Custom Exhaust system and sizes 3 5 and 4 and how it sounds

[Aevus]

Hey guys, I need to share my experience since I didnt take the usual bolt-on route Borla or Corsa catback... And also I have a solid background in acoustic.

Just to clear things up from the start, my goal was not to achieve the loudest possible exhaust system but to have the less restriction and the less exhaust reversion possible, in order to optimize the scavenging effect at high rpm. I'm working on an engine that will peak around 6,800-7,000rpm.

So if your goal is to keep the maximum possible low-end torque, you might want to stick with the usual 3 inches diameter.

My baseline was:

- stock headers
- stock cats (2 each sides)
- stock crossovers (gen 3)
- stock resonator/muffler delete

Basically, the somewhat very restrictive stock cats were acting as the muffler. Helped by the crossovers, most probably. Nice overall sound, perfect if you are on a tight budget. Not loud but not too quiet.


Next was:

- M&M headers
- High flow Magnaflow 3'' cat (1 each sides)
- Vibrant race muffler 3.5'' (model 1796)
- Direct 3.5'' exit

As expected, the thing is loud. We probably can say ''crazy loud''. Didn't soundtest at WOT but I wouldnt be surprised if the thing would make 140db SPL at 1 meter distance, if not more. In-cabin at idle, it shows on my app a 102db rating and even though the app may not be perfectly accurate, it makes sense. Now, the problem is not really the high sound levels, it's the frequencies....


IMG-7475.jpg


As you can see, it's all low and very low frequencies. That's what is commonly called ''DRONE'' noises. The problem is: the Viper engine is huge. Therefore it produces lots of low frequencies. Pretty much like big diesel trucks. I sure can feel in the cabin ''subwoofer'' kind of frequencies, which means 20-30hz. That's very low and very annoying. I did a 3 hours roadtrip and the constant low-freq rumble was deafening, so I had to keep the engine in higher rpms to change the resonant frequency...

Needless to say that is not good. It's a real head-turner, everybody smiles hearing it on idle because it sounds like a Harley-Davidson, but you probably don't want to live with that. And since low-frequencies can be heard from longer distances, your neighbors will hate you.

So basically, the SOLUTION is not a muffler but a resonator. You want to suppress those annoying lower frequencies. And since the engine is huge and the frequencies very low, you need a big and long resonator that will be truly effective.

So I ended up with a Diamond Eye Performance 4'' resonator. 24 inches total lenght, installed after the Vibrant muffler, so its aligned with the seats, just before the exhaust exit. It's a resonator usually used for trucks but it shouldnt stop you from using it on a Viper. If you want to get rid of the annoying noises but don't want a restrictive muffler, the options are very limited.

Soon I will post a video that shows the result, along with a new audio RTA screenshot to show the difference.

[Aevus]

Also I'd like to point out that 65hz is the peak sound pressure level, as shown on the graph, but you can see that levels are still very high even below 30-40hz and maybe there is subsonic frequencies, which are good for IMAX theaters but not so much for road trips...

[Aevus]

Important to note that if you're concerned about noise (SPL) you need to take both frequencies and decibels into consideration.

Let's say you put aftermarket headers that makes 8db louder and then cat delete that ads 12db (not real numbers here, just as an example) you end up with +20db exhaust system, so you will need a muffler/resonator that will cut 20db and/or unwanted (low) frequencies.

Usually a muffler is designed to filter ALL frequencies, equally. In reality, that's impossible. A muffler is a very simple mechanical device, not a pro audio EQ... So it will act partially as a resonator (focused on a narrower frequencies band) but will probably be much more effective than a resonator to make the peak SPL lower.

The resonator, which also is a simple mechanical device and not a pro audio EQ, will also act as a muffler; which means the overall SPL output will be reduced and pretty much on a wide frequencies band. But theoratically, the manufacturer's are designing them to work on a narrow band (low frequencies -drone noise-).

To sum that up, here is an exemple (again not real numbers)

MUFFLER ABC: -13db on peak and -5db on lower frequencies
RESONATOR XYZ: -4db and -18db on lower frequencies.

Usually, you might want to use both.

But if you're okay with a very loud exhaust at WOT, resonator-only might be the solution.
If you really want a big thumpy rumble at idle, then discard the resonator and use a muffler (or cat-only/straight pipe..)

Hope that helps understand better.

- - - Updated - - -

Important to note that if you're concerned about noise (SPL) you need to take both frequencies and decibels into consideration.

Let's say you put aftermarket headers that makes 8db louder and then cat delete that ads 12db (not real numbers here, just as an example) you end up with +20db exhaust system, so you will need a muffler/resonator that will cut 20db and/or unwanted (low) frequencies if you want to be back to stock SPL (and that's probably not even possible, bytheway)

Usually a muffler is designed to filter ALL frequencies, equally. In reality, that's impossible. A muffler is a very simple mechanical device, not a pro audio EQ... So it will act partially as a resonator (focused on a narrower frequencies band) but will probably be much more effective than a resonator to make the peak SPL lower.

The resonator, which also is a simple mechanical device and not a pro audio EQ, will also act as a muffler; which means the overall SPL output will be reduced and pretty much on a wide frequencies band. But theoratically, the manufacturer's are designing them to work on a narrow band (low frequencies -drone noise-).

To sum that up, here is an exemple (again not real numbers)

MUFFLER ABC: -13db on peak and -5db on lower frequencies
RESONATOR XYZ: -4db and -18db on lower frequencies.

Usually, you might want to use both.

But if you're okay with a very loud exhaust at WOT, resonator-only might be the solution.
If you really want a big thumpy rumble at idle, then discard the resonator and use a muffler (or cat-only/straight pipe..)

Hope that helps understand better.

[Aevus]

Where it starts to be tricky, is if you're concerned about racetrack SPL limits.

Problem is: peak SPL will probably happen at higher frequencies at WOT. But lower frequencies travels farther and are omnidirectional.

So you basically have to know where the racetrack's microphones are placed. Since the higher frequencies are more directional, you might have a loud exhaust but be ok if the pipes are turned up or anywhere but directly at the mic direction.

With lower frequencies, you basically have no control since the rumble is omnidirectional and can be heard from longer distances. So a resonator might be the only option. In my case, as you see in the graph above, I don't even need to go to my racetrack to know that I'd be kicked out. Both peak SPL at higher frequencies and low frequencies will trigger the mic's limit. With a resonator I might have a fighting chance.

[13COBRA]

I had to make a post to save you from 5 in a row without anyone else commenting. You're welcome.

[Aevus]

I had to make a post to save you from 5 in a row without anyone else commenting. You're welcome.

Hahaha thanks 13cobra

it shows that much that I'm in vacations?

[Aevus]

Diamond Eye 4'' resonator. Will get the thing installed and heat wrapped, hopefully it will reduce the lower frequencies at least 5-6db and also a bit of the higher frequencies....

[ViperDrive97]

Did you ever get things tested with video/audio?

[Aevus]

Did you ever get things tested with video/audio?

Unfortunately not, I'm still waiting for my race engine to be ready (head porting is taking forever...)

I'm using right now (stock engine) M&M headers + high flow cats + custom straight through 28'' resonator (in/out 3.5'') and I'm not satisfied at all: a mix of ''too quiet'' at idle and lots of drone at cruising speed.

Next thing i'll try is the Diamond Eye 4'' resonator for sure. Since I'll have high duration camshaft and forged internals that will affect the sound, I will wait for the new engine to be installed before touching the exhaust system

[GTS Dean]

I doubt the short block configuration has much, if any effect on the sound. Ignition, cam/valve timing and exhaust harmonics are probably going to be your key drivers.