Question about what causes exhaust noise

[jessman1128]

No, my question isn't "What causes exhaust noise?". I'm not that much of a novice.

My question is a little more specific than that. What engine components/functions have a direct correlation to the volume of the exhaust noise? I am enough of a novice to not understand this. I know that exhaust noise is a result of the exhaust gases being expelled from the engine. (I hope I'm right about that, and didn't just disprove my "I'm not that much of a novice" claim...)

So assuming that is correct, would it be logical to assume that an increase in the amount of gases being expelled would directly cause an increase in the sound volume of the exhaust noise?

And if that is a logical assumption, then what engine components/functions would cause the amount of gases being expelled to increase? Would variations in the air/fuel ratio cause this? If it's running rich, that means it's using too high a concentration of fuel, right? And would that cause more exhaust gases and subsequently a louder exhaust noise?


Or, to use a specific example, if you had a car that sometimes ran with a certain exhaust noise volume, and other times ran with a noticeably louder exhaust noise volume (and switched back and forth between the two volumes), and you knew that the exhaust system was exactly the same the entire time, what would you suspect would cause the variation in exhaust noise? (Assume the exhaust noise differences were observed while the car was in park and idling at normal speed.)

[2.2 Straight six]

number of cylinders, angle of cylinders, firing order etc..

[GreyGoose006]

iirc, its the exhaust valves that have the greatest impact on the way a given engine sounds

[curtis73]

cam timing events have a lot to do with it. When an exhaust lobe ramp is timed so that it opens early, it opens while there is still lots of pressure and some combustion still happening. This effect is also simulated by fast exhaust opening ramps on the cam.

Having headers allows some of the exhaust noise to come right out in the open. The exhaust pulses hit the inside of the manifold/header just the same, but manifolds being made of cast iron deaden the sound a lot more.

Mufflers and exhaust diameter are what I would say causes the biggest change. For a given engine configuration, the most change that can be affected is with the exhaust. Consider it like a big trombone. The engine is like the vibrating mouthpiece. That is unchanged. The exhaust tubing does the exact same thing with vibrations that a trombone does; shapes, alters, and modifies the sound. Of course in a car, the point is to cancel some sound waves along the way. Bigger diameters allow flow and more sound to make it out. Smaller diameters (in exhaust AND trombones) make it sound more treble-y and tinny. Go too small with either and it will sound like a trumpet; blatty and harsh.

Altering the length of either will alter its natural resonance frequency. Hence why trombones have longer tubing than trumpets.

You can't make a V6 sound like a V8 any more than you can make a Waltz in 3/4 time sound like a March in 4/4 time. The pulses are just not divided over time the same way.