Exhaust drone...resonator help

I changed my exhaust around on my 454 4 speed '62 Biscayne. Had the complete 2 1/2" NOISEMASTER kit with the mufflers in the OE position (under front floor). Car sits so low I got tired of dragging the mufflers and didn't really think the muffler sound fits the type of car so I changed it around.
Put straight pipes in place of the mufflers at the front, bought new mufflers (AP Exhaust Xcelerators, straight thru gl*** packed 14" oval case similar to Dynomax Ultra or Magnaflow) and moved the mufflers to behind the rear tires with 45 degree down-turned outlets that peak out just under the rear bumper. Sounds great, deep rumble , sound fits the car, quieter at an idle than the 3 chamber NOISEMASTERs but louder when your are in it hard.

The problem I have now is this HELLACIOUS drone from 2200 rpm- 2800 rpm, it is about 150 hz. I ***ume because there is a long stretch of pipe from the header collector to the muffler behind the rear tire. My wife and two kids about jumped out of the car while going 50 mph when it was doing it. The system is hung with the late model type rubber and rod hangers bolted to the frame in the GM locations.
I realize to correct this I need to install resonators probably somewhere close to the middle of the stretch of each pipe. The OE muffler location where the muffler elimination pipe is located about 1/3 of the distance from the header collector. My issue once again is ground clearence, I can't go any taller than the 2 1/2" pipe diameter so I was considering fabricating my own resonators. I would have to build them wider than the pipe to achieve the volume I need. I know there is a few engineers and exhaust experts on here who can help me out.

Thanks,
Marty McFly

 

Do you have an "H" or "X" pipe between the 2 banks?

 

GreenMtnBoy's on the right track.

Install an H-pipe.

My 32 roadster droned with a pair of gl*** packed Turbo Muffs, not too bad, but it was worse with a pair of two chamber FlowMasters.

The H-pipe pretty much cured the drone.

My aim was sound attenuation and not HP.
An H-pipe does give you a bit more torque down low, but it's not really noticeable on the street.

Pipe size - for sound attenuation - isn't critical.
I run a 2 1/4" H-pipe on a 2 1/2" system.
The smaller diameter H-pipe is a little easier to install than running the same size pipe as the rest of the exhaust system.

Give the H-pipe a shot, lots cheaper than installing a pair of resonaters.
You can always add em later if things are still too loud.

Do you have room for a pair of Hemi-muffs?
They flow well and are quiet.
I'd run em in my roadster if they fit, but I'm fairly limited far as muffler space goes.

 

62 is a x frame cant use a h pipe or x pipe. depending on how close the pipe is to the frame you should put some gl*** pack style resonators right in the middle of the car. another way to do this is put the muffs right BEFORE the rear tires then put resonators in the tail pipes.

 

I was affraid of that.
On my daily-driver '94 Buick wagon with an LT1 and factory true dual exhaust, the resonators are at the very end of the system. The mufflers are before the rear axle, the resonators are at the end with a 12" tailpipe. I'm sure GMs main goal with the Buick was noise suppresion.

 

I have tried to find some way of installing an H pipe but when the driving height is so low I have no place to put it due to the x frame.
Has anyone may their own resonators? I know of the tech info on the homemade muffs but did not see any resonator info except store bought.

M. McFly

 

For a cheap try at a resonator, get some 3" pipe and have it sized down by the local muffler shop to 2 1/2" and weld it in. It acts as an expansion chamber and will break up that long pipe. It is worth a shot. I'd say 12-18" will do you OK. OR go to Tractor Supply and get 2 tractor mufflers, they are almost always a straight through like a Cherry Bomb.

You need to break that sound up. Try an H pipe just before the axle. You aren't looking for power, you are looking to smooth out the noise.

You can also mess with changing the length of your tailpipe, it will affect it.

 

Another way to fix the noise would be to change the point of resonance . You can do this by adding weight to the exhaust. Everything has a point of resonance, this is probably the easiest way to change it and increase it to the point to where you wont hear it. Adding resonators would help also, but this is why on some of the late model vehicles, they have weights clamped to the exhaust pipe.

 

I was thinking about buying 36" of 3 1/2" (2 1/2" x 4") oval exhaust tubing, cutting it in two 16"-18" pieces and putting 2 1/2" ******s on each end and end plates and putting them in place of the muffler eliminator pipes at the OE muffler location to create expansion chambers, but I was wondering if putting angled or perferated plates or packing inside would help break up the standing waves better?

M. McFly

 

That's really interesteing 55heiftan I gotta check that out!

 

I considered adding weight to the exhaust pipe like some of the OEMs have done which is similar to using heavier wood for a speaker cabinet to lower the resonant frequency of the material. The problem is the sound level is so high I was thinking it might take a 5 pound weight to do the job!

M. McFly

 

Even if you have to run a super-bent piece of 1.5 inch pipe to equalize the tubes, it might solve the problem.

The other thing you might be facing is the length of pipe after the gl*** packs.
I know any muffler shop sells funky chrome tips that have 1-2 rows of resonators to even the flow.

 

I put home-made 14" baffles into the straight pipes of my '62. It still has that straight pipe sound but it seems to have fewer of the high notes. It's not too loud unless you are really pouring the fuel to it.

 

If your pipes are a long straight shot, adding a U bend in the section where the muffler was may be enough to break the resonance. If you have enough room ...straight, 90, U, 90 back to straight pipe. Does that make sense?

Tom

 

Could you run a pipe under the Engine to connect the two sides?

It wouldn't be the optimum place for a H-pipe, but it would even out the Exhaust pulses...

 

Btw, 2200 rpm is 36.6 Hz and 2800 rpm is 46.6 Hz. 150 hz is 9000 rpm

 

The weight stuff is not for acoustical fixes, it is for pipes resonating. That is called a tuned damper. Check out a big one on most Rendezvous.

Radiated noise can be a big issue. If your floor is vibrating it is because it has a mechanical resonance. If your head is vibrating, it is most likely due to an acoustical resonance. If your floor vibrates enough, you have a giant speaker, try rolling a window down to see if it stops.

It is the physical pipe shaking that the m***es fix. The other thing is, you may be doing 9000 rpm BUT the cylinder isn't FIRING at 9000 rpm, it fires every OTHER revolution. EACH revolution on a V8 fires 4 times so in the exhaust world we call that the 'primary engine order.' This is usually the core component of low to mid RPM sound. To calculate: RPM/60*Primary Order = Frequency.

A 4 cylinder has 2nd Order as it's primary order.
A 6 cylinder has 3rd Order.
An 8 cylinder has 4th Order.
Etc, etc, and so on and so forth.

2400 rpm/60 * 4 = 160 Hz so your estimate of 150 Hz is about right.

Adding the U bends effectively lengthens the pipe.

Adding the perf tube affects certain frequencies and you can do that.

Adding packing typically affects high frequencies and then you need a real packing material that won't blow out and is resistant to heat and the chemicals released in the combustion process. So, if you want to go that far, you might as well buy one already complete. A 454 isn't going to spin so high that you need to worry about high frequencies anyway.

Another order that usually crops up is half orders. This comes about when you have un-equal length manifolds. Basically, one cylinder is sending out a signal about 1/2 phase ahead of the next one. 2 per side fire so you actually hear 1.5, not 2. To make a nice balance, a cross pipe, h pipe, shared resonator (2 in, 2 out with open internals), or an x pipe will usually smooth this out.

 

On the angled plates inside, don't bother, you'd just increase the back pressure unless you know what you are tuning for. You have a standing wave in that big pipe that you need to get away from. That pipe length must have a natural frequency of close to 150 the way it is. Like blowing over a whiskey bottle. You get that deep note. Now you've taken that bottle and stretched it out. If you break that bottle into little bottles, you have a higher natural frequency for each section and as long as you don't go right in the center, you should be able to kill it.

 

Yea,Wasn't figuring in the 1,2 3, and 4th order

 

Thanks for all the replies so far.

I did alot of amplifier and sound system repair and knew my ear was fairly accurate.

Since this is a low frequency I am trying to eliminate do I need to go as large as I can or would a small resonator say 12" long be able to reduce the frequency, I know this stuff doesn't follow hard fast rules.

Would a H pipe really reduce this resonation enough? I understand they eliminate some uneven exhaust pulses. In this situation do they have enough volume to give those standing waves a place to go? Would the 2 sides be out of phase enough to cancel each other out?

M. McFly

 

I think the problem is in the angle that the exhaust pipes are at.At highway speeds the exhaust pulses are hitting the ground and bouncing back up to possibly vibrate the floor or trunk pan.A friend of mine had a 55 Chevy that did the exact thing at 55-60mph.Drive you right out of the car.Try re-angling the pipes or possibly send them straight back.

 

The pic shows that the H-pipe on my roadster is quite a way to the rear of the optimum performance location.
Which is ok since I'm after sound attenuation and not worried about an HP gain.



Another way around it is to do what some NASCAR cars have done.
A 6" pipe squished down to perhaps 2" or so has sufficient capacity for an H-pipe on a high performance, high rpm engine.

I'd try a 2 1/2" pipe squished down to an inch to an inch and half since you're limited in clearance and being able to get a pipe in.

Another way around the problem would be an inch and a half H-pipe.
The small pipe could do it for sound attenuation.

 

The outlets are under the rear bumper and point behind the car at a 45 degree angle. A straight line from the outlets to the ground means they intercect the groung about 1-2 feet behind the car, so I am driving away from the sound.

I need to post pics, it would be easier to explain the set up and space restrictions I am faced with.

M. McFly

 

Something I have not tried but easier to do:
An H or X will change the frequency, but so will any change in length or pulse sequence.
This includes a pipe attached to the existing straight section, but going nowhere: capped at the end. It resonates in its own length, and changes the frequency of the main tube. What's nice is the cost is very low, and the length is easily adjusted and much easier to hide. A 90° bend right after the connect allows the extension to run parallel to and right next to the main pipe.

 

Ding ding ding. Panic has it. ANY change will change it. Any change in volume will alter the sound. Don't go crazy, just break up that long pipe. If you've already got a place where you've put in straight pipes, alter the volume. Go with a 3" pipe, reduce it down and see what happens. You don't need the tuning volume unless you want it quieter so don't sweat the size unless you really end up with a problem. The H pipe will help because you WILL get cancellation from one side to the other (1357 fire differently than 2468). Like C9 said, you are after sound attenuation, not power.

The tailpipes could be part of it but where your mufflers are is the biggest issue. If you've done stereo installs, think of the size of the cabin you deal with. You stay away from sounds that set them off and vibrate the back windows if you want it to sound good. That is the natural frequency of the cabin. What you are doing with those long pipes and big engine is setting off the natural frequency of the pipe.

If you want to see a cool exhaust, check out the 99-03 or 04 Honda Odyssey tailpipe. It is a flattened oval with an outward curve. The pipe before that going to the resonator is an airgap pipe. It has a small patch of louvers inside it and that airgap is closed on one end. It acts as a 1/4 wave tuner to break up the sound that was set up in that tailpipe. It was setting up a standing wave in the area where the back seat folds down (think Stow and Go) and it was a really low freq. Made that whole van go off the scale. Wait until someone decides they don't NEED OEM replacement on that one!

I worked on that at Walker in 95-97 and tested about 60 different combinations on the dyno to get there. Lots of modelling, lots of calculations, and still had to do a bit of trial and error. Standing waves are a ***** but you can get rid of them.

 

Pipe clamps are your friend. Look for the flat strap type (Torca is a brand name) and use them to try your pipe replacements out first before you weld anything. Having a take apart system is better every time.

 

Word up on that Danimal..the Pipe clamps he speaks of was how i built my exhaust on my 53 chevy..worked like a charm, and i was able to adjust my exhaust , find what fit right , looked right, and most of all Sound Right! before welding it togeather.

and after i found what worked best I put flanges in the system so i could take it apart if necessary for future modifications, to the exhaust or drivetrain near the exhaust.

 

I Work In A Muffler Shop And I Know For A Fact That If You Want To Quiet This Thing Down Inside Use Some Resonators. You Can Get Them In Different Lengths But For Tour Ride I Would Use Some 18in 2.5in Diameter Packs. Imco Makes Some Good Ones That Have Louvers In A Spiral Direction. You Can Put Them In So The Cups Of The Louvers Catch The Exhaust And Will Quiet It Down. Without Having To Redo All Your Exhaust Just Cut Out That Section Where The Frame Is Straight Right In The Middle Of The Car And Weld Those Things In. Thats It You Are Done!
As Far As The Angle Of The Tails Go, As Long As The Actual End Of The Pipe Is Not Under The Car You Should Be O.k. . When The End Of The Pipe Is 45d Underneath The Car You Will Get A Bad Drone.

 

The cups aren't catching the sound, they are letting it into a chamber where it beats itself to death. You've got to watch those on high flow cars because you can get a mean whistle. The other thing is, right in the middle might not do you any good because the standing wave will not see it as a broken pipe. Just off center is probably a better way to go. Think of a jump rope making a wave. Get one standing on it so you have an up half and a down half. There is not action at the center so putting a resonator there won't do anything to it. You've got to hit where it moves. It isn't just volume, it is placement.

 

I installed an hei dist on My el camino and picked up a drone at freeway speed, found the timing at freeway speeds to be less than it should be ,started with changing weights and springs got better ,adjusted vacuum advance for more timing at cruise and drone went away. Guy at work had same drone and he tried the timing change and hooked up his vacuum advance to manifold vacuum and it got rid of his drone also.