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sonic booms from sbc 350?

2K views 24 replies 13 participants last post by  BBCMudbogger 
#1 · (Edited)
I have a 1966 impala with a mildly built 355. duel 3" exhaust with "X" pipe and 40 series flow masters that end right before the rearend. car runs great but i get these sonic boom sounds when cruising at about 3,000 rpms. tack does not jump. is this normal with my exhaust?
 
#7 ·
If you have the spark plug wires 'zip tied' together, cut them apart. A crossfire may not be noticed at idle, but it will be noticed under load. You can fix this by wrapping a 'zip tie' around the plug wires, and wrapping 'zip ties' between the plug wires. Pull these tight, and then snug the outer 'zip tie'.
 
#11 ·
ONE of the 350s (notice i said one, meaning there were several engines in it at one point) i had in my old 72 pickup started doing that after it got hot on a highway trip once. The next time i took it out on the highway it blew up. Not sure what the sounds were caused by but it was related to it getting hot. It never ran right again after that. I hope this isnt the case for you but its possible. I suspect it was a valve problem.
 
#12 ·
You're describing what is typically referred to as exhaust drone. It is very common with 40-series. Depending on how your exhaust is configured, there is always risk of drone, but you can mostly eliminate it with quieter/better designed mufflers.

The drone comes from the exhaust creating sounds that resonate inside the cabin much like subs resonate best at a certain frequency in their box's volume.

The phenomenon is known as a "standing wave" where the length of the frequency bounces back on itself in a mirror image. You can duplicate the same thing in the shower by singing. Start sliding up in pitch and you'll get to a frequency that makes it sound like the shower walls just collapsed on your ears.
 
#14 ·
im running a 389 in my 68 tempest, super comp headers, stainless 3" exhaust with delta 40's and a x pipe.

when im cruzing, and let off the accelerator, it will idle down and will emit a low but distinct "boom".

when i smash the throttle, she runs like a bat outta hell.

sometimes when cruzing on the freeway it will make other people look.

i tried to solve this problem by isolating plug wires, changing timing and even rejetting my carb...

nothing worked, it has a pretty radical cam in it ( 290 dur @ 050, with .525 lift 112 centers).

since it didnt hurt peformance, ive learned to live with it.
 
#15 ·
h0trod389 said:
im running a 389 in my 68 tempest, super comp headers, stainless 3" exhaust with delta 40's and a x pipe.

when im cruzing, and let off the accelerator, it will idle down and will emit a low but distinct "boom".

when i smash the throttle, she runs like a bat outta hell.

sometimes when cruzing on the freeway it will make other people look.

i tried to solve this problem by isolating plug wires, changing timing and even rejetting my carb...

nothing worked, it has a pretty radical cam in it ( 290 dur @ 050, with .525 lift 112 centers).

since it didnt hurt peformance, ive learned to live with it.
I wonder if in your case that could be an (after fire).. unburned fuel collecting and igniting in the muffler?
 
#21 ·
Exhaust backfires can be caused by several things, but I would check exhaust valves first. They might be sticking open/not sealing well. I would also check the timing chain. Usually they start making a lot of noise before they affect the valve timing too much, but if there is a lot of slack it can cause backfiring. Excessively rich mixtures can make backfiring, but you would probably notice black smoke.
 
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