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      08-19-2008, 11:26 PM   #1
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Question: Engine/exhaust sound change based upon rpm

Every once in awhile, I'll accelerate with a fairly good amount of throttle while in a higher gear and lower rpms (yes, I know that's contrary to the whole high-revving M concept, but if I'm not going for max acceleration, I don't always feel the need to downshift to maximize torque). Under such circumstances (such as accelerating hard but still less than full throttle below 2500 rpm's), the engine/exhaust sounds more "burbly" or raucous (hard to describe with words) rather than the smoother, more "monotone" higher-pitched revving sound you hear when accelerating hard in a higher rpm range (such as 3500 or higher).

I'm assuming that throttle/valve settings are different for each of the scenarios listed above, which in turn explains (I assume) the different engine/exhaust sound (rather than any kind of problem with my engine under such circumstances). Just curious if anyone knows the technical explanation for the differing sound under these different conditions... Thanks.
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      08-20-2008, 03:31 PM   #2
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Hmm I dont have an explanation for this but I know what your talking about because its happened to me before
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      08-20-2008, 04:49 PM   #3
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My guess would be that at lower RPMs, some of the mechanical parts make minimal and intermitently audible noise and therefore sounds "burbly". As you climb the RPMs, the engine goes faster, and everything starts making more noise until it all starts making noise at a constant rate therefore sounding smoother.

Who knows. Just guessing.
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      08-20-2008, 05:47 PM   #4
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I'm with you on this one ^. As the engine speeds up, everything becomes more of a hum than single events. Think of a wheel like on wheel of fortune. A slow spin and you'll every little click, and on a fast spin it'll be more of a hum. The same thing goes for an engine.
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      08-20-2008, 07:30 PM   #5
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my Z4 does the same thing
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      08-20-2008, 07:38 PM   #6
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As far as I understand it the sound is created by a pulse in the engine. The faster the engine is going the quicker the pulse will be moving. There is a constant pulse that goes back and forth between the piston and the intake, and another between the piston and the exhaust. The faster and more frequent this pulse is happening the smoother and higher pitch the sound is. At least this would be my guess.
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