cork gaskets at the time and then I got the FelPro blue silicone ones with solid spacers and valvecovers would stay tight.

Late in the season after a 10.71 quarter mile run the oil pressure was down to 10 lbs. and so I parked it. Upon tearing down the center 3 main webs were broken and the only thing holding it together was a thick steel main stud girdle. The main bearings looked good and the rod brgs. looked great and will be reused!

It sure is a bummer when you try to do your best and then?????
Dale C.

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Re: Controlling exhaust vibration and harmonics

Post by PackardV8 » Sun Dec 09, 2012 12:19 pm

The car in question is a C4 with sidepipes.
Once Warren Buffet said he wished there had been a capitalist at Kitty Hawk to shoot Wilbur and Orville Wright because since their inception commercial airlines have been a net capital loss. They've lost more money than they've made.

Same-same for the guy who invented sidepipes. Since the first time I experienced them in the mid-'60s, I've never been in a Corvette, Cobra or Viper with sidepipes which wouldn't make you hate it after an hour on the highway.

Bottom line - you can make the resonance from sidepipes less bad, but you can't make it go away.

Seriously, it vibrates like a massage chair around 16k rpms now! (And, yes, nothing is touching the frame.)
But they did place their hands on the oil pan and said it was "vibrating like an electric sander".
This might be your root problem. If the engine is really out of balance, nothing you can do to the side pipe system is going to make any difference.

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89vette
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Re: Controlling exhaust vibration and harmonics

Post by 89vette » Thu Dec 13, 2012 12:27 am

I guess one of the things I'm not clear about is the ability of a header to induce vibration. I've got a unique...non-run-of-the-mill product made by a smaller company in Los Angeles. It's a 4-2-1 header, of which very few are running these days. One of the benefits of these is the "double" pulse reversion wave. I'm wondering if it's possible that the headers themselves induce THAT much vibration into the system that it feels like the motor? I asked the exhaust shop owner if he put his hands on the headers. He said no -- worrying they were too hot.

Maybe they were WAY worse.

Kinda hard to swap them for a traditional 4-1 header w/o spending lots of money.



Note: If not mentioned in my OP, the headers fire into Magnaflow metallic spun cats (2.5" exhaust). Then, head thru a Magnaflow center cat (3") and over to the side pipes. With the 350, this setup was pretty tame (sound/vibration). But it had OEM manifolds too.

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Re: Controlling exhaust vibration and harmonics

Post by flyingwedge » Tue Jan 08, 2013 4:16 pm

>. I would start with a mechanic's stethiscope. Beyond that, you might consider adding 1+ resonance tubes, is a closed-end tube, fed laterally from the cross-over pipe or pipe before/after muffler. You could look under a Honda S2000, to get a rough idea. They work to cancell antenuation of pressure waves, at certain frequencies. Good Luck, flyingwedge.

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Re: Controlling exhaust vibration and harmonics

Post by 89vette » Tue Dec 10, 2013 5:49 pm

Seems like the addition of resonance tubes would be a crap-shoot (to find the right diameter/length) unless there's some formulation to determine tuning. A local shop has seen cross-over (x/h pipe tubes) actuall increase resonance. I suspect wave tuning is key. Couldn't find any posts/info on this subject.

More discussion on my particular issue/config can be found here.viewtopic.php?f=1&t=37751&start=15

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