What creates "Drone"?

[alnukem]

I have a C3 Corvette & am going replace the "Block Hugger Headers" with long tubes. I am contemplating whether to go the normal 2.5" or bump up to 2.75" exhaust pipes & wonder about the drone, as these cars don't have all the creature comforts that our cars do. It is a 350 10:1 stick car with 3.70 gears & a small 268 solid roller cam. Thanks!

[32valvZ]

Exhaust drone is primarily the result of sound waves traveling through the air and vibration that is transferred through a vehicle's frame, body and components. Exhaust drone vibration may also be a result of sound frequency alignment between the engine and exhaust.

[alnukem]

Does exhaust tube size have anything to do with it?

[Paul Workman]

Originally Posted by alnukem

Does exhaust tube size have anything to do with it?

Yes, but to a lessor extent: length is the primary factor.

The Corsa system uses cancellation techniques to eliminate much of the resonant frequency "drone" by (essentially).

Through the use of select lengths of closed tubes, the system capture the specific (drone) frequency sound waves and reflecting them back on themselves resulting in cancelling the particular frequency...and any frequencies close to the objective wave.

The Corsa system is divided into two sections; the resonator and the muffler - both having frequency resonator chambers of different lengths in order to broaden the range of cancellation (if you will).

[WARP TEN]

While I am not familiar with the exact situation with a 350 in a C3, most GM engines seem to have a couple of points in the the rpm range that produce significant spikes in sound frequencies that create the drone you are thinking of. I am most familiar with the LT5 350 engine. The LT5 has resonance spikes at two primary rpm points, about 1,800 rpms (120 Hz frequency) and 3,400 rpms (230 Hz). The resonance is most noticeable under light to moderate throttle load. When a ZR-1 with stock 3.45 gearing is in sixth gear at about 65-70 mph the engine is running around 1,800. Light throttle application will produce drone there unless there is some mechanism to mitigate it. The mechanism used by the stock exhaust system (and as Paul noted, the Corsa system) is called a Helmholtz resonator. It is essentially a closed container that has an inlet for sounds waves to enter but no outlet for exhaust to flow through. It is designed to have a volume and shape that attenuates sound waves of a specific frequency because the waves bounce around in the resonator and cancel each other out. The stock system and the Corsa have center resonators and rear mufflers. Each incorporates a Helmholtz resonator. The one in the center resonator addresses one of the rpm points and the ones in the mufflers address the other. If you were to cut a stock muffler open horizontally you would find the rear half is a closed Helmholtz resonator with just an inlet pipe open to it. (There is also a small 1/2" diameter pipe that comes out in the second exhaust opening on early mufflers that goes directly through the Helmholtz resonator with no effect on it). In the center resonator is is wrapped around the pipe with an opening or openings to allow sounds waves in. Below is a (rather blurry) picture showing the resonator opening at the front of an older Corsa center resonator through a cutaway in the pipe. As to the effect of pipe diameter on drone, I don't believe it changes the resonant frequencies or their respective rpm points, but from experience I do think larger diameter pipes make the exhaust louder.--Bob

[XfireZ51]

If u notice, the drone does not not occur whenever u hit a certain RPM, but rather it occurs when you may be at a certain rpm range and when
Manifold Absolute Pressure (MAP) is higher rather than lower, ie 55kPa+.
Typically this occurs when going up an incline and u depress the accelerator, opening the TBs and temporarily slowing the airflow until the motor catches up. The exhaust note becomes “sweeter” during trailing throttle or in cases of flat to slight decline conditions. What I have found, and not just w the LT-5, is that I prefer a slightly leaner A/F ratio since it sharpens the exhaust note and tends to mitigate drone. The more “blubbery” ur exhaust sounds when beginning to open the throttle, the more likely your AFR is richer during that period. Tuning the VE(Volumetric Efficiency) and AE(Acceleration Enrichment) for that rpm range will allow engine operation to move further down the kPa scale, an indication that airflow velocity has increased and thereby generating higher vacuum. IOW, the motor is operating at higher levels of efficiency. And that helps to minimize drone. A good exhaust and exhaust valving, like the NPP system, can reduce it further.

[DRM500RUBYZR-1]

Some Observations:
1- Drone is a very subjective phenomena. One may find it unbearable, while another finds it only mildly disturbing, yet another barely notices it at all or is bothered by it, all experiencing the exact same drone.


2- Loud and Drone are too different things, however Drone can be easily masked by Loud, and some find it very pleasant when so masked.


3- Drone did not "exist" back in the original days of C1-C3 cars. It was "Discovered" by people who post on Forums about it's horrific nature. Yes the same Drone, that others may fail to be bothered by.


4- EVERYTHING causes it, effects it, eliminates it, or makes it worse.


5- Consider the multitude of frequency adjustments that you can make to a contemporary audio system. One hundred people will have 100 different adjustment levels, that the other 99 will find "off".
Same phenomena exists with exhaust systems. No one system is the clear winner for all.


6- One can make endless modifications to current day systems, while back in the 60's, everyone was quite satisfied with a $25.00 Glass Pack Muffler.


You cannot go wrong putting on 2.5 to 2.75 pipes on a small block C-3 car. 3 inch is somewhat overkill unless you are pumping out 700 plus horsepower. Many big blocks run fine with headers and 2.5 pipes.


My only suggestion is do not weld the pipes and mufflers until you have driven it for awhile with whatever mufflers that you select. That way if you dislike the sound, you can easily make a change or changes until you get it "right".
And remember, if you weld up a C-3 system, hope and pray you never have to drop it for drivetrain repairs.


Good luck with your effort, and I bet that you will love whichever choice that you make given the car and engine choice!


Keep us posted!
Marty

[32valvesftw]

Its resonance and will be worst at certain rpms, with mine its about 1600 rpm. Diameter has something to do with it but probably not much, I think Corsa handles it with baffles of sorts that break up the sound wave and maybe shorten the attenuation, so the resonance occurs at a speed that does not normally occur in normal driving. Mine got worse with he addition of an X pipe.

[alnukem]

Thanks guys!

[USAZR1]

2.75" exhaust tubing is pretty rare, except in some oem applications, so for all practical purposes, 2.5" and 3" is the sizes you will see most. A good mandrel-bent 2.5" exhaust can easily handle 450rwhp and is much easier to fit.

If you want a nice sound with very little to no drone, check out Dynomax's Super Turbo line of mufflers. They're made in many lengths so you should be able to find a pair to fit a C3.