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158 Posts
is the skunk2 alpha and q300 loud??
The header more changes the tone rather than directly increasing or decreasing the decibel of exhaust. The Q300 is one of the more quiet exhausts for our vehicle.

Skunk exhaust tend to be very loud. Invidia's q300 is quieter thanks to the twin-loop. Full Race is in between the two.

38 Posts
More info as requested:

To change the resonance your system you will have to mount a pipe off to the side, a side branch resonator, somewhere in the piping between the resonator and the exhaust. The location is not critical but rather ideal between the resonator (if you have one) and before the exhaust. Lets go with a 2" pipe. Why 2"? The diameter of the resonator pipe will affect how much "drone" is to be cancelled. Larger is more, however 2" will remove ~70% of your noise and it will fit inside the 3" pipe. IE your welder will like you. There will be a cap on the end of the pipe.

The length of the pipe should be calculated using the speed of sound, ideally we design for a 1/4 wave length of the frequency to be cancelled (dis-regarding all the theory).

First lets calculate the speed of sound. Because the speed of sound changes with temperature you should assume an air temperature of ~100C. Why not 500C? Because your exhaust temperature at the tip (where the drone leaves) is not 500C. If it was, it would be glowing bright red. Usually its very hot to the touch (50C) or maybe burning to the touch (70C), but often not my skin bubbled and fell off (100C). So lets assume 100C to be safe, worst case scenario. The air temperature will be VERY close to the exhaust temperature. So if the exhaust tip is 70C, the air will be very close, not over 100C for a standard length exhaust system.

The formula to calculate the speed of sound vs temperature is:

v = 331m/s + 0.6m/s/C * T (331m.s is the speed of sound at 0C at sea level)

So skipping the math at 100C the speed of sound is 391 m/s. Second you need to figure out your drone frequency. So lets say at 3000 RPM it drones. Ok so 3000 revs per minute = 50 revs per second = 25 exhaust pulses per second (4 stroke so divide by 2) = 100HZ (25HZ*4 Cylinders).

So 100 sine waves (per second) spread out over 391 meters (per second) = 3.91 meters per sine wave = one wavelength.

3.91 meters / 4 (1/4 wave length)= 97 cm = 1/4 wave length.

Thats a long pipe! Well ideally you would have almost no drone if you used a 97cm side branch resonator. Reality check! Thats not gonna happen in the real world.

So be sure to use harmonics of 97cm. Like 97cm/2 or 97cm/4 (24 cm). This will not be a complete noise cancelling affect as the cancelling sine wave will be shifted left or right and not entirely cancelling the wave at 3000 RPM, but it will still make a huge difference. A 10% reduction will sound a world of difference, a 30% reduction and you won't think its the same car.

Sound/Noise is calculated in db, it is logarithmic, and every 3db is a power factor of 2. So even if you only drop the drone a few db, it will make a huge difference in over all drone.

Good luck, and theres no warranty on my math :)

I know this is an old threat but I wanted to share my experience with my set up.
I have a Q300 cat back on a FA5 with stock header + cat. I was experiencing bad drone on highway.
After lots of research, I ended up adding a 2" diameter with an ~ 98 cm branch to my mid pipe {this includes the bend}.

The result. Drone reduce a lot compare to before. However, CERTAIN TIMES the drone comes back but at a less intensity. If anyone is planning on doing it. I would suggest on using a bigger diameter 2.5" or 2.7" which is the same diameter as the Q300. My only concern to that would be ground clearance which is a problem as there is not a lot of space under the car to add a 97 cm pipe.

Hope this help someone.
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