Controlling exhaust vibration and harmonics
Posted: 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!
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!