I have yet to come across a definitive position on noises coming from the exhaust system of engines similar to ours, if not ours exactly.

Should there, at any time, be audible sounds from a healthy exhaust system?

Let healthy be defined as the following:

No leaks of macroscopic proportions, including pinholes around welds, from the exhaust port to the tail-pipe.
No forms of fuel injection such as those necessary for flame exhaust, flame throwing exhaust, or any system designed to produce an intense combustion of alien fuels.
The system is one which extends normally to the tail pipe with the use of catalytic converter(s), muffler(s), and appropriately sized piping of two and a half inches or three whereby the primaries of both headers are of one and five-eighths inches or one and three-quarters inches.
There is no binding of hard, metallic components which are made of materials with blatantly-high COTEs.
The engine is not being operated at a fuel-to-air ratio diverting more than two and a half points from Stoichiometric (1:0.068027210884353741496598639455782).
No small green men with cave-man like clubs banging on the system of any part of the car for that matter. This also includes the person monitoring the sounds. :lala:

Okay, so now that I have put that out there to weed out any of the smart-asses or simply idiotic folk trying to waste bytes, to the more layman's explanation. The bulk of my observations for these abnormal sounds are included in deceleration of the engine rpm, not the vehicle itself. There are so many words, phrases, and general false notions being thrown around about this, I am here to wipe the slate clean and then mark it. There are so many insignificant people saying how much they like it that I am brought to deduce it is not healthy. ...not that I am by any means, significant; how often do people know what it is good for them, or the high performance engine.

Backfire, afterburn, and lean pop, all seem to be used interchangeably among the laymen. I understand the concepts and arguments behind many of those, but maybe someone of a higher order can make a qualitative definition of all of these noises. My primary concern is with the noises that are present during deceleration and the aforementioned question:

Should there, at any time, be audible sounds from a healthy exhaust system?

 

great question i wanna know the answer too

 

Yeah me too...

 

Should there be audible sound from a healthy exhaust at any time??? Of course there should, it's simple physics. Combustion, explosions cause shockwaves, they have to go somewhere so they travel down the exhaust, and a muffler does exactly what it's name implies, it muffles, not eliminate exhaust sounds.

Now if you are wanting mathmatical evidence or something like that, then i think you are S.O.L. with anyone on this website.

 

I was referring to the pops, bangs, and crackles, supposedly caused by a host of theories I have come across. In particular I am referring to those experienced when a revving engine is restricted of air via the gas pedal in a TPI car.

 

Those noises you describe are created from afterfire (I know, I know, another term). Basically all that's happening is unburnt fuel igniting in the exhaust. It's not theory, its fact.

No car will have exhaust free of unburnt fuel. Not our cars, not newer cars, not any internal comustion engine. They just aren't that efficient. Even Formula One cars, the most technologically advanced cars in the world have hydrocarbons in the exhaust.

Upon decel a condition called reversion takes place. Basically a small amount of air is sucked back into the exhaust toward the engine, in reverse, hence the name.

Excess fuel + heat generated from the combustion process + air = ignition = noise in the exhaust.

Exhaust leaks are the #1 culprit. Large single exhausts are second. Large dual exhausts are less likely to create the noise, as only one bank of cylinders dumps fuel into each pipe.

If you are experiencing the noises, double check for exhaust leaks. Anytime I experience the noise it is due to a loose header bolt, cracked weld, etc creating a small leak. Other than that, run a smaller exhaust.

 

I bolted on my 3" SLP cat-back the other day and didn't get the muffler on until a day and a half later. When I had the open 3" pipe in the back, and I revved the engine in park and let off, it would pop a few times. Doesn't do it with the SLP muffler on, and didn't do it with the old 2.75" exhaust either.

 

So the only reason it doesn't happen on factory exhausts is because they are restrictive in comparison to an aftermarket product?

The best I can say is that when you close the throttle blades the system has enough lag so that it is still injecting fuel causing a rich condition. At 3000 rpm, thats 50 revolutions per second, with only enough air for 13. So, I would imagine this is where the access fuel is coming from. Now, the question becomes what does a smaller diameter tube have to do with it? Maybe because in a system with too big of a tube, there is enough air to disturb the perfect vacuum which theoritcally would occupy an area between exhaust pulses. With this problem, you have a space of air next to a space of hot fuel and exhaust. This making any sense to anyone but me? lol Maybe I am just :lala: :lala:

 

Larger pipes usually mean a slower exhaust velocity. Less air coming out makes it easier for the reversion process to take place. If you want more info on reversion check out a marine engine builder site, or someone who is familiar with marine engines. Reversion in a marine engine is common when noobs put big cams and pipes in their V8s looking for more power. Reversion in a marine application is disastrous!