Chop Chop

[Dinosport]

I have a 78 Mali with a small block, Ls swap being put together. So the car currently has headers true dual that exit behind the rear tires. Ive always done my duals all the way out under the rear bumper. Something I noticed is that at cruise speed or maybe just a little about the exhaust makes this chopping sound thats pretty annoying.
I would like to fix that when Im doing the swap but Im not sure what causes it. Is it cheap mufflers? I have no idea what they are as they came with the car? I was going to H pipe it anyway would that have any affect? Or should I just redo the rear of the exhaust straight back like I usually do. exiting under the rear bumper.

 

[83StreetMachine]

It's probably a combination of the two. Cheap mufflers causing a resonance against the floor and inside the cabin of the car. And not having an H or X pipe on the exhaust to balance the flow/scavenging and sound.

If it's driving you nuts just swap out the mufflers for now. I recommend Magnaflow. Or you can wait until you are ready to do your swap.

I recommend just starting fresh with a new exhaust when you do your swap. Get a nice set of mufflers and an X pipe exhaust. And I always exit tail pipes behind the rear tires. It's a much cleaner look IMHO.

 

[motorheadmike]

Muffler design has a huge influence on this. An X-pipe will do a lot to soften the exhaust pulses, more so than an H-pipe. You can add a tuned resonator to each pipe either pre-X pipe or pre-muffler - moving them around can have different effects due to the speed and density of the exhaust gases moving through the pipe (moves faster and is less dense closer to the head)... at least as I understand it. Personally I like having cats in place to normalize exhaust noise... I'd like to say that they hinder performance - but, given the 11.9 time slip I have for our TBSS, and all the 11, 10, and 9 second cars coming out of the factory these days that would be a bald-faced lie.

The wagon has cats too.

 

[TURNA]

Hooker Aero Chamber's

 

[Dinosport]

Muffler design has a huge influence on this. An X-pipe will do a lot to soften the exhaust pulses, more so than an H-pipe. You can add a tuned resonator to each pipe either pre-X pipe or pre-muffler - moving them around can have different effects due to the speed and density of the exhaust gases moving through the pipe (moves faster and is less dense closer to the head)... at least as I understand it. Personally I like having cats in place to normalize exhaust noise... I'd like to say that they hinder performance - but, given the 11.9 time slip I have for our TBSS, and all the 11, 10, and 9 second cars coming out of the factory these days that would be a bald-faced lie.

The wagon has cats too.

Thanks Ill keep that in mind as I plan my swap. As for the cats hindering performance, at one time that may have been true but these days they have become so much better. The key is to get up to date cats and not what the gbody wouldve come with. Ive never had this problem with rear exit exhaust though.

 

[axisg]

So from exhaust pointing behind you ( rear exhaust ) to beside you ( behind the rear wheels ) will change the sound. My vote would be start with H-pipe then maybe dual cats as mike suggested. This made HUGE difference on my friends truck. It was annoying with true duals and just adding the Hpipe and dual cats make it stop setting off car alarms and highway drone so loud you could not hold a conversation.

Good info here

 

[Bar50]

I have Magnaflow straight thru style mufflers (look like stainless turbos). They are on my 78 with a stock 454 and small tube, long, headers. True duals that dump behind the rear tires and an angle outwards towards the curb (muscle car style). Mine sound great at idle, nice deep tone, but, they sound like glasspacks. They have a warble and cackle, just like glasspacks, only they cost like $90 each. I am going back for Flowmasters, but, thats a person opinion.

 

[pagrunt]

This may not be the issue but could it be a exhaust leak? I know when the headers started to leak I got a popping noise when ever I let off on the gas. Maybe a loose pipe joint or bad seam/weld on the muffler?