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Discussion Starter · #1 ·
Why a new thread for this exhaust?

Because even though there have been a number of good reviews, that accurately describe the great aspects of this exhaust. I think there are still unanswered questions by some members about whether this is the right system for their car.

I want a replacement exhaust that makes the engine more audible at medium to wide-open throttle, particularly as the RPM’s rise towards the red line. But it needs to be as close to factory as possible when rolling around town or out on the highway on light throttle. I think this would make the 2.0 Giulia a little easier and more engaging to drive with the paddles &/or when you put it in D mode. A bonus would be to make the system less flow restrictive so that the full benefits of any potential intake or ECU tweaks can be realized. I pretty sure I have seen posts expressing a desire for essentially this feature set in a replacement exhaust.

I have been waiting to write this for a week as I got a bit more familiar with this exhaust. Also to drive in different situations not just commuting on the highway.

So how close does the Centerline Corsa come to what I want?

At medium or more throttle and 3,500 plus RPM’s its awesome you really get to hear the MultiAir 2.0 and the tone has dropped a couple of octaves. Throttle blips on downshift bring a smile every time. It snarls and snaps on overrun. Paddle shift points can now easily be done by ear. Anytime you are asking the engine to try it’s a great noise. Even at idle the sound is completely in keeping with what your brain says a car that looks like the Giulia should sound like.

On the highway at cruse hovering around 2,000 at light throttle the iPhone dBm meter says the volume level hasn’t changed. So we are golden, right?... Unfortunately, no.

The issue is that the system is strongly resonant at least to my ears right around 2,000 RPM. And the sound it makes is really low, down at the point where you more feel it than hear it. People sitting in the back seat do feel it. Definitely not a mechanical vibration, my install has lots of clearance everywhere. If you want to know what back seat passengers are experiencing just fold the back seat down and you can feel the sound.

What about on surface streets?

The 2,000 RPM resonance becomes a much larger fraction of the car noise at surface street speeds. Not enough to interfere in any way with talking to other people in the car including those in the back. But you are definitely strongly aware of the engine sound all of the time. From outside the perception is different, its quite a refined growl and not obnoxiously loud.

What I would love is a way to reduce the 2,000 RPM resonance. I called Centerline to ask whether there was any way to purchase a center section with a different resonator and was told there is no plans for anything like at this point. If I could cut the 2,000 rpm resonance by 6dBm I would have no problem unconditionally recommending this exhaust to anyone. But as is, the Corsa exhaust definitely asks that you give up an inaudible engine at surface street speeds and highway cruse and that isn’t for everyone.


Why no audio clips?

Simple answer I don’t have a microphone and very few people have speakers that will come anywhere close to giving you a good idea of what this exhaust actually sounds like.

Am I keeping it on my Giulia?

Yes, family members convinced me that I was being overly sensitive about other people’s perception of the sound and that I should embrace the rumble as part of driving a Giulia. I do love 90% of the sound the Giulia now makes and it does seem to pull a little more in the 4,000 RPM plus. The new exhaust tips are great visual improvement over the shrouds the Giulia came with. If your inner child is all grown up this probably isn’t your exhaust. But if hearing the MultiAir 2.0 growl and snarl makes you laugh out loud, it definitely is.
 

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Discussion Starter · #2 ·
Install hints

The instructions ask you to initially hang all sections with the clamps loose and I think this is important. I would also suggest that you slacken the nuts for mount of the very first rubber hanger because small changes in alignment here make big differences further back. With everything a little loose the whole system can find the point of minimum tension in all the hangers and this seems to provide good clearance everywhere and minimal chance of rattles.

Tip protrusion, mine definitely stick out further than some of the install pictures but I’m happy with the position. See attached photos.
 

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Thanks for the great review, Alan. I had been on the fence about this system and seems our priorities in sound are similar. In taking to the centerline folks (who were really nice, honest/straightforward) I wasn't convinced that this system was completely drone free at the lower rpm range in realistic conditions. They were kind enough to talk me out of ordering after a great discussion about my taste. For me personally, I love to hear the engine note and enjoy an exhaust that accompanies it but doesn't resonate over everything else. I'd be okay with that in high revs but certainly not anything very deep in the lower range. The coolest thing about this car is that it can be very silent/tame then instantly summoned for the beast that it is with the right DNA programming to boot.

I get this impression from the madness products as well from the clips and design attributes but there isn't a whole of info on them. I think I'm going to go with the Remus option and thier modular approach. Start with the resonated axle back (a design element missing on other systems) and add the center pipe (which deletes the center resonater if more sound is desired).

I'm not concerned about adding a lot more power to this car as it's already fast enough to create trouble! I doubt that 3inches over 2.5/2.75 will matter much for those looking for a modest gain through other mods.
 

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I've had mine on my Ti Sport now for 5 days.. I agree with Alan that it growls at low RPMs - but I'm ok with this as it's personal preference. At cruising speeds it seems not much of a change from before - maybe a bit more masculine. But on acceleration and above 3000 rpms - oh my, this thing barks and sounds like a "real" Italian car. Absolutely love it and it could be in my head but seems to accelerate a tad quicker when punching the throttle.

I had mine installed at the Ferrari/Maser dealership - the service guys loved it - said it had a bit of 4C to it now. But keep in mind these are guys who are working on high performance cars all the time so even with the Corsa exhaust the Giulia is still pretty tame to them.

My advice - if you want to unleash your inner Italian then go for it. If you want the option of toning things down around town to a quiet, refined sound then this may not be the system for you.
 

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Yes, family members convinced me that I was being overly sensitive about other people’s perception of the sound and that I should embrace the rumble as part of driving a Giulia. I do love 90% of the sound the Giulia now makes and it does seem to pull a little more in the 4,000 RPM plus. The new exhaust tips are great visual improvement over the shrouds the Giulia came with. If your inner child is all grown up this probably isn’t your exhaust. But if hearing the MultiAir 2.0 growl and snarl makes you laugh out loud, it definitely is.
Thanks for the honest assessment and discussion.

The issue of "drone" has become important lately - mainly due to the new generation of 8+ speed automatics that quickly shift into the highest gear possible for fuel economy. The ultra-low RPM driving conditions this produces often coincide with natural resonances produced by the engine. In discussions with Magnaflow, this is basically an issue industry-wide, and a big part of the reason OEM systems have become so quiet.

We did spend a lot of time working on the issue and testing different exhaust configurations. One prototype design had a large rear muffler, which effectively killed all resonance, but it also killed most of the sound at higher RPMs. We just didn't feel this design was different enough from the OEM system to justify producing the system, but may look at it again in the future.
 

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Why a new thread for this exhaust?What I would love is a way to reduce the 2,000 RPM resonance. I called Centerline to ask whether there was any way to purchase a center section with a different resonator and was told there is no plans for anything like at this point. If I could cut the 2,000 rpm resonance by 6dBm I would have no problem unconditionally recommending this exhaust to anyone. But as is, the Corsa exhaust definitely asks that you give up an inaudible engine at surface street speeds and highway cruse and that isn’t for everyone.
Alan - I have a prototype center section with *NO* resonator at all. I can send this to you if you would like to use it as a blank canvas and experiment with installing a different resonator to fine tune the system to get exactly what you want. Shoot me a PM or email at joe [at] centerlinealfa [dot] com and we can arrange that for you.
 

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As far as I am concerned it is the perfect system for me. Exactly what I was looking for. It can be a beast or it can be tame. It all depends on your right foot.
 
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This is why I enjoy this forum

This thread is a perfect example of what Makes Guliaforums deeply satisfying. Straight talk. Clarity. Transparency. Engaged members and Vendors.

Bravo all.

BTW can't wait to get my Corsa on. Soon I hope!
 

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After spending sometime in our 2.0L Giulia over the past week. It sounds very good.

It has a nice rumble at lower RPMs and a nice vibrant growl when you get into heavier engine loads.

As stated before it really depends on how far down you have your right foot.

You can still sneak around without be obnoxious and when you want to have fun, it sounds wonderful with a very distinct Italian note.

 

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Thanks for the honest assessment and discussion.

The issue of "drone" has become important lately - mainly due to the new generation of 8+ speed automatics that quickly shift into the highest gear possible for fuel economy. The ultra-low RPM driving conditions this produces often coincide with natural resonances produced by the engine. In discussions with Magnaflow, this is basically an issue industry-wide, and a big part of the reason OEM systems have become so quiet.

We did spend a lot of time working on the issue and testing different exhaust configurations. One prototype design had a large rear muffler, which effectively killed all resonance, but it also killed most of the sound at higher RPMs. We just didn't feel this design was different enough from the OEM system to justify producing the system, but may look at it again in the future.
So what is the end result. Does the system produce drone at any point?
 

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The instructions ask you to initially hang all sections with the clamps loose and I think this is important. I would also suggest that you slacken the nuts for mount of the very first rubber hanger because small changes in alignment here make big differences further back. With everything a little loose the whole system can find the point of minimum tension in all the hangers and this seems to provide good clearance everywhere and minimal chance of rattles.

Tip protrusion, mine definitely stick out further than some of the install pictures but I’m happy with the position. See attached photos.
Can you take a picture of the weld area in the right side tipd? Thanks for the pictures!!
 

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So what is the end result. Does the system produce drone at any point?
Chipps - In my opinion, "no".... what do you get however, is a pretty deep growly system when you're in 10mph - 15mph neighborhood driving. Especially when the engine is cold. It's the anti-Prius in that respect - in suburbia a neighbor will hear you leaving in the morning.
 

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Steady cruising is at very acceptable db's. You won't know it's there unless you're paying attention to it. If a sports exhaust note is not a priority, pass on it. Otherwise jump on this.
 

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Discussion Starter · #15 ·
Can you take a picture of the weld area in the right side tipd? Thanks for the pictures!!
Will do but it may be a little while until I can get the time to take the diffuser off again. I still haven't completely got orientated on the photo you have posted, I see where it touches I'm just not visualizing exactly where on the Giulia it is. My tips do sit at least an inch further away from the driver though and at that point there weren't any places where it looked like anything would touch and rattle.
 

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Discussion Starter · #16 ·
The drone is gone

I wanted to share information about the change I have made to eliminate the 67 Hz resonance at 2,000 RPM in my Centerline Corsa exhaust. The change gives this system exactly the mix of characteristics I want and I think other Giuliaforum members might be interested. Or at least entertained by what I have done. It’s still very much a sports exhaust that makes it easy and fun to drive the Giulia with the shifter paddles. I definitely spend more time in manual shift mode now. The burble at idle and all the great noise above 2,200 RPM are still present. But it’s now quieter on the highway and surface street commute.

Web searches quickly showed that unsurprisingly the issue of resonance / drone is present in many, perhaps all, muffler delete exhausts. It’s definitely not anything specific to this system. It also became clear there are a number of ways people try to mitigate the problem. I picked one that uses basic physics to actively cancel the unwanted noise without adding a restriction to the flow like a muffler. However, in the postings on other forums that I have been able to find, there is a lot of confusion mixed in. There is also number of university research papers filled with finite element analysis and impractical physical designs for car exhausts. I’m not claiming expertise but I’m hoping I can at least provide an accurate explanation of how this method works and why I didn’t pick alternatives. But the full explanation requires a bunch of background, which gets lengthy and may be of no interest. I’m going to start with a still long summary that skips almost all of the background and details. If there is any interest I'm happy to add some additional posts with a more detailed explanations and the calculations.

The Centerline Corsa exhaust on the 2.0 Giulia is resonant right around 2,000 RPM which for a 4 stroke 4 cylinder means 67Hz. Looking at the FFT plot below the Sound Pressure Level, SPL, is ~20dB greater than almost anything else. The issue is that an increase of 20dB means approximately 100 times the acoustical energy. The next loudest is the spike at ~140Hz and the 67Hz peak is still 10dB (10x) greater than this. The 67Hz is both heard and also felt as vibration through the seat backs, particularly for passengers in the rear seats. An important note is that this happened for me at small throttle openings perhaps 10 to 20% just to change speed on the highway not hard acceleration.



The solution I picked was a ¼ wavelength / branch resonator aka J tube to cancel out this frequency. Fancy name for a second piece of exhaust pipe T’d into the main system but with a closed end. Pressure pulses split at the T, the pulse that runs down the resonator bounces off the closed end and returns back to the T. At a small range of frequencies determined by the length of the pipe the total distance traveled is ½ a wavelength and the reflected pulse cancels out the next pulse in the main exhaust. Fortunately, the 2.0 Giulia transmission tunnel has space for a second exhaust pipe. This is the drawing I took to my local speed shop / custom muffler place.



This is what they fabricated which other than the connection to the elbow being welded not clamped is just what I asked for.


They did a careful job to make sure the system was as straight as possible and had clearance everywhere. There have been no rattles or clunks.


View from the back of the car, the additional hanger they fabricated can be seen and they did a good job matching the factory ones.

And the result is that at 2,000 RPM the FFT chart looks like the picture below. You need to ignore the fact that I had the microphone gain set differently so the overall sound levels are different. The key is that at 67Hz the SPL / loudness is no greater than the surrounding frequencies. I appear to have got close to 20 dB of reduction, which exceeds what I expected. I can’t show it in a single chart but I now see a small peak that corresponds to the RPM moves across the spectrum as I rev the engine but doesn’t change dramatically in size at 67Hz.



Based on the diagram above you can see I asked for an adjustable end cap. The reason for this is that the exact length required depends on the temperature of the air in the resonator. This is because the speed of sound through air changes with temperature and I didn’t have a measured value to use just a guess.



This is the adjustable end cap in the shortest possible position as it came back from the fabrication shop. Nice welding but provided less adjustment than I hoped for.

I was pleasantly surprised that my first guess at length was great when the exhaust was cold and for the first ½ hour of driving but once fully hot there was just a hint of resonance returning. So, I lengthened the tube by approx. 1” by sliding the clamped-on end cap, this fixed the fully hot issue. But I have a second longer end cap on order just to try and find the too long point. I’m grateful to Joe from Centerline for supplying the blank center section which made it easier to create this experiment.

I know the classic question with exhaust systems are, what does it sound like. Because I now have a way of connecting an external mic to the iPhone here are some recordings from inside the cabin with the microphone strapped onto the passenger headrest. I figured that was as close as I could get to real world driving experience with this system. I need to mention that I also have the Eurocompulsion V1 intake that I think enhances the audible ‘bark’ during upshifts. But 95% of the noise is the Centerline Corsa exhaust.

It’s a little tricky to be able to get an idea of the relative sound level for clips like this so I have mentioned common background noise that can be heard in a couple of the clips to give you an idea. First time I have used this audio clip posting service since I can’t find a way to go it direct with the Giuliaform. I checked and the links seem to work without having to create an account on the service but if there is a problem I will find some other way of posting them.

At cruse on highway trying to work the RPM’s right around 2,000 RPM’s. You can hear the turn signal near the beginning as a reference level. I have the settings->cluster->warning buzzer level set to low BTW. This is basically demonstrating the absence of drone so pretty boring.
2000 RPM highway w turn signal

Another 2,000 RPM on the highway, in this one you can hear the tires crossing patches in the road. So again, a pretty boring clip but wanted to show the drone is gone.
2000 RPM highway

Now the fun stuff just to show it still growls when you want it to
Accelerating from rest in N mode
Accel in N mode

Accelerating in D mode with a little bark at the shifts
D mode with bark

Accelerating hard in D mode in the rain - which is the background noise that you hear building up particularly at the end.
D mode in rain
 

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Alan definitely put in some time on this and the science behind the 1/4 wave resonators is very cool. Thanks for putting in the effort to document this for us!!

We've experimented with using them; they did not fit into the design brief of the original Corsa exhaust but if there is enough interest we may develop another variant in the future.
 

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Excellent work Alan! Nothing beats solving a specific problem or issue by logical thinking, measurement, analysis, engineered solution and re-measurement to validate result followed by fine tuning. Excellent work with the Helmholtz resonator.

I have an axle back muffled system with factory resonator front section. I have very low pitch resonance (sounds like a bass speaker humming on medium volume) between 1000-1500rpm. To my ear I feel it peaks at 1250 rpm so the resonance is only an issue during cold start OR the auto-box up shifting to highest gear especially in A mode which sometimes at speeds below 60mph drops revs to just under 1500rpm when I can hear the resonance or when changing gears (auto or manual shift mode) at low town speeds when revs drop momentarily. As you describe it I can feel a slight reverberation from the back of the cabin.
In both N and D modes there is no noticeable resonance at all while cruising on the highway as the revs are higher than 1500rpm. I may experiment with the 1/4 wave-tube to cancel out the peak.

Few questions if I may:
"2.0 Giulia is resonant right around 2,000 RPM which for a 4 stroke 4 cylinder means 67Hz." - I am assuming you arrived at 67Hz by:
RPM * 2 (2 exhaust pulses per rev) * (1/60) ?
Which for me at 1250rpm = 41.7Hz, which is a very low frequency that could explain the very low hum I hear from the rear of the cabin.

I am interested to know the formula you used to calculate the Overall length of the tube and measurement for the Tee-in position to understand how you worked it out.
What I am also not sure of is the positioning which in your case is before resonator but in my case needs to be further back just before the muffler as the resonance only appeared with the axle back muffler?

My understanding was the 1/4 wave resonator is fitted as straight as possible or J shaped at 90 degrees? Yours while it Tee-in 90deg then runs parallel?
Also what mic and software did you use to measure the sound levels?


Great work. Thanks.
 

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Discussion Starter · #20 ·
Calculations

Triumph23,

Pleased to provide some background to see whether that helps you come up with a solution for your lower frequency. Which will be harder because the required 1/4 wave resonator is going to be longer. Apologies my measurements are all for inches and Fahrenheit but I'm sure you can adapt them for units that make sense to you.

RPM to Hz
• You multiply by ½ the number of cylinders to get the number of exhaust pulses per engine revolution.
• You divide by 60 to turn revolutions per minute into revolutions per second because frequency is measured in cycles per second.

Hz = (RPM x ½ the number of cylinders) ÷ 60
67 = (2000 x 2) ÷ 60

Measurement tools
I am fortunate to have access to a microphone with a flat frequency response. By getting an adaptor to plug it into an iPad and an application called FFT plot I had a portable audio spectrum analysis tool. Which in and of itself is pretty miraculous.

The microphone isn’t super fancy
But it is an omni directional flat frequency response that I think is exactly what is needed in this situation

This is the amplifier / adapter I am using to connect an XLR mic to a 3.5mm jack on the iPad

Hz to resonator length

It’s a ¼ wave resonator so the resonator length = wavelength / 4

And the wavelength = speed of sound ÷ frequency

And the speed of sound depends on the density of the air which is dependent primarily on the temperature of the air. There are a number of online calculators for the speed of sound including this one.Speed of sound calculator

These calculators tend to give the speed of sound in feet per second ft/s, it’s easier for me to think of the resonator length in inches so my formula becomes

resonator length in inches = ( speed of sound in ft/s ÷ frequency in Hz ) x 3

But what temperature to use? No exhaust gas flows through the resonator. But it will be heated both by being connected to the main exhaust at the front of the car and heat radiated because its close for most of its length. Adding to the complexity, the heating clearly won’t be even and is likely to be affected by an unknown amount by the outside air temp. Even if I could measure the existing system after driving the car I would still have no idea what the heating effect on the resonator was going to be. I also had no idea how much how sensitive the length calculation was to change in temperature.

So, I used the online calculator to give me a speed of sound for temperatures from 50 F to 200 F in 50 degree increments. And then used a spreadsheet to calculate and plot the resonator length for all those temperatures.


Then I decided to rearrange the formula in the spreadsheet to try and understand how the frequency response of resonator changed with temperature. I also added multiple lengths of resonator. Looking at the data that way has helped me think more clearly about how to change the length based on the changes I was hearing and measuring.



Here is another version of the first chart but redone for 1250 RPM - 42 Hz. which is you current estimate


It's going to be really hard to find space for a resonator that is ~85".
 

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