Glass Packs as Resonators

[FirstEliminator]

Hey guys,

There is an awful droan noise from the exhaust system when crusing about 65 mph. If I go past 70, it fades away. I have heard that adding a glass pack in the tail pipe will vastly reduce the droan. The exhaust is in a 69 Cougar with a 351w. It is 2 1/4 from the cast manifolds. Then into a Y that feeds to single 2 1/2 pipe, into a 2 1/2 Walker Turbo muffler then continues as single 2 1/2 over the axle and out. Cougars originally had single 2 1/4 exhaust the whole length in a 2 barrel application. The car is restored pretty much stock with a little it of pep worked in here and there.

A long time ago I remember reading on some cherry bombs that if it is installed the direction where the inner louvers will catch the exhaust it will make it quieter. Turning it around where the air flow skims over the louvers will make better flow with louder sound. The new 18" round glass packs I got from Summit have an arrow suggesting the air flow should skim over the louvers. Will it make it quieter to turn these around. If so, is there any problem with doing this?

All in all, I'd like to get rid of the droan. With this particular car, I like the A/C on, windows closed, cruising in quiet where I can talk to the passengers without yelling.

Before cutting and welding I'd like to hear from those that have conquered the droan noise at cruising speed.

thanks,
Mark

[sportsroof]

IMHO, yes it will help. I did similar to my '70 Mustang with 460CJ years ago. Basically motorsport here (UK) is all noise tested and I was afraid of failing noise with just with one muffler each side (dual exhaust). I made up some small 'resonators' like mini glass packs and fitted one each side just before the tail pipe exits. Dropped the noise noticeably and passed noise tests!
Can't answer your question about the louvres, I've heard conflicting reports. I just use mufflers with the perforated tubes, not the louvred type. HTH
Cheers, Martin

[Belgian1979]

I have a bullet style glass pack muffler in the middle of the car on the 2 exhaust pipes. No drone. Noise is nice but rather loud, albeit not obnoxiously loud.

[Geoff2]

From my experience, every combination is different. What works on Bill's car may cause drone on Joe's car. Even axle ratio can affect the exh because the engine load changes with a change in axle ratio. You just have to experiment. Case in point. I recently rebuilt an engine that had headers on it. Only internal change was a slightly hotter cam [ a factory cam ] that still provided a smooth 600 rpm idle in gear. The owner didn't want the hassle of headers, so he fitted factory exh manifolds; the car is just for touring/cruising. With these two changes, & no other changes, it now has exh drone, where as before it didn't...

[Belgian1979]

From my experience, every combination is different. What works on Bill's car may cause drone on Joe's car. Even axle ratio can affect the exh because the engine load changes with a change in axle ratio. You just have to experiment. Case in point. I recently rebuilt an engine that had headers on it. Only internal change was a slightly hotter cam [ a factory cam ] that still provided a smooth 600 rpm idle in gear. The owner didn't want the hassle of headers, so he fitted factory exh manifolds; the car is just for touring/cruising. With these two changes, & no other changes, it now has exh drone, where as before it didn't...

I've seen drone mentioned on some exhaust mufflers more than on other. Most notably Flowmaster.

Another thing that plays a role is the amount of cabin sound profing. In the pre 80's this wasn't what it should be. Different times, different demands.

[DaveMcLain]

Placing an inexpensive glasspack or similar type of straight through absorptive muffler right at the end of the pipe can be very effective at muffling and reducing drone without sacrificing performance.

I have a set of those really small Schoenfeld mufflers from Speedway that I sometimes will stick on a header while dyno testing. Most people are amazed at how much they cut down the noise without sacrificing much if any performance.

[John Ross]

I believe the droning noise that people complain about is when the engine is at a specific RPM, and the noise made is the result of that sound bouncing around the car's specific interior volume and shape in such a manner that it is annoying. Change ANYTHING about the combo, and the drone will either disappear or shift to a different speed/RPM.

My '96 Viper coupe had a stock drivetrain & exhaust, and an annoying drone at 75MPH in 6th gear on the highway. Opening the back hatch ONE INCH made the drone go away.

I'd bet that if you change almost anything about your exhaust system, it will improve your situation.

JR

[panic]

Sometimes what you're hearing is noise caused by a specific mounting point, the muffler buzzes and the hanger makes the floor vibrate etc.
Adding a support, weakening one (drill holes through the strap, isolate it with a rubber donut), or moving one may help.

[digger]

Most of the glass packs I've used are perforated tube so not directional. They do work well when you get some length and volume.

[Belgian1979]

Most of the glass packs I've used are perforated tube so not directional. They do work well when you get some length and volume.

Agreed. But it depends on what you want to have in terms of sound. Smaller volume attenuates at lower speeds but gives off brutal sound at higher engine speeds.

[FirstEliminator]

Has there been any preference between the glass packs with the perforated tube versus the internal louvers?

thanks,
Mark

[strokersix]

If you don't have room for full length glasspacks you can section them with a chop saw and weld them back together to make shorty mufflers. I did this on my '72 Nova. Mild bbc, headers, dual pipes into single flowmaster mounted in stock location, dual tailpipes exiting under the bumper. It was louder than I liked. Adding the shorty glasspacks in the tailpipes quieted it down a bit.

[Circlotron]

Sometimes what you're hearing is noise caused by a specific mounting point, the muffler buzzes and the hanger makes the floor vibrate etc.
Adding a support, weakening one (drill holes through the strap, isolate it with a rubber donut), or moving one may help.

+1
Sometimes a straight strap can be a problem because you are asking it absorb vibes by stretching which depending on the material, they don't always do very well. Much better to bend the strap at a right angle so one end has a vertical fastener passing through it, the other end a horizontal faster. Best to attach the strap or whatever to a chassis rail or similar rigid object. The floorpan is somewhat flexy, especially the further you go towards the centre. It makes the entire floor act like a loudspeaker cone. Same deal with transmission crossmember attachment points. Best ones reach right to the sides of the car where rigidity is greater, not to barely either side of the transmission tunnel.

Also, how far the tailpipes(s) extend beyond the rear of the car has quite an effect too. Easy to attach temporary extensions and check for any improvement.

[digger]

Most of the glass packs I've used are perforated tube so not directional. They do work well when you get some length and volume.

Agreed. But it depends on what you want to have in terms of sound. Smaller volume attenuates at lower speeds but gives off brutal sound at higher engine speeds.

i ran two short hot dog cylindrical resonators in parallel part way down the exhaust on a twin exhaust system and it was terribly droney at low rpm.
So i designed a bigger volume one basically joined them like an oval twin in and out straight through muffler and it was significantly better. due to space restrictions the length had to remain about the same and it was maybe a 2" longer body. i am not sure if having the two pipes being able to someone communicate with each other (each pipe is connected to the same volume stuffed with glass and stainless wool) had any effect.there was an existing rear muffler that remained the same

[ptuomov]

There's a long recent thread on various related topics:

viewtopic.php?f=1&t=49628