So I am having some trouble tracking down the source of my vibration/resonance problem. I had a new 2" dual exhaust put on my 54 ford with 12" bullet style glasspacks. Sounds awesome! Problem is that when I am just cruising (not accelerating or decelerating) I get this resonance that I can hear as much as I can feel it in the seat of my pants. It only occurs when I am in 3rd gear and starts at approximately 30 mph and gets progressively worse as my speed goes up. The resonance goes away as soon as I push in the clutch or go hard on the gas. I cannot reproduce the vibration when I am out of gear. From what I have read through my search, some of the possible causes could be: a standing wave in the exhaust pipe causing resonance, any part of the exhaust touching the body/frame, broken exhaust mounts, possible driveshaft imbalance/bad u joints. So far I have checked that all of my exhaust mounts are good and solid (but not binding), I have had an H-pipe installed to try to cancel out the standing wave, I have checked the exhaust for contact with the body/frame, and I have even gotten the driveshaft balanced with new u-joints. So far nothing has helped. I have also heard that by adding weight to the exhuast system, you can change its natural frequency and hopefully get rid of the resonance/vibration. Does anyone have any experience with this? Should I just use some exhaust pipe clamps? Where should I put them? Are there any other possible causes that I have not considered? Any help would be greatly appreciated.
Sounds like a harmonic resonance problem. Not sure the exact difinition, but basically your exhaust could be emitting a tone that is in line with something else on your car causing it to also create it's own tone. So the way to fix it would be to change the tone of the exhaust. Not sure how this could be done. Try the weights, or I'm sure somebody has a better suggestion
Just a thought but I chased the same type of sound/ vibration for over 4 months on a big block S-10. After changing everything I could think of, it was the ring gear.
It started with the new system, Correct? I have read about it many places and seen it mentioned several other places, that if your pipes are exactly the same, left and right, it most likely will cause a harmonic resonance problem. Having a difference designed in the pipes, IE: having one muffler moved forward or aft, both pipes running down the same side, thicker wall section of pipe ect. any notable difference, eliminates the compound resonance by each one having there own freq range that do not multiply the resonance and involve other parts of the car into the noise.
Shorten one of your exhaust pipes and see if it changes. Pipes of equal length will sometimes do this.... or add resonators.
both pipes are not the same length. my 54 has a 239 with the 3 speed overdrive. It used to have the cross over pipe that linked the two exhaust manifolds together. In order to put true dual exhaust, i could not exit the rear of the driver side manifold because of the shift linkage/steering box. Instead, the driver side exhaust pipe exits the front of the manifold, goes down under cross member, then out the back. All together, the driver side exhaust pipe is probably a good 5-6 feet longer than the passenger side. Since the passenger side exhaust pipe is pretty much the same configuration as stock, I dont think that side is causing the vibrations. I have a feeling that the driver side exhaust pipe is the one that is giving me the problem.
I had the same problem with a 66 Chevy I had back in 1972 . It was the tone of the mufflers at certain RPM and ONLY in gear ! Change the mufflers or use a small resonator to change the pitch .
If using polyurthane bushing as hangers on the exhaust they could be to hard or stiff. That would be an easy check/fix.
What are the tailpipes suspended from? If they did like some lazy muffler shops do and welded a bolt to the middle of the metal pan that sits under the back seat and hung the tailpipe hanger from that, it could be the vibration of your tailpipes is rattling that big sheet metal pan like a giant diaphragm. When the tailpipe vibrates at the rpm that the big pan under your back seat naturally resonates at, the whole pan will start to drone really loud at that frequency. If it's something like that, maybe you can just move the tailpipe hanger to another location -- hanging it off of a crossmember on the frame or something. Putting some sound dampening material like Dynamat on the floor under the carpets will also help out a lot to keep the sheet metal from ringing. Some big wide flat or pan shaped panels will ring like a drum if they're not dampened enough with some type of soundproofing material.
Had a friend who owned a 55 Chevy with this problem.The exhaust terminated in "scavenger style"pipes that ended just under the rear axle and were aimed slightly down.At any speed above 30 mph it would set up a vibration due to the exhaust bouncing off the ground and the pulse wave ricocheting off the trunk floor.At 60 mph you couldn't stand to be in the car as it would actually give you a headache! I would try using a small section of flexible pipe that will slip over the end of the tail pipes and move it around a bit to experiment.I bet you will get a change in resonance.
An H pipe in exhaust does very little, if anything, to help balance exhaust flow. Replace with an X style cross pipe. Tail pipes over or under differential? If over may be causing too much back pressure. Try running short pipes at muffler-direct exhaust away from pointing straight down-kicks up dust, etc. Or, have pipes end/exit at drivers side of car before rear wheel. Do not run one to each side of car.
The Jalopy Journal
