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Question about rumbling/backfiring in the mufflers


47heaven

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A while back, after I had my second pipe put on, I noticed that when I would coast in third or second gear I would hear the small rumbling and backfiring in the mufflers. I would say it's more of a rumbling/popping sound. Not as loud as the rapping on the lowriders, but you can hear it. I asked Norm (Normspeed) about it and he said that he wouldn't worry about it...that it sounded cool. He's right, it does have a good sound to it, but I'd really like to know if this is normal or this a sign that something needs to be adjusted. I was told it was unburned gas igniting in the glasspack or muffler. Is that supposed to happen?

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That backfiring is caused by an incomplete fuel burn in the cylinders, which causes the fumes to explode somewhere in the exhaust system. It's exactly the same popping you hear from a two-stroke dirt bike. (It's also the reason that flame arrestors are required on dirt bikes.)

Check for an air leak in your exhaust system. If the backfires are intense and/or happening at or near the exhaust manifold you could get valve damage. If they're occuring way back in the mufflers and barely a rumble, smile and enjoy the sound.

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We ust to do that with our model A on purpose. You can retard the spark manually on an A bone and then let oft the gas then just wait a second or so and blame!!!! Kind of cool but sure can would do number on the old muflers.

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Question here though, is it actually backfiring or just the normal popping and rumbling of the glass packs. That's the problem we run into when on a forum or listing to a description. Some people have different ways of explaining things and that makes it hard to know for sure if it's ok or not.

I will say this much though. Best set of pipes and glass packs I ever had on a car was the 57 Ford I owned in 1964. Had a 1963 Police Interceptor engine under the hood with dual glass packs out the rear. Rev that thing up and let off the accelerator real fast and you never heard such a sweet (loud) rumble and popping out the tail pipes. Music to the ears without the radio. Would wake up the dead.:D But.........it did not backfire. The popping or plopping was the normal sound of the glass packs. Actually, they were the Cheery Bomb mufflers.

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I'll jump in because I actually heard heard Darin's car in action but this is not an objective opinion because I like a motor with a little bit of attitude. :D

Darin's motor is one sweet running flattie. A fresh Dodge 230 in a P15, 3 speed with OD. A single fresh Ball & Ball carb. He has a split exhaust manifold, dual Smittys, and stock diameter (1 1/2" or so) pipes to the rear bumper.

When we were coming down outta the hills, maybe a 3,000 ft descent or more, using the motor for compression braking, throttle closed, she rumbles and pops sorta randomly, just like a nice set of dual glasspacks will. Around town when warmed up and rowing through the gears she might give a little rap at higher rpms or occasional single pop between gears. Not a backfire. I like it a lot!.

That said, I heard her after we retarded the timing about 6 degrees from where Darin's regular mechanic had marked the flywheel. A story for another thread, it's a 12 volt HEI distrib. Darin was interested in fine tuning his motor to run as cool as possible in hot weather in hilly terrain so we tried a little less initial advance.

All I can say is, it's all in what your ear likes. Back in the day, that would have been a six that people noticed. I wish my 47 2 door sedan in high school sounded that mean. But, a decent muffler shop could switch back to some quieter mufflers pretty quick, and someone here would probably love those Smitties:)

I wish I had access to a digital camcorder again, the one I had was a loaner.

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Question here though' date=' is it actually backfiring or just the normal popping and rumbling of the glass packs. That's the problem we run into when on a forum or listing to a description. Some people have different ways of explaining things and that makes it hard to know for sure if it's ok or not.

I will say this much though. Best set of pipes and glass packs I ever had on a car was the 57 Ford I owned in 1964. Had a 1963 Police Interceptor engine under the hood with dual glass packs out the rear. Rev that thing up and let off the accelerator real fast and you never heard such a sweet (loud) rumble and popping out the tail pipes. Music to the ears without the radio. Would wake up the dead.:D But.........it did not backfire. The popping or plopping was the normal sound of the glass packs. Actually, they were the Cheery Bomb mufflers.[/quote']

That rumbling you're hearing is back-firing. All a backfire is, is a fuel detonation outside of the piston cylinder. Where it occurs and how severe it is determines whether it's an undesirable situation.

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That rumbling you're hearing is back-firing. All a backfire is, is a fuel detonation outside of the piston cylinder. Where it occurs and how severe it is determines whether it's an undesirable situation.

Dan,

The rumble in my 57 Ford wasn't a backfire. It was the Cherry Bomb mufflers making the sound. I think Normspeed just used a better word for describing it. It's more of a rapping sound coming out of a nice set of cool sounding pipes, when idling or letting off the accelerator. That's music to a hot rodder. If we wanted to get a backfire back in the day, we would just turn the ignition key on and off real fast, while rolling in gear to create a backfire.

I also have a slight rap in my P15 with the single Smithy that's on it. Nothing like on that 57 Ford I mentioned before though. If that noise in the 57 Ford had been caused by backfiring the engine would not have lasted about 10 years of backfiring. I only had the car a few months after putting that Police Interceptor engine in it because I bought the new 64 Chevelle not long after. However, my father bought the Ford after I bought the Chevelle because he needed another car and knew the 57 was in tip top shape. He kept that car for about 10 years without any engine problems. Also, that car would roll after the engine swap I made. You could hit passing gear in that thing at any speed and it would lock in regardless of how fast you got it going. It wouldn't kick out of passing gear until you let off the accelerator. Was a real sleeper in a street race.:D

P.S. I did not make the engine swap myself. I had it done by a local service station in Louisville, KY. I just bought the engine, etc. to make the swap. The service station was Puckett's Service station and they were known in the area at the time for building race cars. So.........that Ford was tuned perfectly.

If you remove the muffler from any car you will get the same rapping/plopping etc. running a straight pipe on a perfectly tuned car. It's not backfiring since nothing changed except the muffler being removed. Same applies to the glass pack mufflers, they just create the sound, some better than others.

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Hey guys...thanks for all that feed back. I really do like the sound it makes. I mean, it sounds like a "guy's" car. I just wanted to be sure that it wasn't going to cause some damage down the road. I will do the checks on the spark plugs..etc., just to make sure all is well. And it really is more of a popping and rumbling sound, rather than a backfire, now that I've heard it better.

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Some folks pay big bucks to get the sound you have Darin. Enjoy.

I must admit that for the most part Norms Coupe gave a somewhat correct answer to the backfire question. This is the second or maybe third correct answer Norm has given to questions on this forum:D

A backfire is timing or valve-related problem and when it occurs fire will exit the top of the carburetor. Hence the engine is firing backwards with reversed exhaust flow.

When an engine is under acceleration the air/fuel ratio is somewhat leaner than when an engine is decelerating. So the rich mixture not burning completely in the combustion chamber creates the popping in the exhaust.

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I've got warranty papers for this same popping exhaust from when my 94 dakota was new. Now this is a computer controlled v6 with sensors etc to make the fuel mixtures perfect etc. Couple times it went to the dealer and they said it was normal for a manual tranny vehicle. So its been popping on deceleration since new and hit 174K last weekend. Darin keep driving!

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