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Old 04-26-2009, 04:39 PM
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sonic booms from sbc 350?

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?

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Last edited by brian400ex; 04-26-2009 at 05:24 PM. Reason: wrond description of exhaust
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Old 04-26-2009, 05:48 PM
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What the heck is an exhaust "sonic boom"? A backfire?
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Old 04-26-2009, 07:46 PM
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Resonance...????

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Old 04-26-2009, 09:20 PM
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i got 44s and they eco/boom/beat my mind to peices at at about 3000, ratlles the truck and the car next to me.

is it constant or once in a while?
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Old 04-26-2009, 09:22 PM
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they are going to make alot of diffrent noises. exspecially when there are no tail pipes. Cole
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Old 04-26-2009, 09:28 PM
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i just have turn downs but my pipes run all the way to the rear, then muff,then dump.
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Old 04-26-2009, 09:46 PM
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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'.
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Old 04-26-2009, 10:01 PM
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Thats awesome! Why would you want to NOT make a (sonic Boom?
I want to know how you are doing it so I can do it.
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Old 04-26-2009, 11:03 PM
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Oh yea, killer loud exhaust, one more reason I need 5 grand for hearing aides!
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Old 04-26-2009, 11:12 PM
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Just cross the #1 and #3 plug wires on the distributor. It sounds like a single cylinder with no exhaust.

I had one like this towed in for diagnosis

I almost forgot about this.

Last edited by carsavvycook; 04-26-2009 at 11:15 PM. Reason: Should have added
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Old 04-27-2009, 04:06 AM
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its pretty constant at 3000 rpms. almost sounds like if u had a set of sub woofers in the trunk they r not loud but noticeable. plug wires are pretty new. u stomp on it and put a load on it u cant hear these "booms".
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Old 04-27-2009, 06:05 PM
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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.
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Old 04-27-2009, 11:01 PM
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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.
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Old 05-04-2009, 10:29 AM
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any other ideas? you hear the usual exhaust tone and then you hear a lil "pop" mixed in there. sounds like sonic booms. kind of muffled. not a high pitch sound.
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Old 05-04-2009, 12:02 PM
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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.
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